FERMILAB-CONF-23-008 SLAC-PUB-17717 January 2023

Report of the 2021 U.S. Community Study
on the Future of Particle Physics
(Snowmass 2021)

Summary Chapter

2021 – 2022 Snowmass Steering Group:

Joel N. Butler1, R. Sekhar Chivukula2,
André de Gouvêa3, Tao Han4, Young-Kee Kim5,
Priscilla Cushman6 (APS Division of Particles and Fields),
Glennys R. Farrar7 (APS Division of Astrophysics),
Yury G. Kolomensky8 (APS Division of Nuclear Physics),
Sergei Nagaitsev1 (APS Division of Physics of Beams),
Nicolás Yunes9 (APS Division of Gravitational Physics)

Snowmass 2021 Frontier Conveners:

Stephen Gourlay10, Tor Raubenheimer11, Vladimir Shiltsev1 (Accelerator),
Kétévi A. Assamagan12, Breese Quinn13 (Community Engagement), V. Daniel
Elvira1, Steven Gottlieb14, Benjamin Nachman10 (Computational), Aaron S.
Chou1, Marcelle Soares-Santos15, Tim M. P. Tait16 (Cosmic), Meenakshi
Narain17†00footnotetext: †deceased, Jan. 1, 2023., Laura Reina18, Alessandro
Tricoli12 (Energy), Phillip S. Barbeau19, Petra Merkel1, Jinlong Zhang20
(Instrumentation), Patrick Huber21, Kate Scholberg19, Elizabeth Worcester12
(Neutrino), Marina Artuso22, Robert H. Bernstein1, Alexey A. Petrov23 (Rare
Processes), Nathaniel Craig24, Csaba Csáki25, Aida X. El-Khadra9 (Theory),
Laura Baudis26, Jeter Hall27, Kevin T. Lesko10, John L. Orrell28 (Underground
Facilities), Julia Gonski29, Fernanda Psihas1, Sara M. Simon1 (Early Career)

Editor:  Michael E. Peskin11

Abstract

The 2021-22 High-Energy Physics Community Planning Exercise (a.k.a. “Snowmass
2021”) was organized by the Division of Particles and Fields of the American
Physical Society. Snowmass 2021 was a scientific study that provided an
opportunity for the entire U.S. particle physics community, along with its
international partners, to identify the most important scientific questions in
High Energy Physics for the following decade, with an eye to the decade after
that, and the experiments, facilities, infrastructure, and R&D needed to
pursue them. This Snowmass summary report synthesizes the lessons learned and
the main conclusions of the Community Planning Exercise as a whole and
presents a community-informed synopsis of U.S. particle physics at the
beginning of 2023. This document, along with the Snowmass reports from the
various subfields, will provide input to the 2023 Particle Physics Project
Prioritization Panel (P5) subpanel of the U.S. High-Energy Physics Advisory
Panel (HEPAP), and will help to guide and inform the activity of the U.S.
particle physics community during the next decade and beyond.

1 Fermi National Accelerator Laboratory, Batavia IL 60510 USA
2 University of California, San Diego, La Jolla CA 92093 USA
3 Northwestern University, Evanston IL 60208 USA
4 University of Pittsburgh, Pittsburgh PA 15260 USA
5 University of Chicago, Chicago IL60637 USA
6 University of Minnesota, Minneapolis MN 55455 USA
7 New York University, New York NY 10027 USA
8 University of California, Berkeley CA 94720 USA
9 University of Illinois at Urbana-Champaign, Urbana IL 61801 USA
10 Lawrence Berkeley National Laboratory, Berkeley CA 94720 USA
11 SLAC National Accelerator Laboratory, Menlo Park CA 94025 USA
12 Brookhaven National Laboratory, Upton NY 11973 USA
13 University of Mississippi, University MS 38677 USA
14 University of Indiana, Bloomington IN 47405 USA
15 University of Michigan, Ann Arbor MI 48109 USA
16 University of California, Irvine CA 92697 USA
17 Brown University, Providence RI 02912 USA
18 Florida State University, Tallahassee FL 32306 USA
19 Duke University, Durham, NC 27708 USA
20 Argonne National Laboratory, Lemont, IL 60439 USA
21 Virginia Tech, Blacksburg, VA 24061 USA
22 Syracuse University, Syracuse NY 13244 USA
23 University of South Carolina, Columbia, SC 29208 USA
24 University of Calilfornia, Santa Barbara CA 93106 USA
25 Cornell University, Ithaca NY 14853 USA
26 University of Zürich, 8006 Zürich SWITZERLAND
27 SNOLAB, Lively, ON P3Y 1N2 CANADA
28 Pacific Northwest National Laboratory, Richland, WA 99354 USA
29 Columbia University, New York NY 10027 USA

###### Contents

  1. 0 The Future of U.S. Particle Physics: Summary of Snowmass 2021
    1. 1 Introduction
      1. 1 The Snowmass Process, Introduction and History
      2. 2 Introduction to Snowmass 2021
      3. 3 Overview of the Snowmass Report
    2. 2 High-Level Conclusions from Snowmass 2021
      1. 1 Large-Scale Future Proposals Discussed at Snowmass 2021
      2. 2 Medium- and Small-Scale Future Experiments and Projects
    3. 3 A Science Driver-Oriented View
      1. 1 Use the Higgs Boson as a Tool for Discovery
      2. 2 Pursue the Physics Associated with Neutrino Mass
      3. 3 Identify the New Physics of Dark Matter
      4. 4 Understand Cosmic Acceleration: Dark Energy and Inflation
      5. 5 Explore the Unknown: New Particles, Interactions, and Physical Principles
      6. 6 Flavor as a Tool for Discovery $-$ Possible New Driver
    4. 4 Brief Summaries of Frontier Reports
      1. 1 Accelerator Frontier [5]
      2. 2 Community Engagement Frontier [6]
      3. 3 Computational Frontier [7]
      4. 4  Cosmic Frontier [8]
      5. 5 Energy Frontier [9]
      6. 6 Instrumentation Frontier [10]
      7. 7 Neutrino Frontier [11]
      8. 8  Rare Processes and Precision Measurements Frontier [12]
      9. 9  Theory Frontier [13]
      10. 10  Underground Facilities & Infrastructure Frontier [14]
      11. 11 Snowmass Early Career Report [15]
    5. 5 Special Cross-cutting Topics
      1. 1 Prospects and Evaluation of Future Colliders
        1. e+e- Colliders Forum [16]
        2. Muon Collider Forum: main findings and recommendations [17]
        3. Implementation Task Force (ITF) analysis [18]
      2. 2 Dark Matter Complementarity [19]
      3. 3 Synergies and Complementarities with Astrophysics and Nuclear Physics
        1. High Energy Physics and Astrophysics
        2. High Energy Physics and Nuclear Physics
    6. 6 Summary

##  Chapter 0 The Future of U.S. Particle Physics: Summary of Snowmass 2021

###  1 Introduction

Particle physics seeks to identify the elementary constituents of matter and
the forces and principles by which they interact. By looking at matter at the
smallest distance scales, physicists have identified the interactions that
operate at the subatomic level: the strong, weak, and electromagnetic forces –
the last two of which are unified into a single electroweak theory. The force
carriers of these interactions, the photon, the $W$, and $Z$ bosons of the
electroweak theory, and the gluons of the strong interactions, along with the
quarks and leptons, the fermionic components of matter, form the basis of the
Standard Model (SM) of particle physics.

The last missing element in the SM was the agent of electroweak symmetry
breaking, which gives mass to the $W$ and $Z$ bosons as well as the quarks and
leptons. The new scalar particle discovered at the Large Hadron Collider (LHC)
by the ATLAS and CMS experiments in 2012 is produced and decays in a manner
that suggested that it is the “Higgs boson”, the excitation of an elementary
scalar Higgs field which accommodates the simplest mechanism for electroweak
symmetry breaking. In the decade since then, ATLAS and CMS have shown that the
couplings of this new boson to the heaviest SM particles, the top quark, the
tau lepton, and the $b$-quark, all of which are third-generation fermions, as
well as to the $W$ and $Z$, are all consistent with its interpretation as the
Higgs boson. While there is plenty of room for new physics and there is much
work to be done to fully study this remarkable, unique scalar particle, its
discovery demonstrates that the SM is at least approximately correct and
accurately describes elementary particles and their interactions at energies
up to a few hundred GeV.

While the SM explains many aspects of the fundamental physics world with
impressive accuracy, it is nonetheless silent on several issues of fundamental
importance, including the “small” mass, 125 GeV, of the Higgs boson, which is
unstable to radiative corrections from interactions at higher energy scales;
the excess of matter over antimatter (the baryon asymmetry of the universe);
why the strong interaction does not exhibit CP violation; the mechanism behind
the tiny yet nonzero neutrino masses; the issue of flavor, namely why are
there three generations of quarks and leptons with disparate masses and
mixings and why is the mixing pattern observed in the lepton sector so
different from that in the quark sector; the origin and nature of dark matter
and dark energy; the mechanism for cosmic inflation in the early Universe; how
gravity fits into the other fundamental interactions and how our general
relativistic understanding of space and time can be reconciled with quantum
mechanics. These are profound questions whose answers will fundamentally
change our understanding of the physical world. Since the answers to these
questions cannot be found within the SM, there must be new Beyond the Standard
Model (BSM) particles and interactions that explain these aspects of nature.

We have many theoretical ideas that address some or all of these problems but,
for now, no unambiguous experimental guidance on which ones are correct. Hints
do exist, including the tension between some theoretical calculations of the
anomalous magnetic moment of the muon and the measured value, anomalies in
neutrino physics exhibited in short-baseline experiments, a possible
disagreement between the most recently published, most precise measurement of
the $W$ boson mass and its theoretical value, and several anomalies in quark
flavor physics. With more and more precise data, some of these may disappear
or have explanations within the SM, while others may evolve into puzzles whose
solutions provide vital hints about BSM physics. Along the way, new anomalies
might emerge, potentially pointing in unforeseen directions. The next big clue
may be hiding within data already collected at the LHC or about to be
collected during the planned High Luminosity LHC (HL-LHC) era or in the rare
decays and properties of known particles. Ongoing or future oscillation
experiments may reveal more surprises in the neutrino sector. The new physics
may be at masses higher than we can currently reach or weakly coupled enough
that it has yet to manifest itself in a way we are prepared to notice.

At present we have come to understand that we are confronted by a vast range
of possibilities for BSM physics. For example, the particle that might
constitute dark matter can have a mass that spans 90 orders of magnitude. This
is an exciting challenge. There is room for innovative ideas and a vast
unexplored territory to investigate. There are extraordinary new accelerators
and detectors that we can build, new and different observations we can make,
along with enormous computing power and new techniques that permit us to
collect and analyze unprecedented amounts of data using innovative, more
powerful software tools. These new techniques enable us to probe the
boundaries of the SM to unprecedented accuracy and extend the search for BSM
physics to higher energy scales and weaker couplings. For these reasons, we
believe that we will find the trail of the new physics that lies beyond the
SM. This will take the efforts, imagination, and creativity of the community
that participated in Snowmass 2021, joined by a large and even more diverse
new generation of bold thinkers and intrepid doers. We, therefore, present the
following vision that emerged from Snowmass:

> Lead the exploration of the fundamental nature of matter, energy, space and
> time, by using ground-breaking theoretical, observational, and experimental
> methods; developing state-of-the-art technology for fundamental science and
> for the benefit of society; training and employing a diverse and world-class
> workforce of physicists, engineers, technicians, and computer scientists
> from universities and laboratories across the nation; collaborating closely
> with our global partners and with colleagues in adjacent areas of science;
> and probing the boundaries of the Standard Model of particle physics to
> illuminate the exciting terrain beyond, and to address the deepest mysteries
> in the Universe.

We conclude from our long community study that, in the spirit of Snowmass, we
stand ready with great enthusiasm to “delve deep, search wide, and aim high”
to advance our understanding of the Universe!

####  1 The Snowmass Process, Introduction and History

This report presents the main conclusions of Snowmass 2021, the most recent of
the U.S. High Energy Physics (HEP) Community Planning Exercises, sponsored by
the Division of Particles and Fields (DPF) of the American Physical Society
(APS), with strong consultation from the aligned APS Divisions of Nuclear
Physics, Astrophysics, Gravitational Physics, and Physics of Beams. These
exercises have occurred roughly every seven to twelve years since 1982. The
goal of these community studies has been to identify the most important
scientific questions in HEP for the following decade, with an eye to the
decade after that, and the facilities, infrastructure, and R&D needed to
pursue them. Although focused on the U.S. HEP program, HEP is a global
enterprise and international participation was encouraged. Many scientists
from outside the U.S. have contributed to Snowmass 2021. HEP has important
connections to many other fields of physics and physicists from all related
fields have participated. For many years these efforts concluded with a
workshop in the summer in Snowmass, Colorado and so these exercises became
known as the Snowmass Summer Study and Workshops. The last such exercise was
held in 2013 [1]. Its concluding workshop was for the first time held outside
of Snowmass at the University of Minnesota. However, we retained the name
“Snowmass” to connect it to and commemorate the previous studies.

In 2013 and all previous editions, the planning exercise focused on scientific
issues without considering detailed budget constraints or establishing project
or funding priorities. All scientifically credible ideas and proposals were
welcome. These science exercises were each followed by a subpanel of High
Energy Physics Advisory Panel (HEPAP) that advised the funding agencies on
which of the many proposed experiments and projects should be carried out,
given realistic budget constraints. In 2013, the output of the Snowmass study
provided input to the HEPAP subpanel called the Particle Physics Project
Prioritization Panel, “P5”, which was charged by the U.S. Department of Energy
(DOE) and the National Science Foundation (NSF) with the task of recommending
plans for U.S. investments in particle physics under specific budget scenarios
for the coming decade. P5 began its work in the fall of 2013 and produced a
report [2] with its recommendations in the spring of 2014.

At the start of its deliberations, P5 examined the findings of the Snowmass
study and identified five main science Drivers for HEP:

  1. 1.

Use the Higgs Boson as a Tool for Discovery,

  2. 2.

Pursue the Physics Associated with Neutrino Mass,

  3. 3.

Identify the New Physics of Dark Matter,

  4. 4.

Understand Cosmic Acceleration: Dark Energy and Inflation,

  5. 5.

Explore the Unknown: New Particles, Interactions, and Physical Principles.

It also made 29 recommendations concerning the projects and we have been
constructing the favored ones, some of which are shown in Fig. 1, since 2015.
The two-step approach to planning, the community Snowmass study, centered on
the science, and the subsequent P5 prioritization, also taking into account
budget constraints, led to a clear understanding and broad acceptance of the
2014 P5 report by the HEP community and beyond.

Among the projects assigned the highest priority by P5 was participation in
the High Luminosity Upgrade of the Large Hadron Collider and the ATLAS and CMS
experiments at CERN; the LSST/Rubin Observatory; and the construction of PIP-
II SRF proton linac at Fermilab and the LBNF/DUNE (Long Baseline Neutrino
Facility/Deep Underground Neutrino Experiment) long baseline neutrino
experiment. The full suite of experiments enabled by the projects recommended
by the 2013/14 Snowmass/P5 process was chosen to enable U.S. high energy
physicists and their partners to examine the properties of matter at the
tiniest distance scales ever probed and set the stage for the exciting
experimental investigations of the decade just past and into the decade to
come.

Figure 1: Examples of some of the projects prioritized in the Snowmass 2013/P5
2014 process. LBNF/DUNE: a) cavern excavation; b) “open cut” to receive
excavated rock; c) interactions recorded by Proto-DUNE-SP; d) Neutrino
Platform at CERN; Muon g-2: e) muon storage ring; ATLAS Upgrade: f) New Small
Wheel; Mu2e: g) calorimeter support frame; h) Tracker planes; CMS Upgrade: i)
Phase 1 upgrade pixel installation; Vera Rubin Observatory: j) schematic.

####  2 Introduction to Snowmass 2021

In 2019, with the construction of many of the projects supported by the
2013/14 planning process well underway or in an advanced stage of planning, as
shown in Fig. 2, the U.S. High Energy Physics Advisory Panel (HEPAP)
recognized the need for a new community study to develop the initiatives in
the decade starting in 2025. DPF began preparation for Snowmass 2021 community
study in 2019. This “Snowmass 2021” HEP Community Planning Exercise formally
began with a “kick-off” meeting at the 2020 APS April Meeting and was followed
by a Community-wide Planning Meeting (CPM), in October 2020, in which many
activities, meetings, and workshops were mapped out. The exercise was to have
concluded in July of 2021 with a final workshop in Seattle hosted by the
University of Washington. The COVID-19 pandemic severely disrupted our plans
and forced us to pause work between January and September of 2021, especially
to lighten the burden on our younger scientists, many of whom had to take care
of and school their young children. The final workshop in Seattle was delayed
until the summer of 2022 and the start of the P5 panel was also delayed by one
year by the DOE and NSF. We resumed work for Snowmass in September of 2021
and, despite the continuing, but now diminished, challenges of COVID-19, our
community accomplished an amazing amount of work and was well prepared for the
Seattle meeting in July of 2022.

Snowmass 2021 was organized into ten working groups, or “Frontiers”:
Accelerator (AF), Community Engagement (CEF), Computational (CompF), Cosmic
(CF), Energy (EF), Instrumentation (IF), Neutrinos (NF), Rare Processes and
Precision Measurements (RPF), Theory (TF), and Underground Facilities and
Infrastructure (UF). These Frontiers comprise a broad array of ground-breaking
scientific research topics and the underlying technology and infrastructure
needed to execute them, as well as, through the CEF, a forum to examine how
the U.S. HEP community can become more representative of and responsive to all
members of our community and can engage with society as a whole. Each Frontier
divided its work into several Topical Groups (TGs). The number of TGs varied
by Frontier from 6 to 11 and there were approximately 80 overall. A call for
two-page “Letters of Interest (LOIs)”, due by August 31, 2020, resulted in
over 1500 submissions. These were assigned to the TGs (recognizing overlaps).
Each Topical Group encouraged its members to produce contributed papers more
completely describing their ideas and plans. Often, several submitters of LOIs
joined together to produce a single contributed paper. Over five hundred
contributed papers, also known as white papers, were produced! The work
incorporated input from the “European Strategy for Particle Physics” [3] and
other national and international planning exercises.

Early Career physicists have been formally represented at Snowmass since 2001
and gained even more formal recognition in 2013. In 2021/22, a Snowmass Early
Career (SEC) organization was formed to assist young physicists in
contributing to the Snowmass process and to bring their issues into the
community study. The SEC was treated comparably to a full Frontier. There were
also important connections among all the Frontiers, and we implemented a
strong team of about 80 cross-frontier liaisons to make sure nothing fell
between the cracks. International and interdisciplinary participation was
encouraged. Our goal was for everyone to be able to participate and for all
voices to be heard.

Leading up to the Seattle meeting, both before and after the pandemic pause,
there were many TG workshops/meetings, cross-frontier/cross-TG
workshops/meetings, and many studies carried out by groups and individuals. By
the spring of 2022, nearly 500 white papers had already been submitted. By
January 2022, it was possible for people to travel to and attend meetings with
a relatively low risk of contracting COVID, and about half the Frontiers held
“all-frontier” meetings with both in-person and remote participation, with the
other Frontiers relying completely on videoconferencing. The purpose of these
meetings was to begin the synthesis of the white papers and the formulation of
the topical group reports.

Figure 2: Status of projects supported all or in part by DOE in operation
(green), under construction or in production (blue), or still under design and
project definition (yellow), shown at the Snowmass Community Summer Study,
July 2022.

The final discussions and synthesis of all the white papers and other
materials developed for Snowmass took place at the Community Summer Study and
Workshop (CSS) at the University of Washington in Seattle, from July 17-26,
2022. Vaccines and other measures enabled us to conduct a safe, hybrid
meeting, with an in-person attendance of about 700 and about 650 participants
connecting remotely, as pictured in Fig. 3.

The daily CSS program typically started at 8 am and ran until 7 pm. Days 2-8
were packed with parallel sessions dedicated to Frontier and cross-frontier
meetings in the mornings and three 90-minute-long plenary sessions in the
afternoons in which each Frontier presented talks designed to inform the
entire community of its main plans and conclusions. There were also panel
discussions of issues of common interest to the whole community. Day 1 and
days 9 and 10 consisted of all-plenary sessions that included special
presentations of plans and planning processes by many leaders of U.S. and
international institutes, laboratories, and funding agencies. Also, on days 8
and 9, each Frontier summarized its vision for the future and its main
conclusions and recommendations, including the projects and facilities that
would be needed to realize the vision.

On the final day of the CSS, Priscilla Cushman of the University of Minnesota
put it all in perspective with her inspiring talk, “Snowmass Highlights and
Message Distillation”. As the audience held its breath, JoAnne Hewett of SLAC
National Accelerator Laboratory and chairperson of the U.S. High Energy
Physics Advisory Panel, announced that the new P5 chairperson is Hitoshi
Murayama from UC Berkeley.

A full schedule for the Seattle meeting, along with talks and eventually
recordings, can be found on the Snowmass Indico site [4]. Short versions of
the Frontier Summaries [5] – [14], the report of the Snowmass Early Career
Scientists [15], and discussions of selected cross-cutting and
interdisciplinary aspects of HEP [16, 17, 18, 19] are included below.

Figure 3: Snowmass Community Summer Study, University of Washington, Seattle,
a photograph showing in-person attendees in the center and panels in the upper
right- and left-hand corners showing some of the remote participants.

####  3 Overview of the Snowmass Report

This Snowmass Report represents the summary of the Snowmass 2021 Community
Planning Exercise. It is organized around the work of the ten Frontiers and
the SEC. Each Frontier and the SEC conducted its work in meetings of various
kinds, mostly using Zoom because of COVID-19, but with some hybrid meetings
with an in-person component as the pandemic began to abate in the spring of
2022. By the late spring, the Topical Groups in each Frontier and the SEC
began preparing their reports. All participants in the work were encouraged to
read and comment on the various drafts. Revisions were made and eventually, a
final consensus report emerged. By the time of the CSS, Topical Group reports
of each Frontier were being synthesized into Frontier Reports. These also went
through a process of drafting, followed by meetings, commentary, and iteration
to achieve a consensus.

The remainder of this Chapter consists of some very high-level conclusions, a
discussion and assessment of the five science Drivers defined by P5/2014 and
their applicability to HEP in the next decade and beyond, a brief summary of
each of the ten Frontier reports and the SEC report, some examples of cross-
cutting HEP activities, and some concluding observations. In the full Snowmass
Report, this summary chapter is followed by eleven chapters presenting the
full reports of the ten Frontiers and the SEC. These are followed by chapters
that discuss options for future particle colliders; the search for dark matter
as an example of how the Frontiers are working together, and with adjacent
disciplines, to solve one of our most important puzzles; and the close
connections between particle physics, astrophysics, and nuclear physics.
Finally, there is a brief summary and conclusion for the whole Snowmass
process. Key contributors are acknowledged at the end of the report.

All Snowmass Frontier summaries, Topical Group summaries, and white papers are
accessible from the Snowmass Proceedings website [20]. The Snowmass Report
[21] is also available from that site.

###  2 High-Level Conclusions from Snowmass 2021

There was broad agreement at Snowmass 2021 on the general principles needed to
have a successful U.S. HEP program in the future.

  * •

In 2014, P5 formulated five science Drivers that were meant to summarize and
organize the scientific questions that HEP seeks to answer. These five Drivers
have guided U.S. HEP for nearly a decade with great success. We have made
tangible progress toward addressing them, and have used that progress to
formulate the steps for the next decade that are outlined in this report.
There was consensus in Snowmass 2021 that these Drivers were still appropriate
for the next decade.

    * –

A proposal was made by the Rare Processes and Precision Measurements Frontier
that the physics of flavor, currently included without explicit acknowledgment
in the science Drivers, be specifically named, given its unique
characteristics and opportunities. One way of accomplishing this would be for
flavor physics to become a sixth Driver.

  * •

The completion of existing experiments and the construction and operation of
approved projects, including those prioritized by the 2014 P5 such as the HL-
LHC, LBNF/DUNE, PIP-II, LSST/Rubin Observatory, and Mu2e programs, along with
our many midsize and small experiments, are critical for addressing the
science Drivers for the near term and for much of the next two decades.

  * •

As existing approved construction projects come to a completion, a broad and
complementary set of new projects should be considered a high priority to
maintain the continuity of our investigations and opportunities for new
discoveries.

    * –

A list of the large-scale (estimated total project costs of approximately
$500M or larger) new projects or programs discussed at Snowmass is shown in
Table 1 and is discussed in section 1 and in the Frontier summaries that
follow. The proposals are examined in detail in the Snowmass 2021 Frontier and
Topical Group reports.

    * –

The portfolio of HEP projects should continue to include a healthy breadth and
balance of physics topics, experiment sizes, and timescales, supported by a
dedicated, robust, ongoing funding process. Medium- and smaller-scale projects
are discussed more fully in section 2, the reports in this volume, and the
corresponding Topical Group reports.

  * •

Robust support for physics research programs at universities and national
laboratories is essential to operate existing and planned experiments, analyze
the data they collect, plan and construct upgrades and new experiments and
projects, and educate the next generation of researchers and technical
experts.

  * •

Greater support for the infrastructure and enabling technologies, namely
accelerators, computation, detectors, and instrumentation, is essential. Both
R&D directed to specific future projects and generic research should be
supported in these critical enabling technologies, as well as in new areas
such as quantum science and machine learning.

    * –

Some guidance to HEPAP and the funding agencies in how the work of the
enabling Frontiers should be coordinated and advanced would be very valuable.

    * –

When R&D projects produce successful tools, where appropriate, plans should be
developed to convert them to products and support them throughout their useful
lives.

  * •

HEP benefits from collaboration with adjacent scientific disciplines, such as
nuclear physics, accelerator technology and beam physics, astronomy and
astrophysics, gravitational physics, and atomic and molecular physics, and
from interactions with industry, and contributes to them in return.
Opportunities to strengthen and expand such collaborations are mutually
beneficial and should be pursued, and new opportunities for collaboration are
arising in other areas such as artificial intelligence and machine learning,
and quantum information science and sensing.

  * •

The HEP community should strengthen connections with its early career
researchers, foster their professional success both within and outside of
academia, and ensure their voices are well-represented in physics and
community planning.

  * •

HEP should take a cohesive and strategic approach to promote diversity,
equity, and inclusion in high-energy physics, in collaboration with funding
agencies, universities, adjacent scientific disciplines, APS/DPF, and others.

    * –

The community should institute a broad array of practices and programs to
reach and retain the diverse talent pool needed for success in achieving our
scientific vision and to address the persistent under-representation of women
scientists, LGBT+ scientists, scientists who are Black, Indigenous, and people
of color (BIPOC), and scientists with disabilities.

  * •

The HEP community must engage in a coordinated way with five other
interrelated communities: academia, the education communities providing
instruction from kindergarten through postdoctoral training, private industry,
government policymakers, and the broader society. A structure for formulating
a coordinated approach to achieve these goals should be created and provided
with the resources needed for success.

####  1 Large-Scale Future Proposals Discussed at Snowmass 2021

Although costs and budgets of future projects or programs were not considered
in detail during the Snowmass process, the new proposals discussed were
divided into categories based on the approximate total projected cost –
“large”, greater than $\sim$$500M, “medium”, between of order $75M$-$$500M,
and “small”, less than or of order $75M.111These monetary thresholds were used
in the Snowmass study for discussion purposes only, and are not aligned with
any budgeting process. The costs are intended to represent the U.S. share of
the projects. The identification of large proposals, and their related
timescales, is particularly important for the combined Snowmass/P5 process
since the impact that they have on planning is so significant. In contrast,
“medium” and “small” projects and proposals, though extremely important, of
great scientific value, and essential to maintain the diversity and vibrancy
of the U.S. HEP program (see section 2), have a much smaller effect on overall
resources. In Table 1 we list the future large proposals discussed during the
Snowmass process for the coming two decades to address the essential
scientific goals that were identified.

As illustrated in Table 1, the future large-scale projects/programs that were
discussed during Snowmass 2021 fall into four scientific themes – broadly
corresponding to the scientific questions addressed by the energy, neutrino,
cosmic, and rare process Frontiers – and can be classified as occurring in the
coming decade or the following one. The specific scientific goals and
capabilities of the projects included in this table are discussed further in
this document and the Frontier Reports that follow – and even more thoroughly
in the related Topical Group Reports. This table is not a timeline, rather
large projects are listed by the decade in which the preponderance of their
activity is projected to occur. Projects may start sooner than indicated or
may take longer to complete, as described in the frontier reports. The
projects listed were considered during Snowmass 2021 without prioritization or
consideration of any future budget scenarios.

Decadal Overview of Future Large-Scale Projects
---
Frontier/Decade | 2025 - 2035 | 2035 -2045
Energy Frontier | U.S. Initiative for the Targeted Development of Future Colliders and their Detectors
| Higgs Factory
Neutrino Frontier | LBNF/DUNE Phase I & PIP- II | DUNE Phase II (incl. proton injector)
Cosmic Frontier | Cosmic Microwave Background - S4 | Next Gen. Grav. Wave Observatory∗
Spectroscopic Survey - S5∗ | Line Intensity Mapping∗
Multi-Scale Dark Matter Program (incl. Gen-3 WIMP searches)
Rare Process Frontier |  | Advanced Muon Facility
Table 1:  An overview, binned by decade, of future large-scale projects or
programs (total projected costs of $500M or larger) endorsed by one or more of
the Snowmass Frontiers to address the essential scientific goals of the next
two decades. This table is not a timeline, rather large projects are listed by
the decade in which the preponderance of their activity is projected to occur.
Projects may start sooner than indicated or may take longer to complete, as
described in the frontier reports. Projects were not prioritized, nor examined
in the context of budgetary scenarios. In the observational Cosmic program,
project funding may come from sources other than HEP, as denoted by an
asterisk.

Some notes on each of the scientific categories in the table:

  * •

In the Cosmic Frontier, a coordinated multi-scale dark matter program would
combine direct, indirect, and cosmic probe experiments to explore the large
dark-matter landscape (and, in total, rise to the “large” project category).
Note that an expansion of underground facilities at SURF may be required as a
component of this program. In the observational Cosmic program, projects may
leverage funding from sources outside of HEP itself, as denoted by the
asterisks in the table. Both CMB-S4 and Gen-3 WIMP searches (previously DM-G3)
were endorsed as promising future directions by the previous Snowmass/P5
process.

  * •

In the case of the Energy Frontier, and as emphasized by the Accelerator and
Theory Frontiers as well, the goal should be to position the U.S. HEP program
to support construction of an Higgs Factory as early as 2030, and to
subsequently be prepared to host or participate in the construction of a
multi-TeV (muon or hadron) collider. In total, these investments (referred to
as a “U.S. Initiative for the Targeted Development of Future Colliders and
their Detectors” in Table 1) rise to the level of a large-scale project. In
the early phase accelerator work should prioritize an $e^{+}e^{-}$ Higgs
Factory (such as ILC, CLIC, FCC-ee, CEPC, C3, or HELEN), a parallel effort
should focus on multi-TeV colliders for the longer term, and some work on
advanced accelerator R&D should continue. Targeted detector R&D for the Higgs
Factory is required in the early phase, with a smaller detector R&D component
related to multi-TeV colliders. In the later phase, as an $e^{+}e^{-}$ Higgs
Factory construction is taking place, accelerator and detector R&D effort on
multi-TeV colliders will need to increase.

  * •

For the Neutrino Frontier the highest priorities are the completion of
LBNF/DUNE Phase I in the coming decade (with the corresponding PIP-II
upgrade), and the construction of DUNE Phase II (with the corresponding proton
source upgrade) in the decade following. DUNE Phase I and Phase II are
described briefly in section 4.7 and more completely in the Neutrino Frontier
report. The completion of the DUNE science program was identified as a high
priority of the previous Snowmass/P5 process.

  * •

For the Rare Process and Precision Measurements Frontier, the “Advanced Muon
Facility” for studies of muon decays, muon conversion, and muonium
transitions, may require coordinated improvements or modifications to the FNAL
proton complex.

####  2 Medium- and Small-Scale Future Experiments and Projects

Medium- and small-size experiments and projects are an important component of
the current and proposed program. In the past, experiments with these scales
have made significant measurements and important discoveries, opening up new
areas of scientific exploration. Furthermore, because of their timescale and
size, these experiments offer unique leadership and training opportunities for
younger physicists and allow for greater diversity in the experimental
particle physics ecosystem.

In the Snowmass process, many medium- and small-scale experiments have been
presented – too many and too diverse a group to easily categorize and
summarize. Here we mention a few broad classes corresponding to the four major
experimental directions, though this list is not comprehensive.

  * •

Cosmic Frontier: Many small- and mid-scale dark matter searches are needed to
cover the huge parameter space, including models of ultralight dark matter
such as the QCD axion, in which dark matter could reside. Various expansions
of astroparticle observatories such as IceCube Gen2, AugerPrime, the Cerenkov
Telescope Array, and the Southern Wide-field Gamma Ray Observatory are also
underway or planned, to probe fundamental physics beyond accelerator energies.
While mostly supported outside of DOE, these can benefit HEP by probing
fundamental physics in regimes not accessible to accelerators.

  * •

Energy Frontier: A series of medium-scale “auxiliary experiments” are being
pursued using the products of collisions that will already be available in the
ATLAS, CMS, and LHCb interaction regions to study parts of phase space that
are not covered by those experiments. Examples of experiments being considered
include: FASER$\nu$, Mathusla, CODEX-b, FACET (a forward extension of CMS),
and those associated with the Forward Physics Facility (FPF).

  * •

Neutrino Frontier: A case was made at Snowmass for a comprehensive short-
baseline neutrino program; a program to measure many basic neutrino scattering
processes that we have never before been able to measure precisely; a campaign
to measure the proton-nucleus cross sections needed to understand fluxes in
neutrino beams; a search for new neutrino interactions; the use of
astrophysical neutrinos as messengers of physics and astrophysics; and several
other topics.

  * •

Rare Processes and Precision Measurements Frontier: Upgrades are proposed to
two projects to study $b$-quarks in which the U.S. currently participates,
LHCb at CERN and Belle II at KEK in Japan. The U.S. community working in
heavy-flavor physics has a long tradition of leadership and innovation. LHCb
is currently commissioning its Upgrade I, which features an innovative purely
software trigger, and is planning a second upgrade that should lead to an even
higher sensitivity and a broader physics reach in the HL-LHC era. Belle II and
SuperKEKB are considering staged upgrades to reach higher luminosity.
Individually and even more in combination, the synergistic efforts of LHCb and
Belle II will lead to an unprecedented breadth of exploration of heavy quark
physics as a tool of discovery. The pair add unique studies of rare tau
decays, exotic QCD states, and other fundamental quantities with an overall
program that spans our Frontier.

New projects are proposed to study the decays of particles containing light or
strange quarks, charged lepton flavor experiments to study flavor violation
and lepton flavor non-universality, and dark sector beam dump experiments to
search for new feebly-interacting particles. Additional directions discussed
include an upgrade to the Mu2e experiment, and the development of storage
rings to search for a proton electric dipole moment.

A balanced portfolio of these mid- and small-scale experiments from all four
Frontiers is a vital component of a healthy and vibrant U.S. HEP program. An
important recommendation of Snowmass is that resources be allocated to allow
for a sufficiently large and diverse selection of important small- and medium-
scale experiments to go forward in the coming decade.

###  3 A Science Driver-Oriented View

These are exciting and challenging times for fundamental particle physics.
During the last several decades of intense experimental and theoretical
research, we have come to rely on the Standard Model of particle physics — a
very predictive, relativistic, spontaneously-broken quantum gauge field theory
— in order to describe microscopic phenomena at the smallest accessible
distance scales and highest accessible energy scales. Its most remarkable
prediction – the existence of a Higgs particle associated with a fundamental
scalar field responsible for electroweak symmetry breaking – was established
in the last decade, while innumerable other nontrivial predictions and
relationships have been confirmed, sometimes with extraordinary precision.

There are, however, big questions in fundamental particle physics to which we
currently do not know the answer. These cannot be addressed with the current
version of the SM and call for new particles, new interactions, and, perhaps,
a qualitatively different description of nature at very small distance scales.
There are several potential answers, but at this point, we know very little
about these potential new particles and interactions. There are different
research directions and inquiries we expect, in tandem, to guide the way.
These were organized by the last P5 into science Drivers [2]. Here, we make
use of the science Drivers in order to discuss the progress made on the big
questions in particle physics over the last decade and to guide the research
directions particle physics should pursue in this decade. In the last
subsection, we present a proposal from the Rare Process and Precision
Measurements Frontier to highlight the physics of flavor, possibly as another
Driver.

####  1 Use the Higgs Boson as a Tool for Discovery

A fundamental aspect of the SM of particle physics is that the electroweak
gauge interactions are spontaneously broken to electromagnetism, resulting in
masses for the $W$ and $Z$ bosons, while the photon remains massless. The
model requires that, in addition to the gauge bosons themselves, along with
the quarks and leptons, new particles and interactions must also be present to
account for electroweak symmetry breaking. The simplest possibility for a
symmetry-breaking sector, hypothesized in the 1960s when the standard
electroweak theory was developed, is to introduce a fundamental scalar field
(a Higgs doublet) whose potential energy is adjusted so that it acquires a
vacuum expectation value. The simplest one-doublet Higgs model makes very
specific, testable, predictions. In particular, the model predicts that there
will be a single neutral scalar particle (the Higgs boson) in the spectrum
which couples to all massive particles – the quarks, leptons, and $W$ and $Z$
bosons – with definite strengths related to the masses of those particles.

A decades-long campaign in pursuit of the physics behind electroweak symmetry
breaking resulted in a breakthrough in 2012, with the discovery of a new
scalar particle at the LHC. The properties of this new particle are consistent
with the Higgs boson of the simplest one-doublet Higgs model. In the last
decade, the LHC experiments have measured many of the properties of this
putative Higgs particle, including its charge, spin, CP properties, and mass,
while putting stringent limits on its decay width. Existing measurements of
the particle’s couplings with the quarks, leptons, and electroweak gauge
bosons are all consistent, within current experimental errors (typically of
order $10\%-20\%$), with the predictions of the standard Higgs models.

These experimental results indicate that the physics of electroweak symmetry
breaking is, at least approximately, consistent with the simplest Higgs model
hypothesized in the 1960s. However, is this minimal model the whole story?
Several questions remain unanswered and we have only just begun to reveal how
nature chooses to spontaneously break the electroweak symmetry. For example,
we currently lack experimental knowledge of the form of the Higgs potential.
This information is encoded in the couplings of the Higgs particle to gauge
bosons, fermions, and itself. Implications from these investigations directly
inform our understanding of the universe. For example, whether our vacuum is
fundamentally stable or only metastable is tied to the Higgs potential. On the
other hand, the nature of the electroweak phase transition in the early
universe, which may provide the key to understanding why the universe has so
much matter in it, also depends on Higgs properties and interactions.

The existence of what appears to be a fundamental scalar invites other
questions. For example, how are the Higgs particle mass and electroweak
symmetry breaking scale so different – by sixteen orders of magnitude – from
the scales associated with gravitational phenomena? In the absence of new
particles and interactions, it was expected that these two energy scales would
naturally be the same. On the other hand, in the SM, the masses of the quarks
and leptons arise from their couplings to the Higgs boson. Hence, the physics
of the Higgs boson is associated with establishing the different “flavors” of
matter which, as discussed below, is a mystery in and of itself. Information
on what might lie beyond the SM is also potentially encoded in the couplings
of the Higgs boson if they are measured with enough precision. For example,
increased accuracy in the measurement of the SM Higgs couplings to other SM
particles can be naively translated into sensitivity to heavy new physics; 10%
precision is sensitive to new scales up to 1–2 TeV while a precision below the
percent level would be sensitive to new physics with masses of order 10 TeV of
higher. Alternatively, the Higgs boson may serve as a portal to a so-far
hidden sector of new physics and be uncovered through rare “invisible” Higgs
decays.

The HL-LHC will reach percent level precision on most couplings and aims at
constraining the Higgs potential through a measurement of the Higgs-boson
self-coupling with a projected precision of about 50%. Future colliders, both
Higgs factories (mainly $e^{+}e^{-}$ colliders with energies at the $Z$-pole,
near the $ZH$ threshold, and up to 1 TeV) and multi-TeV colliders (either muon
or hadron colliders providing partonic center of mass energies of 10 TeV or
above) are the two main avenues that will allow precision measurements of
Higgs properties and interactions and, at the highest energies, potentially
provide direct access to such physics. Studying the Higgs at Higgs factories,
coupled with commensurate improvements in precision collider phenomenology,
may uncover the scale of new BSM physics and the minimum target energy for
multi-TeV machines.

####  2 Pursue the Physics Associated with Neutrino Mass

By the end of the twentieth century, decades-old puzzles associated with the
measurements of the fluxes of neutrinos from the Sun and the atmosphere were
decisively solved by a collection of gargantuan detectors of neutrinos from
different sources. These revealed, rather surprisingly, that neutrino masses
are tiny but definitively not zero, contradicting the assertions of the
minimal version of the SM. Our understanding of nature is such that all
fundamental masses are dynamical – they arise as a consequence of interactions
among the massive particles and the mechanism of electroweak symmetry breaking
– and we currently know nothing about the origin of nonzero neutrino masses
other than the fact that new particles and interactions must exist to explain
it. These new particles associated with neutrino masses can be very light or
very heavy – from sub-eV to YeV ($10^{24}$ eV) – and either charged or
neutral, opening an avenue to discovery in a broad range of experiments from
low-energy neutrino oscillation experiments to astrophysical neutrino
detectors to high-energy colliders.

Nonzero neutrino masses were established through the discovery of neutrino
oscillations, the phenomenon that describes how lepton-flavor quantum numbers
are not conserved but evolve – oscillate – in space and time. The early
twenty-first century has witnessed rapid experimental and theoretical progress
in our understanding of neutrino oscillations. We are quickly moving from the
exploratory phase to the precision era, in which oscillation experiments will
be able to answer the questions of how the neutrino masses are ordered and
whether neutrinos and antineutrinos oscillate differently. The mass ordering
directly informs models of neutrino mass generation, and CP violation in
neutrinos may play a role in the matter-antimatter asymmetry of the universe.
Precise, over-constraining measurements of neutrino oscillations, which will
require improved theoretical predictions of neutrino-nucleus cross sections,
will test our current understanding of neutrino masses and mixings and allow
us to look for more new physics in the neutrino sector. Moreover, neutrino
experiments have a unique sensitivity to a wide range of weakly- or feebly-
interacting new particles due to the high beam luminosity and sensitive, large
detectors.

Beyond measurements of neutrino oscillations, another fundamental question we
need to address is the nature of the neutrinos themselves – are they Dirac or
Majorana fermions? If neutrinos have Majorana masses, that would imply that
lepton number is not an exact symmetry of nature. Searches for neutrinoless
double-beta decay are the most powerful probes of lepton-number violation, and
an ambitious program aimed at improving existing sensitivity by at least one
order of magnitude is underway. On another front, precision measurements of
the kinematics of beta decay continue to probe the absolute neutrino mass
scale and there are vigorous R&D efforts aiming at an order-of-magnitude
improvement in sensitivity by the end of the decade. Finally, neutrinos impact
the evolution of the universe and their masses leave an imprint on the large-
scale structure of the universe. Cosmology-informing large-scale surveys
already constrain the magnitude of the sum of neutrino masses. Future
enterprises (including CMB-S4) aim at “discovering” non-zero neutrino mass by
being sensitive to the minimum possible sum of neutrino masses consistent with
oscillation measurements – modulo more surprises – by the end of the decade.

The degrees of freedom responsible for nonzero neutrino masses can manifest
themselves in a variety of observables, many of which are not directly related
to neutrinos. For example, next-generation precision measurements of charged-
lepton properties (e.g., the muon $g-2$ and the electron electric-dipole
moment) and the measurement of exceedingly rare charged-lepton processes
(e.g., $\mu\to eee$ and $\mu\to e\gamma$ decays or $\mu\to e$-conversion in
nuclei) will inform the neutrino mass puzzle; the same might also be true of
precision measurements of leptonic neutral-current decays of heavy mesons. The
current and next-generation of high-energy colliders are also capable of
directly producing the new particles responsible for neutrino masses as long
as they are neither too heavy nor too weakly coupled. Finally, cosmic surveys
are not only sensitive to the values of the neutrino masses but are also
impacted by new neutrino properties and interactions.

####  3 Identify the New Physics of Dark Matter

Precision measurements of the properties of the universe and its constituents
at different distance scales – from galactic scales to clusters of galaxies,
to the cosmic microwave background – combined with our understanding of
gravitational interactions, indicate that most of the matter in the universe
is not made SM out of particles (protons, neutrons, electrons, and neutrinos).
We know very little about this so-called dark matter but there are many
hypotheses regarding its nature. It can be stable or unstable and it can be a
fundamental particle or a composite object. It can be very light (less than a
zeV) or macroscopically heavy (dozens of solar masses). It can interact with
ordinary matter a lot, a little, or only gravitationally.

Given the enormous parameter space of possibilities, a diverse experimental
and theoretical effort is required in order to learn the identity and nature
of dark matter and to measure its properties and interactions. If the dark
matter mass is, roughly, between 100 MeV and a TeV and it interacts with SM
particles via some new “weaker-than-weak” interactions, it can be detected in
very large, very quiet underground experiments. So-called WIMP searches have
been pursued for a few decades and the technologies have evolved to allow for
multi-ton experiments that have examined many orders of magnitude of parameter
space in detail with more improvements to come. The next generation of these
direct-detection searches will aim at ${\cal O}(10^{2})$ tons of target
material. These are multi-purpose experiments that are also sensitive to
neutrinos from the Sun, the atmosphere, and supernova explosions. Dark matter
is also expected to accumulate in the centers of galaxies and stellar objects,
where it can self-annihilate or decay. These processes may lead to cosmic
rays, gamma rays, neutrinos, or other cosmic messengers that can be detected
on Earth (or in orbit around the Earth or the Sun) in a diverse variety of
observatories. For lighter dark matter particles, new detector technologies
and search strategies have opened the door to a wealth of distinct
experiments. The last decade witnessed an explosion of qualitatively different
searches for the QCD axion and ultra-light axion-like dark matter particles,
often taking advantage of the development of new quantum devices. Meanwhile,
new cosmic observational probes have been developed to study the properties of
dark matter via its gravitational effects on cosmology and astrophysics.

Indirect searches for dark-matter-mediated processes in the cosmos have also
evolved and diversified since the last P5, making use of the current
generation of gamma-ray telescopes, X-ray telescopes, cosmic-ray detectors,
and neutrino telescopes. Some of these facilities are underground, some are on
the Earth’s surface, and some are in orbit. Many of these detectors have
upgrade programs underway, in different stages of R&D or implementation.

Alternatively, rather than searching for the dark matter present in the
universe, the new particle(s) associated with the dark matter can also be
produced in particle physics experiments. Experiments at high energy
colliders, including the LHC, the upcoming High Luminosity LHC, and next-
generation lepton and hadron colliders are well-positioned to inform the dark
matter puzzle through their ability to produce and detect new particles. High-
intensity experiments, including the facilities where intense neutrino beams
are produced (for example, J-PARC, in Japan, or the upcoming LBNF/DUNE target
and near-detector complex at FNAL), allow one to search for light new, long-
lived particles that are produced in different (via weaker than SM
interactions) ways and which may be observed in the near-detectors that serve
the neutrino oscillation experiments. Our understanding of different ways to
search for light, very weakly coupled dark matter particles has evolved
dramatically in the last decade and multiple experiments sensitive to “dark
sectors” have emerged and are under consideration.

####  4 Understand Cosmic Acceleration: Dark Energy and Inflation

Cosmic surveys allow us to measure the expansion rate of the universe as a
function of time. In the 1990s, these unexpectedly revealed that today, the
expansion rate of the universe is accelerating. We know very little about the
fundamental physics – referred to as dark energy – responsible for this
phenomenon. The simplest solution is the existence of a nonzero cosmological
constant. Our current understanding of fundamental physics is essentially
silent about the cosmological constant; simple estimates yield a value that is
120 orders of magnitude too large. There are also dynamical hypotheses for the
origin of dark energy, associated with the existence of new fundamental
fields. Learning more about the nature of dark energy is a challenge and a
priority for fundamental physics in this and the upcoming decade.

Precise cosmic surveys of different cosmological eras using different tracers
are needed in order to measure the expansion rate of the universe as a
function of cosmological time. These include large-scale-structure
measurements that will be facilitated by the Vera Rubin Observatory and next-
generation measurements of the cosmic microwave background – including its
polarization. These observations are positioned to study the possible
dynamical evolution of dark energy over a wide range of redshifts. On the
theory side, there are several attempts to explain and interpret the physics
of dark energy, which may be related to our efforts to understand quantum
gravity.

There is also very compelling indirect evidence for inflation, a period of
exponential expansion of the universe almost immediately after the Big Bang.
The dynamics behind inflation is also virtually unknown. Measurements of the
polarization patterns in the cosmic microwave background may determine the
absolute energy scale of inflation, etched in the form of primordial
gravitational waves, while optical spectroscopic and CMB measurements of the
matter power spectrum will reveal clues about the scattering dynamics of this
ultra high energy epoch as imprinted in the large scale structure of the
universe. The direct observation of gravitational waves, a revolutionary
discovery in 2015, has motivated studies to broaden the sensitivity and
frequency reach of gravitational wave experiments. Applications to fundamental
physics, including uncovering the physics of inflation along with the
exploration of other early-universe phenomena (phase transitions, the imprint
of ultra-heavy fields, etc.), are under intense investigation both
theoretically and experimentally.

####  5 Explore the Unknown: New Particles, Interactions, and Physical
Principles

Particle Physics research operates at the boundary between the known and the
unknown. New facilities allow us to explore new distance and energy scales or
unveil unexpected very rare phenomena. Broadly speaking, all particle physics
experimental enterprises are sensitive to unanticipated new phenomena and have
the potential to uncover new particles, new interactions, or even new
principles that guide our description of fundamental physics. Exploring the
unknown is at the heart of experimental and theoretical particle physics
research.

The direct exploration of nature at the smallest distance scales requires
high-energy collisions of fundamental particles. Very high energy collisions
allow for the production of new, heavy particles, as long as these interact
with the known fundamental particles. The highest laboratory center-of-mass
energies are currently found at the LHC, and experiments there at the 1 TeV
partonic energy scale are a central piece of particle physics research for the
next decade and beyond. Access to higher center-of-mass energies in the future
requires extensive R&D efforts in accelerator technologies, superconducting
magnets, detector technologies, computing and data analysis, etc. Two options
to reach the 10 TeV partonic energy scale have been identified: muon colliders
with multi-TeV center-of-mass energies and proton colliders with one order of
magnitude higher energy (around 100 TeV).

New, light particles can also be discovered in lower-energy experiments. For
example, new neutral particles that mix with the electroweak neutrino partners
of the SM charged leptons can be discovered via neutrino oscillations – they
allow for new oscillation frequencies – and are the subject of intense
experimental exploration in this decade (building on measurements made during
the last few decades). Precision measurements of neutrino oscillations are
also sensitive to new neutrino–matter interactions. New, light particles can
also be discovered through their influence on the evolution of the Universe,
as revealed by precision cosmology.

Precision measurements and the searches for rare processes are sensitive to
the quantum effects of hypothetical new particles and interactions. One
special class of rare-process searches involves those that are forbidden in
the SM. In the SM (when neutrino masses are zero) baryon number and lepton
number conservation are the consequence of accidental global symmetries. The
conservation of baryon number forbids proton decay and
$n\leftrightarrow\overline{n}$ oscillations while lepton-number conservation
forbids neutrinoless double-beta decays. While neutrino masses may ultimately
be evidence that the lepton number is not conserved – experimental evidence is
needed to prove this, as discussed above – there are no indirect or direct
hints that baryon number conservation is violated. Searches for baryon-number-
violating processes (which are predicted by most Grand Unified theories) serve
as, sometimes, the deepest probes we have of new phenomena at very small
distance scales. Very large neutrino detectors also allow the most sensitive
searches for proton decay and other baryon-number-violating nuclear processes.
With bigger and better-instrumented neutrino detectors our sensitivity to
proton decay is expected to increase by at least one order of magnitude in the
coming decade.

####  6 Flavor as a Tool for Discovery $-$ Possible New Driver

At Snowmass 2021, a new Driver was proposed by the Rare Processes and
Precision Measurements Frontier: Flavor as a tool for discovery. In the last
P5, this topic was understood as included in a subset of the other science
Drivers. However, new experimental capabilities and the ability for flavor
probes to be sensitive to ultra-high-energy physics suggest that new physics
in the flavor sector might be uncovered in the coming decade, as described
below.

All matter particles come in sets of three, called generations. There are
three types of charged leptons, three types of neutral leptons (neutrinos),
and three different pairs of quarks whose members have charges $+2e/3$ and
$-e/3$. These are postulated to have all the same properties relative to one
another except for their masses, which are known to span several orders of
magnitude. We don’t understand this triplication of the matter particles nor
is there a compelling mechanism behind the disparate masses of the different
generations. In the SM, only the weak interactions allow for interactions
among different generations. In the quark sector, inter-generational mixing is
known to be small or very small. In the lepton sector, instead, inter-
generational mixing is large or very large (albeit difficult to access in
practice because neutrino masses are tiny). We don’t understand how quark
mixing and lepton mixing are related, if at all. Since the discovery of the
muon many decades ago, the solutions to these flavor puzzles continue to elude
the particle physics community.

A known consequence of the three generations, plus nontrivial mixing, is that
SM interactions can account for the violation of CP invariance, a phenomenon
that is well established in the quark sector. However, while the search for
new CP-violating phases is still an active field of investigation in quark
flavor, determining whether (and how much) CP invariance is violated in the
lepton sector is an open experimental challenge for the current and next
generation of neutrino oscillation experiments and among the top priorities
for the LBNF/DUNE and Tokai-to-Hyper-Kamiokande projects. Understanding the
physics of CP violation is a necessary ingredient for addressing another big
question in fundamental physics: why is there so much more matter in the
universe than antimatter? Cosmological observations and our theoretical
interpretations indicate that, in the absence of new particles and
interactions, the universe should be mostly empty, except for photons and
neutrinos. The presence of an overabundance of matter implies a primordial
matter-antimatter asymmetry. Furthermore, since the electroweak interactions
are known to violate baryon number at temperatures above the electroweak phase
transition, this asymmetry must be generated dynamically in the early
universe, through a process known as baryogenesis. CP violation is a necessary
ingredient for baryogenesis. The CP violation present in the quark sector is
not sufficient to explain the matter-antimatter asymmetry in the universe.
Whether the potential violation of CP invariance in the lepton sector is
sufficient to account for baryogenesis is a fundamental particle physics
problem that can be definitively addressed in the coming two decades.

Precision measurements of rare processes involving different generations, when
complemented by precise theoretical calculations, are powerful probes of new
phenomena. Searches for the virtual effects of new particles in flavor or CP-
violating observables can, in many cases, provide access to the physics at
scales higher than those directly accessible at high-energy collider
experiments. These include precision measurements of CP-violation in Kaon,
$B$-meson, and $D$-meson mixing, and flavor-changing neutral current processes
involving Kaons (e.g., $K^{0}\to\pi^{0}\nu\overline{\nu}$), $B$-mesons (e.g.,
$B^{+}\to K^{+}\mu^{+}\mu^{-}$), muons (e.g., $\mu\to e\gamma$), and taus
(e.g., $\tau^{-}\to\mu^{+}\mu^{-}e^{-}$). These observables are the subject of
active research at accelerator facilities around the world and are important
targets for next-generation experiments. Some can be probed with current
experiments like LHCb and the Muon $g-2$ experiment at Fermilab. Others will
be investigated in the near future at Belle II. Depending on what the near
future brings, dedicated next-generation experiments will be required.

At lower energies still, searches for permanent electric dipole moments of
fundamental particles – electrons and muons – or baryons – protons and
neutrons are also very clean probes of new particles and interactions and
would reveal the existence of new sources of CP violation.

###  4 Brief Summaries of Frontier Reports

The main body of this Snowmass Report consists of the full reports of each of
the ten Frontiers and the SEC report. These range from about 10 to 70 pages in
length. In this section, we present a brief summary of each Frontier report,
giving an overview of the material and the main high-level conclusions. For
the main physics Frontiers, they discuss the large- and mid-range projects
that are proposed for consideration by P5 for construction or development in
the next two decades. While the brief summaries of the full Frontier reports
are presented in alphabetical order, the Frontiers may be thought of as
divided into two groups, “enabling frontiers” and “physics frontiers”.

The enabling frontiers, namely Accelerator, Community Engagement,
Computational, Instrumentation, and Underground, provide the basic foundation,
infrastructure, R&D, and support on which facilities and experiments are
constructed and operated and that ultimately are necessary to produce the
physics. These Frontiers include experts in accelerator physics,
instrumentation, and computation. These experts may be engineers, skilled
technicians, computer scientists, or physicists. The line between these
Frontiers and the physics frontiers is not sharp and individuals and projects
may cross the boundaries. Since working scientists are the bedrock of our
field, we have included the Community Engagement Frontier as an enabling
technology. The Theory Frontier is both an enabling technology for HEP
experiments and a highly significant physics research endeavor in its own
right, producing fundamental new knowledge. For that reason, we include it
among the physics frontiers.

The physics frontiers are the ones that carry out the high energy physics
program, from proposal to final physics analysis and publication, and whose
projects and experiments have been the main subject of P5’s prioritization.
These include the Cosmic, Energy, Neutrino, and Rare Processes and Precision
Measurements Frontiers, and for the reasons presented above, Theory. These
correspond closely to the Three Frontiers of Snowmass 2013, Energy, Cosmic,
and Intensity, except that in 2021 the Intensity Frontier was split into the
Neutrino Frontier and the Rare Processes and Precision Measurements Frontier
to reflect the growth in the Neutrino program worldwide and in the U.S. in the
last decade. These Frontiers set the physics goals and work with the enabling
frontiers to ensure that the necessary infrastructure and facilities will be
available to achieve them. An excellent example of this is the interaction
between the Energy Frontier and the Accelerator Frontier, apparent in the two
corresponding summaries in this section and further described in Section 5.

With ten Frontiers, we were concerned that communications would be difficult
and stove-piping could be a problem. We implemented a very complete system of
inter-frontier liaisons to ensure good interactions among the Frontiers. At
the Seattle meeting, many morning parallel sessions were dedicated to bringing
together subsets of the Frontiers or their Topical Groups that benefited from
interacting with each other. The work presented here is informed by the
studies done by the Frontiers both individually and in collaboration.

####  1 Accelerator Frontier [5]

High-energy accelerators have been a major enabling technology for HEP and
allied fields since the 1950s. The experiments envisioned by the Rare Process
and Precision Measurement and Neutrino Frontiers rely on high-intensity proton
beams to provide controlled sources of pions, kaons, neutrinos, and other
particles for study. These same facilities also provide the opportunity to
produce new feebly interacting particles related to the dark sector (a focus
also of the Cosmic Frontier) in beam dump experiments. In contrast, the Energy
Frontier requires accelerator complexes to fill storage rings of high-energy
colliding beams of protons, as well as electrons and possibly muons (and their
antiparticles), to allow for a thorough exploration of the properties of
matter at the shortest possible distances. Despite past successes, the
increasing size, cost, and timescale required for modern and future
accelerator-based HEP projects arguably distinguish them as the most
challenging scientific research endeavors being considered.

The goal for the Accelerator Frontier is to answer questions such as: What is
needed to advance the physics? What is currently available (state of the art)
around the world? What new accelerator facilities could be available in the
next decade (or next-to-next decade)? What R&D would enable these future
opportunities? What are the time and cost scales of the R&D and associated
test facilities, as well as the time and cost scale of the final facilities?

Since the previous Snowmass/P5 exercise, the LHC Phase-1 accelerator upgrades
at CERN have been completed and the planning for Phase-2 upgrades (HL-LHC) is
well underway. Construction of the PIP-II proton linac for the Phase I
LBNF/DUNE neutrino program at FNAL is underway, and planning for a multi-MW
beam power upgrade for Phase II is beginning. In addition, numerous new high-
energy colliders in various stages of maturity have been proposed. There are
several $e^{+}e^{-}$ Higgs Factory concepts, including linear colliders such
as the ILC (which has a complete TDR) and others such as CLIC, C3, and HELEN,
and storage-ring proposals such as FCC-ee at CERN and CEPC in China. Finally,
there is increasing interest in energy-frontier “discovery machines” with
partonic center-of-mass energies of order 10 TeV, including a muon collider
and a proton-proton collider, FCC-hh.

To properly support HEP scientific goals the planning of accelerator
development and research should be aligned with the strategic planning for
particle physics and should be part of the P5 prioritization process. The U.S.
HEP accelerator R&D portfolio presently is presently managed by the general
accelerator R&D program (GARD) and contains no collider-specific scope. This
creates a gap in our knowledge base and accelerator/technology capabilities.
It also limits our national aspiration for a leadership role in particle
physics in that the U.S. cannot lead or even contribute to proposals for HEP
collider facilities. To address the gap, the accelerator community proposes
that the U.S. establish a National Integrated R&D Program on future colliders
in the DOE Office of High Energy Physics (OHEP) to carry out technology R&D
and accelerator design for future collider concepts. This program would aim to
enable synergistic engagement in projects proposed abroad (e.g. FCC-ee/hh,
ILC, and IMCC). It would also support the development of design reports on
collider options by the time of the next Snowmass and P5, perhaps 7 – 10 years
from now, particularly for options that can be hosted in the U.S., and the
creation of R&D plans for the decades past 2030.

In addition to the collider-specific program envisioned above, general
accelerator R&D is also needed. Specific needs identified include developing
high-power targets for multi-MW beams for the Phase II LBNF/DUNE upgrade and
for muon collider sources; continuing development of high-gradient super-
conducting and normal-conducting RF cavities for particle acceleration at
linear colliders; high-field magnets for high-energy proton and muon
colliders; wake-field acceleration methods to enable ultra-high energy
colliders; and basic experimental, computational, and theoretical studies of
accelerators and beam physics to enable the optimized design and control of
future facilities. These topics require a substantial increase in accelerator
R&D support, and the Accelerator Frontier recommends that the U.S. strategic
goals and roadmaps of GARD program thrusts be updated.

Finally, workforce development through the strengthening of recruitment,
education, and training, especially of women and other underrepresented
groups, and the expansion of U.S. accelerator beam test facilities to maintain
their competitiveness with respect to worldwide capabilities, are essential to
realizing the vision of the Accelerator Frontier

The Accelerator Frontier of Snowmass 2021 has undertaken a thorough
investigation of the current state and possible future plans that we summarize
below, for which details may be found in the Frontier and Topical Group
Reports (and references therein). The questions facing the frontier were
addressed in seven Topical Groups led by internationally recognized
researchers.

Beam Physics and Accelerator Education Funding for fundamental beam physics
needs to be reinvigorated, including enhancing investment in both general and
directed accelerator research. These investments should be paired with
strengthening and expanding U.S. accelerator test beam capabilities.
Furthermore, the U.S. accelerator physics community must comprehensively and
strategically build a strong and diverse workforce of accelerator physicists
to realize HEP’s long-term plans.

Accelerators for Neutrinos The neutrino physics program is calling for next-
generation, higher-power, megawatt and multi-MW-class super-beam facilities,
such as $>$2MW beam power upgrade of the Fermilab proton accelerator complex.
Challenges needing to be addressed include developing extremely robust targets
to withstand the enormous thermal and mechanical stresses from multi-megawatt
beams; major improvements to the performance and power efficiency of the
linacs, synchrotrons, and accumulator rings needed for a super-beam
accelerator complex; and new approaches to control instabilities and minimize
beam losses.

Accelerators for Electroweak/Higgs Physics The world HEP community has called
for a Higgs/Electroweak Factory as the next large accelerator project. At
present, a dozen Higgs/ElectroWeak Factory proposals are under consideration,
including circular $e^{+}e^{-}$ colliders such as CEPC in China and FCC-ee at
CERN and linear colliders such as CLIC (CERN) and the ILC. Two recently
proposed Higgs factories can potentially be shorter than 7 km and fit the
Fermilab site $-$ C3 and HELEN. Major conclusions of this topical group (AF3)
are: of all the Higgs/EW Factory proposals considered, only a small number of
them are ready or close to a construction phase $-$ namely, the ILC, CLIC,
FCC-ee, and CEPC. Most of the other proposals are in the conceptual design
stage. These proposals should focus on the main R&D tasks to move forward to a
CDR and, later, to a TDR. A detailed estimation of the Technical Readiness
Level, risk factors, technology validation, cost reduction impact, performance
achievability, and timescale are presented in detail in the Collider
Implementation Task Force report [18].

Multi-TeV Colliders The Snowmass process has emphasized the emerging need to
explore the Energy Frontier with particle collisions at significantly higher
energies than LHC. At present only a few design concepts have achieved a level
of design maturity to have reliable performance evaluations based on prior R&D
and design efforts. These options reach only slightly above the 1 TeV: the
linear $e^{+}e^{-}$ collider, the ILC (1 to 3 TeV), FCC-eh ($\sim$1.2 TeV),
and CLIC (3 TeV). Critical project risks have been identified and subsystem-
focused R&D is underway where necessary. Other emerging accelerator complexes
offer unique opportunities to reach the very much sought 10 TeV parton energy
scale, such as FCC-hh (100 TeV p-p) and a high-energy muon collider (6 – 14
TeV), which is more compact and has the potential to be more power efficient.
Both concepts still require significant basic R&D and design effort to reach a
maturity level to be properly evaluated (see the Collider Implementation Task
Force report [18]).

Accelerators for Physics Beyond Colliders and Rare Processes The highest
priority here is the optimum exploitation of the PIP-II 800-MeV
superconducting RF CW-compatible linac under construction at Fermilab. The
LBNF/DUNE experiment will need only a small fraction of the total potentially
available PIP-II proton beam power (bunched). The proposed RPF experiments,
such as rare muon decays and dark matter searches with beam dumps, could make
use of the PIP-II CW beam capability, after facility upgrades, but the bunches
would have to be reconfigured to meet the varying needs of each experiment.
Additionally, a new 0.8 – 1 GeV proton bunch compressor ring (PAR) has been
proposed to accumulate the beam from the PIP-II linac and extract it in
intense bunches at rates of 100 – 1000 Hz.

Advanced Accelerator Concepts This Topical Group focused on understanding the
limits, the state of the design concepts, and the status of R&D for plasma and
structure-based wakefield acceleration. At this time, there are no parameter
sets for a plasma or structure-based linear collider that self-consistently
address the known accelerator physics challenges. To move forward, a vigorous
R&D program will be required. That should be motivated by a better
understanding of the desired parameter ranges for a collider. The U.S. is in a
good position in this respect with several state-of-the-art beam test
facilities mainly dedicated to research in beam physics and the advanced
accelerator field, including FACET-II, BELLA, ATF, AWA, and FAST-IOTA as well
as at numerous universities.

Accelerator Technology - RF, Magnets, and Targets/Sources The GARD roadmap
continues to serve as community-developed guidance for RF technology R&D, but
based on recent progress it would benefit from some mid-course corrections
detailed in the topical group report. The U.S. Magnet Development Program
(U.S.-MDP) focuses on fundamental accelerator magnet R&D providing key magnet
technologies that benefit all future accelerators. Discussions and initiatives
emerging from Snowmass 2021 motivate enhanced scope and a significant increase
in resources to prepare accelerator magnet technology for applications in
future hadron and muon colliders and for fast-ramping accelerator facilities.
The next generation of multi-MW high-power targets for future accelerators
will use more complex geometries, novel materials, and new concepts. There are
challenges in high-intensity high-brightness beam sources for future
accelerators. They are particularly formidable for sources of beams with
special characteristics, such as polarized electrons and ions or ultra-small
emittances, and for tertiary and secondary particles, such as muons and
positrons. Normal-conducting and superconducting RF technology R&D will
develop toward higher gradient and higher quality factor cavities and novel,
more power-efficient RF power sources.

U.S. High-Energy Physics depends crucially on the success of the Accelerator
Physics Frontier, and the analyses and plans presented here provide a
framework for a bright future.

####  2 Community Engagement Frontier [6]

In Snowmass 2013, the U.S. high-energy physics community incorporated a
working group on Communication, Education, and Outreach (CE&O). The 2013
experience made clear that it is crucial to address community engagement for
the health of U.S. HEP. Therefore, in Snowmass 2021 community engagement was
expanded into a full-fledged Community Engagement Frontier (CEF), co-equal
with the physics Frontiers and with a much broader scope than CE&O.

A major conclusion of the Community Engagement Frontier is that the climate of
U.S. High-Energy Physics needs to be improved: The HEP community should
institute a broad array of practices and programs to reach and retain the
diverse talent pool needed for success in achieving our scientific vision.

Of particular concern is broadening the representation of the U.S. high-energy
physics community. Those from marginalized communities who do join the U.S.
HEP community are, all too often, subject to alienation, microaggressions, and
other forms of subtle and overt discrimination. HEP needs to address these
persistent issues of underrepresentation by employing the use of robust
strategic planning procedures including a full re-envisioning of our workplace
norms and culture to prioritize eliminating the barriers and negative
experiences faced by our marginalized colleagues. In addressing the needs of
marginalized physicists, HEP communities must engage in partnership with
scholars, professionals, and other experts in several disciplines, including
but not limited to anti-racism, critical race theory, and social science.
Progress on these issues will require the implementation of new modes of
community organization and decision-making procedures that promote agency and
leadership from all stakeholders within the scientific community. In addition,
all HEP communities should create structures to fully open career path
opportunities to everyone and conduct event planning to ensure events are
accessible to all community members, including those with disabilities.
Finally, as demonstrated vividly during the past two years of COVID-19
lockdowns, to retain our junior colleagues, research institutes and
universities should do more to maintain the highest standard in work-life
balance and mental health of staff.

A diverse pool of candidates cannot be expected in the HEP community if
educational efforts are not made as far back as the K – 12 and university
undergraduate levels, to nurture the pipeline. In this regard, the HEP
community should encourage stronger participation in HEP collaborations by
faculty and students from non-R1 academic institutions; continue and expand
work with K-12 teachers and students to create supportive local communities to
nurture student interest in math and science; foster expanded pre-university
and university programs for international student training, especially in
partnership with colleagues in developing countries.

Engagement with developing countries needs improvement. Universities,
laboratories, and HEP societies should improve and sustain international
outreach, partnerships, schools, workshops, conferences, training, short
visits for research, and development of research consortia; mechanisms should
be developed to facilitate the participation of colleagues from developing
countries. Large international research collaborations should improve efforts
to facilitate the integration and participation of research groups from
developing countries and support efforts to foster HEP in these countries.

Our field cannot absorb all the early career members that it produces, so
funding agencies, national laboratories and universities should work together
to provide more education and career opportunities for engineering and
industry-focused research within and outside HEP, and update degree programs
to match better the skills needed and career opportunities required in today’s
HEP and related fields. These steps should be complemented by developing
effective alumni networking tools and programs, to deepen connections with
industry.

Technology transfers between HEP and industry are necessary for society to
receive the benefits of HEP research. Funding agencies and laboratories should
improve policies and programs to foster technology transfers and facilitate
collaborative programs with industry on targeted technology development
beneficial for both HEP and society.

While HEP has many excellent Communication, Outreach, and Engagement programs,
our community needs to enact structural changes to foster broader, deeper, and
more effective participation in community engagement, through strengthened
policies such as considering community engagement work in hiring, promotion,
and grant decisions. Individual scientists should encourage others, including
peers, mentees, and students, by participating in public engagement and
discussing its importance. Our community also needs to transition from an
ethos of conducting outreach and communication with the public, to a culture
of engagement in relationships with the public. This should be done by
building lasting relationships with the full breadth of all our supporting
communities (especially those that have been historically excluded) that are
not based on transactional interactions, but rather real two-way partnerships
that consider the needs and interests of the audience and include its members
in program design.

The HEP community should provide resources to sustain and grow the annual HEP
Congressional advocacy efforts. HEP groups should coordinate efforts by
laboratories and universities to extend advocacy to the federal executive
branch, and state and local governments. These efforts are particularly
important in disseminating the results of the current Snowmass/P5 exercise.
HEP should strengthen partnerships with other science and physics societies on
advocacy for non-HEP funding issues.

Finally, HEP must take greater responsibility for its impacts on climate
change by addressing and mitigating these impacts through DOE project policies
and individual community member actions.

The expanded Community Engagement Frontier included seven Topical Groups
defined primarily by the broad and diverse issues to be addressed, and a brief
summary of their work is given below.

  * •

Applications and Industry: The charge for the topical group was to develop
strategies to strengthen HEP/Industry relationships. Members of this topical
group produced five contributed papers examining three different modes of
collaborative partnership, and cooperation on three different specific areas
of technology: “Programs enabling deep technology transfer from national
labs”; “Application-driven engagement with universities, synergies with other
funding agencies”; “Big industry engagement to benefit HEP: microelectronics
support from large CAD companies”; “Transformative technology for FLASH
radiation therapy”; and “Nurturing the industrial accelerator technology base
in the U.S.”.

  * •

Career Pipeline and Development: This working group was not simply about
making early career scientists aware of different opportunities, but also
about changing the culture of HEP career paths. Two topics were studied,
namely: “Facilitating Non-HEP Career Transition” and “Enhancing HEP research
in predominantly undergraduate institutions and community colleges”.

  * •

Diversity, Equity, and Inclusion: The DEI topical group focused on issues and
projects related to Diversity, Inclusion, Equity, and Accessibility; all of
which are essential not only to professional success in our field but to
develope a better society at large. Ultimately, twelve contributed papers were
developed, which may be categorized as follows: “Accessibility in High Energy
Physics: Lessons from the Snowmass”; “Lifestyle and personal wellness in
particle physics research”; “Climate of the Field: Snowmass 2021”; “Why should
the United States care about high energy physics in Africa and Latin America”;
“Experiences of Marginalized Communities in HEP”; “In Search of Excellence and
Equity in Physics”; and “Strategies in Education, Outreach, and Inclusion to
Enhance the U.S. Workforce in Accelerator Science and Engineering”.

  * •

Physics Education: The Physics Education topical group, examined the role that
physics education at all levels plays in advancing the field of HEP. Two goals
were identified as critical for the long-term health of the field: attracting
students across all demographics to the study of physics, and providing them
the education, training, and skills they will need to pursue any career in
STEM or related fields. The working group contributed papers are:
“Opportunities for Particle Physics Engagement in K-12 Schools and
Undergraduate Education”; “Transforming U.S. Particle Physics Education: A
Snowmass 2021 Study”; “Broadening the Scope of Education, Career and Open
Science in HEP”; “The Necessity of International Particle Physics
Opportunities for American Education”; “Data Science and Machine Learning in
Education”; and “U.S. CMS - PURSUE (Program for Undergraduate Research Summer
Experience)”.

  * •

Public Education and Outreach: The Public Education and Outreach working group
focused on enabling members of the physics community to effectively
communicate about scientific research through public engagement. The
contributions to this working group resulted in two contributed paper topics,
namely, “The need for structural changes to create impactful public engagement
in U.S. particle physics” and “Particle Physics Outreach at Non-traditional
Venues”.

  * •

Public Policy and Government Engagement: The topical group on Public Policy
and Government Engagement was tasked with conducting a review of all current
interactions between the HEP community and government offices and individuals.
Three contributed papers produced by the topical group document this work:
“Congressional Advocacy for HEP Funding”; “Congressional Advocacy for Areas
Beyond HEP Funding”; and “Non-congressional Government Engagement”.

  * •

Environmental and Societal Impacts: This topical group focused on ideas and
projects related to how particle physics research impacts society and the
environment. Five contributed papers were developed and submitted to this
group, focusing on three areas: environmental impacts of particle physics;
interactions of different laboratories with their local communities; and
nuclear non-proliferation.

The recommendations made by CEF are to improve and sustain strategic
engagements with our communities in order to draw support for and strengthen
the field of particle physics (an inward-focused goal carried forward from
Snowmass 2013) while playing key roles in serving those communities (an
outward-focused goal added for Snowmass 2021). It is critical that we as
individuals agree on the importance of all working together to address CEF
issues in HEP. DPF should work in collaboration with all HEP stakeholders to
develop a structure for implementing the recommendations contained in the CEF
report.

####  3 Computational Frontier [7]

Software and computing (S&C) is essential to all high-energy physics
experiments, accelerator and detector design, and many theoretical studies. It
is a key enabler of all the other Frontiers and all science Drivers, requiring
physics research along with expertise in computer science to address the
complex and unique challenges of our field. The escalating demand for
computing resources is a result of the need to do more sensitive and more
precise experiments, using higher intensity beams, higher luminosity
colliders, and stronger sources, to collect more astrophysical data over wider
fields and farther into the past (higher red-shifts), and to do more precise
theoretical calculations. While experiments may last for many decades and
their experimental hardware may be upgraded a few times during this period,
the computing hardware they use, driven by the commercial sector, may change
every half-decade or even more frequently. The software for the detectors and
facilities evolves continuously to respond to operational issues, but
undergoes larger changes to respond to major detector upgrades, and must also
adapt on shorter time scales to utilize and exploit the latest computing
hardware and software and computing infrastructure changes. In the last
decade, limited computing resources have gone much farther than expected due
to methodological innovation, but it is highly likely that analyzing all the
data to be acquired in the next decades will stress the community’s financial
and human resources. While we cannot predict with any accuracy what the
specific S&C environment will look like in 2035 or 2045, we can try to create
an R&D environment for computing hardware and software that will allow HEP to
adapt to whatever changes occur so it can utilize the most cost-effective
methods.

S&C had a prominent role in the 2013 Snowmass report. However, in the ten
years since then, many new features and trends appeared that were absent or
hardly noticeable then. These include

  * •

The use of heterogeneous accelerators such as GPUs and FPGAs, off-loading
parts of calculations. These devices come from the commercial sector, with
significant implications for the way we develop and maintain software.
Computing has entered a new “post-Moore’s law” phase. Speeds-ups in processing
now come from the use of parallelism within computations rather than higher
clock speeds. This paradigm shift has brought up the enormous challenge of
adapting or re-engineering almost every piece of HEP software, a process that
is heavy in labor and requires rare and expensive expertise.

  * •

The reliance on community hardware resources such as High-Performance
Computing (HPC) centers and the Cloud rather than dedicated experiment
resources to do production and very compute-intensive application. HPC
facilities are not specifically designed for HEP experiments, thus using them
effectively requires significant investment in R&D to address technical
challenges related to task factorization, I/O, storage, and utilization of
external services.

  * •

The pervasive use of artificial intelligence and machine learning, AI/ML, in
nearly every aspect of our software. Hardly mentioned in the 2013 report,
these revolutionary machine-learning approaches are transforming the way we
work.

  * •

The appearance of potentially disruptive technologies, such as quantum
computing. These are R&D projects, and we still need to understand which, if
any, HEP computing needs they satisfy.

It continues to be true today, as it was in 2013, that S&C efforts are neither
funded nor managed like projects in the manner that major facilities and
experiments are. They are funded by various streams including dedicated
resources for large experiments, shared resource pools provided for general
use by small and large groups, and national computing centers provided by
national funding agencies for large, multidisciplinary projects and
communities. Increasingly, HEP researchers are being encouraged to use these
computing centers, as well as commercial resources available in the Cloud.

This new computing environment requires R&D and novel approaches to address
the long-term development, maintenance, and user support of essential software
packages and cross-cutting R&D efforts supported from proof of concept to
prototyping and production. Additionally, strong investment in career
development for software and computing researchers is needed to ensure future
success. All of our computing tasks in HEP are likely to be affected by these
changes, so common solutions across HEP, other sciences, government, and
industry are going to be necessary.

The high-level conclusions of the Computational Frontier are:

  * •

The creation of a standing “Coordinating Panel for Software and Computing”
under the auspices of the APS Division of Particles and Fields, mirroring the
Coordinating Panel for Advanced Detectors (CPAD) established in 2012. As with
CPAD, this would, among other things, require a modification of the DPF
bylaws. A suggested mandate is:

Purpose: Promote, coordinate, and assist the HEP community on Software and
Computing, working with scientific collaborations, grassroots organizations,
institutes and centers, community leaders, and funding agencies on the
evolving HEP Software and Computing needs of experimental, observational, and
theoretical aspects of the HEP programs. The scope should include research,
development, maintenance, and user support.

  * •

Support of software packages The U.S. HEP Community should take a leading role
in the long-term development, maintenance, and user support of essential
software packages with targeted investment. Support of software for HEP has
been uncertain in the U.S. An excellent example is GEANT4, which does not
receive dedicated support in the U.S. at present and yet is critical to most
HEP programs, where it is used in facilities prospect studies, detector design
and optimization, computing infrastructure stress tests, software development,
and physics analysis. Event generators are another example.

  * •

Support for R&D Through existing, reshaped, and expanded programs, R&D efforts
cutting across project or discipline boundaries should be supported from proof
of concept to prototype to production.

  * •

Support for necessary personnel to enable the use of heterogeneous resources
Support for computing professionals/researchers and physicists to conduct code
re-engineering and adaptation will enable the most effective use of
heterogeneous resources.

  * •

Training, career development, and EDI Strong investment in career development
for HEP S&C researchers will ensure future success.

These recommendations of the Snowmass Computational Frontier resonate with the
European Strategy, spelled out in their recent report[3].

In 2013, Computing was organized along frontier lines. In Snowmass 2021,
Computing instead organized itself in Topical Groups that are all cross-
cutting in nature to reflect the need for common solutions to computing
challenges. The seven Topical Groups are Experimental Algorithm
Parallelization, Theoretical Calculations and Simulation, Machine Learning,
Storage and Processing Resource Access, End User Analysis, Quantum Computing,
and Reinterpretation and Long-term Preservation of Data and Code. The
following is a summary of important observations and conclusions from each of
the seven Topical Groups of the Computational Frontier.

Experimental Algorithm Parallelization This topical Group covers event and
physics object reconstruction for all experiments and comparable work for
surveys. Workable and portable solutions must be found and supported to
exploit heterogeneous computing platforms. In parallel, targeted optimizations
of key algorithms for experiments, which often provide the largest speedups,
must also be developed and supported. Software frameworks and common tools
must evolve and adapt to the new computing landscape to enable the usage of
parallel algorithms in production environments of big and small experiments.
Interdisciplinary collaborations and programs will play a critical role.
Training opportunities for early-career researchers are essential, and career
opportunities for researchers who focus on algorithm development need to be
created. To enable the long-term sustainability of HEP software and provide
job security to researchers in this area, software development should be
supported according to a model similar to that for detector projects, or as
part of detector projects. Funding should continue beyond the R&D phase of
algorithm exploration and extend into the development, production, and
maintenance phases. Funding agencies and research teams have to commit to
sustainable long-term effort for mission-critical software products: big
experiments/surveys need multi-year, rather than annual, computing resource
allocation planning for the use of national facilities.

Theoretical Calculations and Simulation Simulations and theory calculations
are used in nearly all aspects of HEP. For this study, the topic was divided
into six domains: cosmic calculations, particle accelerator modeling, detector
simulation, event generators, perturbative calculations, and lattice quantum
chromodynamics (QCD). Each area has specific needs, but there is also
significant commonality, leading to the following general conclusions.
Hardware accelerator-friendly portable common tools are essential. Universal
programming interfaces should be developed to facilitate portability. CPU
clusters will still be necessary to provide general-purpose computational
cycles for tasks that do not map well to accelerators but these must be
carefully sized to the actual needs. Automation of memory hierarchy will
remove a burden from the scientific programmer and enhance productivity. Best
practices for the development of common software should be encouraged. Working
with funding agencies, new arrangements should be established for long-term
support and maintenance of common software tools. Collaboration of computer
scientists, applied mathematicians, nuclear physicists, and others relevant to
HEP computational and physics model development should be encouraged and
supported. Programs should be developed, including joint lab-university
tenure-track appointments in S&C, to generate academic opportunities for young
researchers in computation in academia. Training for graduate students and
postdocs in AI/ML should be provided.

Machine Learning Machine Learning ML/AI techniques are firmly embedded in HEP
and their use is still growing. Areas where dedicated resources are required
for ML to continue to expand and flourish include:

  * •

Dedicated HEP-ML Research While industry is driving research in ML, there are
specific areas where its use in HEP requires dedicated solutions, such as in
uncertainty quantification, validation, and interpretability. Ideas such as
anomaly detection and simulation-based inference may enable analyses that are
not currently possible. ML may also be of use in the design, quality
assurance, and control and calibration of instrumentation and in control at
accelerator facilities and experiments, especially where ultra-low latency is
required for correction. Much of this work is inherently interdisciplinary.

  * •

Software and hardware needs HEP now relies on industry standards such as
TensorFlow, SciPy, and PyTorch. There is also a diverse set of hardware that
supports ML, most involving GPUs, which are mainly available at computing
centers. Small hardware setups available through research grants would help
stimulate research. At the same time, industry continues to develop more
advanced hardware accelerators whose development the HEP community should
track and participate in so that its needs can be met. The HEP community
should be trained in the use of demand services as a way to access the latest
custom hardware.

  * •

Training and Personnel ML/AI should be part of the standard training program.
Courses should be available in graduate school and in summer schools. Training
materials and paths should be readily available. Many of the issues connected
to the use of ML are statistical in nature so researchers need formal training
in that as well. It is also important that career paths be available to
researchers with these broad and cross-cutting capabilities.

Storage and Processing Resource Access This group concentrated on the R&D
challenges that must be addressed in the next decade so that HEP can take
advantage of the newest developments, which will most likely come from
industry. It did not focus on the required capacity for computing or storage.
The complex HEP workflow is divided into six areas – storage, processing, edge
services, AI/ML hardware, analysis facilities, and networking. Each of these
is examined in detail in the report and four overarching themes have emerged:
efficiently exploit specialized compute architectures and systems; invest in
portable and reproducible software and computing solutions to allow
exploitation of diverse facilities; embrace disaggregation of systems and
facilities; and extend common interfaces to diverse facilities. HPC
facilities, often several different ones, will be needed to do major
processing workflows for the analysis of experiments. There are four
recommendations for work that will facilitate the use of these diverse
capabilities for processing HEP data: HPC facilities should revisit their
resource access policies to allow more flexible allocations and job executions
so that more HEP projects can benefit from the large computing facilities;
investment in the development of portable software is key to maximizing the
efficient utilization of diverse processing resources; research is needed to
determine the trade-off between dedicated HEP computing facilities and general
access computing facilities such as the HPC center, Grid and Cloud resources;
and infrastructure development will be needed to support better data
management frameworks across different types of facilities.

End User Analysis Two analysis ecosystems are used in HEP, one based on ROOT,
hosted by CERN, with U.S. support, especially in the area of I/O, and the
other based on Python, which is a set of analysis tools developed primarily to
host analysis software, including ML/AI, developed outside of HEP. One
challenge is that increasingly running on massive amounts of data will be done
at HPCs. Particle physics analysis problems usually require relatively light-
weight computations to be done repeatedly on massive numbers of events, often
referred to as high-throughput computing (HTC). High-Performance Computing
center execution environments are usually not needed for HEP analysis and may
introduce constraints and overheads that make it difficult to use them
efficiently. Actions that need to be taken to serve the needs of the HEP
community for long-term, sustainable end-user analysis are identified:

  * •

Develop both ROOT-based and Python-based analysis ecosystems and maintain
interoperability between them.

  * •

Critical elements of the ROOT evolution plans include the development of
improved columnar data formats and multiple I/O implementations.

  * •

Analysis facilities for low latency columnar analysis in the context of the
Python ecosystem will be essential for future experiments.

User-friendly data provenance and metadata storage systems that can be easily
integrated into typical analysis tasks should be developed. Scaleable analysis
models should be developed so users can perform small interactive tests and
also can run over large datasets using a single interface. Pipelines
compatible with long-term preservation should be built into the structure of
analysis systems. Collaborative software is an important element of the
analysis software stack of an experiment and includes documentation, messaging
between users, discussion forums, software version control, bug tracking, and
document workflow management. Host laboratories should provide a full stack of
these services to their experiments, large and small. Documentation and
training must be produced for analysis software understood as an ecosystem,
not as a disconnected set of packages.

Quantum Computing Quantum computing is an exciting new technology that can be
shown to outperform classical computers in certain applications. However,
current quantum computers (QCs) are relatively small and suffer from
vulnerability to noise. This Topical Group focused solely on the use of re-
programmable quantum hardware. It is not clear where it will be advantageous
to employ QC in HEP, but there are a number of benchmark problems where QC is
expected to be useful and perhaps even transformational, including lattice
QCD, event generation, and data analysis. Currently, there is no consensus on
the best hardware for implementing quantum computers and there are several
software packages available. Access to QC hardware is also a problem.
Nevertheless, we should support research by HEP physicists, in collaboration
with industry and academia, to explore this rapidly developing field which may
eventually offer substantial benefits to HEP. The National QIS Research
Centers are a significant step in this direction.

Reinterpretation and Long-term Preservation of Data and Code This group
considered how data, simulations, analyses, and codes needed to do an analysis
can be preserved so that the analysis could, for example, be repeated well
after the experiment that took the data ended. The purpose might be to verify
the original analysis, try to improve it using new methods or concepts, use
the data for some new analysis, or combine it with some other analysis.
Recommendations in this area include:

  * •

Ensuring that all running and in-preparation experiments have a strategy and
resources for the long-term preservation of data and analysis capabilities.

  * •

Investing in shared cyber-infrastructure to preserve these data and support a
comprehensive analysis from various experiments and surveys – both active and
completed – in order to realize their full scientific impact. The
infrastructure should support the requisite theoretical inputs and
computational requirements for analysis as well as metadata and APIs to track
provenance and provide incentives for participation.

  * •

As a specific example, there should be a data archive center to preserve
Cosmic Frontier datasets and simulations, and facilitate their joint analysis
across different computing centers. Preservation needs are not restricted to
experimental data and analyses. As an example, for many years, the lattice QCD
community has had a tradition of sharing the expensive-to-produce gauge
configurations that can be used for many physics analyses.

Software and computing technologies traverse all areas of HEP and are evolving
rapidly. It is hard to imagine what the state of the art will be at the time
of the next Snowmass in about a decade. It is our hope that the establishment
of a Coordinating Panel for Software and Computing will serve the community on
timescales commensurate with the rapid evolution of S&C technologies.

####  4  Cosmic Frontier [8]

The Cosmic Frontier is focused on understanding how elementary particle
physics shapes the behavior and evolution of the universe, and how
observations of the universe inform our understanding of physics beyond the
SM. In the moments just after the Big Bang, fundamental physics governs the
production of the particles and energy fluctuations that give rise to the
current universe. The properties of elementary particles also govern the
behavior of the most energetic processes observed in the universe today,
including supernovas. The physics questions which guide the cosmic frontier
include:

  * •

What is the nature of dark matter?

Does dark matter interact with ordinary matter, perhaps via the Higgs boson or
another mediator? Is dark matter bosonic and does it exhibit wave-like
properties? Does the dark sector itself have non-trivial dynamics, and does
dark matter interact with itself? How was dark matter produced in the early
universe? Does the nature of dark matter have general implications for physics
beyond the SM?

  * •

What is the nature of dark energy?

Is the observed acceleration in the expansion rate of the universe due to a
new energy component or does it require modifications to general relativity?
If dark energy is a new energy density component, is it a cosmological
constant or is it a dynamical quantity changing in time? When did dark energy
become important in the history of the universe?

  * •

How is the inflationary paradigm realized in nature?

What is the energy scale of primordial inflation? Does inflation have dynamics
that manifest themselves as an observable imprint on the primordial
distribution of matter fluctuations? Detect or place limits on “B-modes” to
discriminate among theories of the Big Bang. Why were there two eras of
acceleration in the history of the universe, one early and one late?

  * •

How can we use cosmic observations to learn about BSM physics?

Did BSM degrees of freedom influence the thermal history of the universe? What
is the scale of neutrino masses? Can cosmological observations distinguish
between the ordinary and inverted neutrino mass hierarchies? What do
gravitational waves reveal about early Universe dynamics and phase
transitions? What do Nature’s highest energy particles produced in cosmic
sources tell us about BSM particles and interactions?

A major thrust of the future Cosmic Frontier program is building the next
generation of cosmological probes. Cosmic surveys “aim high” at observables
spanning almost the entire 13.8 billion-year history of our Universe. The next
big project in this arena is CMB-S4, a system of telescopes to study the
cosmic microwave background and address the mystery of cosmic inflation, which
is expected to operate through to at least 2036. Additional projects that
would start after 2029 are Spec-S5 (the follow-on spectroscopic device to
DESI), a project to carry out line intensity mapping (LIM), and planning
efforts to increase the sensitivity of gravity wave detection by at least a
factor of 10 ($10^{3}$ in sensitive volume) beyond what will be achieved by
LIGO/Virgo.

In the Cosmic Frontier survey ecosystem, each type of program brings unique
strengths to the portfolio, including shape measurements that can only come
from imaging, precise and robust redshift measurements that require
spectroscopy, and multi-messenger data that exploits new gravitational wave
detection capabilities; by also spanning a range in wavelength (e.g.,
Optical/IR vs. mm-wave vs. 21-cm), information from different redshift ranges
can be accessed. While each survey will deliver groundbreaking results on its
own, no single experiment can meet all of the discovery thresholds without
complementary information from other programs.

A second thrust of the Cosmic Frontier is a suite of experiments to explore
the physics of dark matter (DM). The space of DM models encompasses a dizzying
array of possibilities representing many orders of magnitude in mass and
couplings, making the DM program one of the most “interdisciplinary”
investigations in high-energy and particle physics. The Cosmic Frontier’s DM
program will “delve deep, search wide” by employing a broad portfolio of
small/medium-scale direct and indirect detection experiments, as is required
to search optimally for each decade in dark matter mass. Furthermore, an
expanded Cosmic Probes program will strive to identify the properties of dark
matter via cosmic surveys, which is highly synergistic with the other science
targeted by those surveys.

The DM research program enhances adjacent areas of science. New DM detector
technologies will require cross-disciplinary collaborations to access the
expertise, technology, and facilities of neighboring fields of study such as
AMO, condensed matter physics, quantum information science, and gravitational
physics. Similar synergy between fields is present when searching for indirect
effects of dark matter scattering, annihilation, or decay in galactic halos,
where HEP provides a critical role in developing instrumentation for
astroparticle observatories. Equally important, collaboration with the broader
non-HEP astronomy and astrophysics community is essential for understanding
the astrophysical backgrounds in DM searches, while at the same time alerting
those communities to possible non-gravitational DM impacts.

The conclusions of this Frontier arise from the work of the seven Topical
Groups of this Frontier, which were organized into three general categories as
described below.

Dark Matter

Our understanding of the landscape of dark matter theories has evolved
significantly in the past several years, as theoretical exploration has better
defined the boundaries of what models are consistent with observations. As of
Snowmass 2013, the classification of dark matter candidates was largely based
on the particle physics features of the underlying models. Since then, the
focus has shifted toward exploring a wide range of possible phenomena in an
effort to understand how well existing experimental searches cover the space
of possibilities, and how new experimental opportunities provide sensitivity
to regions of theory space that are not captured by the current program.

  * •

Particle-like Dark Matter: In conventional models of GeV-TeV mass dark matter
associated with solving the electroweak gauge hierarchy problem, WIMPs with
electroweak couplings to SM particles would be naturally populated at
approximately the correct density by the freeze-out mechanism. Large second-
generation WIMP detectors based on a variety of scattering targets, combined
with indirect searches for high-energy annihilation products, have excluded
$Z$-mediated couplings to SM particles for WIMP masses up to $\sim$TeV.
Experiments are now probing weaker couplings such as those mediated by Higgs
boson exchange and exploring new techniques to be sensitive to light particle
dark matter (1 eV to 1 GeV).

  * •

Wave-like Dark Matter: Provided that gravitational clumping of a single dark
matter species is responsible for the formation of all galaxies and galactic
substructures, the sizes of the dark matter halos provide a quantum-mechanical
lower bound on the mass of dark matter of order $10^{-22}$ eV. For dark matter
mass less than 100 eV, the mode occupation number must exceed unity in order
for there to be enough dark matter to account for the local gravitational
well. Hence dark matter in the mass range from $10^{-22}$ eV to $10^{2}$ eV
must be wave-like bosonic fields. A high-priority target in this mass range is
the QCD axion, and perhaps the most significant technological development in
the field of terrestrial dark matter searches since the last Snowmass is the
demonstration by ADMX and HAYSTAC of experimental sensitivity to the invisible
QCD axion made possible through the use of quantum sensing technologies. A
comprehensive search for the QCD axion is now within reach, but this program
will require significant investment in high-field magnets which serve as the
axion scattering target.

  * •

Cosmic Probes of Dark Matter: Cosmic probes of dark matter, which seek to
determine the fundamental properties of dark matter through observations of
the cosmos, have emerged as a promising means to reveal the nature of dark
matter, and are valid even when the coupling between dark matter and normal
matter is extremely weak (e.g., as weak as gravity). Rubin LSST has enormous
potential to discover new physics beyond the prevailing cold dark matter
paradigm. Microlensing measurements will directly probe primordial black holes
as a component of dark matter. The planned CMB-S4 and future Spec-S5 and
CMB-S5 projects will also extend access to rich dark matter particle physics.
These probes also constrain neutrino physics. The current cosmological limit
on the sum of the neutrino masses is $\sum m_{\nu}<0.12$ eV as obtained using
cosmic microwave background, baryon acoustic oscillations, supernova Ia, and
large-scale structure measurements. CMB-S4 will reach sensitivity $\sum
m_{\nu}<0.02$ eV, and if the data indicate a mass sum less than 0.11 eV, this
would rule out the inverted neutrino mass ordering which predicts a higher
mass sum.

Dark Energy and Cosmic Inflation

Developing an understanding of the cause of cosmic acceleration in the modern
universe would revolutionize our understanding of fundamental physics.
Cosmology research in the twenty-first century is dominated by large cosmic
survey experiments carried out by worldwide collaborations comprising hundreds
of members from dozens of institutions. The goal is to study both early and
late universe phase transitions and the new particles and interaction dynamics
governing new fundamental scales of nature. New measurements will establish
the values of key parameters, including the equation of state parameter of
dark energy, the rate of expansion of the universe today, and the amplitudes
of fluctuations in the density of matter in the universe.

  * •

The Modern Universe: The community’s input to this Topical Group jointly
describes a multi-probe experimental program that provides a rich, deep, and
flexible portfolio that provides powerful constraints on cosmic acceleration.
The program includes a powerful new Stage V spectroscopic facility (also
referred to here as Spec-S5) that would pursue larger and deeper surveys
enabling transformational advances in our understanding of both eras of
accelerated expansion in the history of the universe – the early inflationary
epoch and the late dark energy-driven one.

  * •

Cosmic Dawn and Before: The scientific goals of this Topical Group are to
study inflation and to search for new physics through precision measurements
of the Cosmic Microwave Background (CMB) from the early universe.
Polarization-sensitive experiments like CMB-S4 offer unique windows on the
B-mode patterns, which reveal the absolute energy scale of inflation and will
continue to provide precision measurements of the other key observables.
Upcoming cosmic surveys including DESI and the envisioned Spec-S5 project will
provide much larger datasets, using precise redshift determinations to map the
matter distribution in all 3 dimensions instead of being constrained to a
single 2-dimensional slice at the surface of the last scattering. A new
technique of line intensity mapping (LIM) of the 3-d distributions of neutral
hydrogen or other gas will possibly access the largest range of redshifts to
provide the highest statistics measurements of the large-scale structure.

  * •

Complementarity of Probes and New Facilities: This topical group presented a
multi-faceted vision for the cosmic survey program which provided a path to
understanding the causes of early- and late-time acceleration by leveraging
ongoing surveys, developing and demonstrating new technologies, and
constructing and operating new instruments The previously mentioned large
CMB-S4 project and the next large Stage V Spectroscopic Facility (Spec-S5)
should play a key role in advances over the next decade and beyond. The vision
also includes potential future surveys that will be enabled by near-term R&D
and small pathfinder initiatives; these projects would employ new technologies
and techniques such as 21-cm and mm-wave Line Intensity Mapping (LIM),
Gravitational Wave Observatories (GWO), and future broadband CMB imaging
(CMB-S5).

Cosmic Probes of Fundamental Physics

Cosmic Probes of Fundamental Physics encompass a multitude of approaches:

  * •

Very high energy particles (ultra-high energy cosmic rays, gamma rays, and
neutrinos): The discovery of TeV$-$PeV astrophysical neutrinos opens up unique
opportunities to probe the neutrino sector at energy scales not accessible
with laboratory neutrino beams; high-energy neutrinos from IceCube were used
to discover the Glashow resonance and measure the high-energy neutrino-nucleon
cross section and inelasticity distribution. UHECR air showers probe hadronic
interactions at center-of-mass energies an order of magnitude beyond the LHC;
they have been used to determine the p-p cross section and have established
significant failings in state-of-the-art modeling of hadronic interactions.
Upper limits on UHE gammas and neutrinos constrain GUT-scale physics.

  * •

Gravitational waves and neutron star observations:  The LIGO-Virgo
collaboration has observed many examples of binary black holes, BH-neutron
stars, and binary NS mergers, revealing an unexpected population of black
holes with masses up to several hundred $M_{\odot}$. LV and NICER have
measured the radii of neutron stars, forcing a re-evaluation of the QCD
equation of state at very high density. Future measurements with a next-
generation gravitational wave observatory, will nail down crucial aspects of
QCD and probe some scenarios of inflation and phase transitions in the early
universe.

  * •

Multi-messenger astrophysics: The dawn of a new astrophysical multi-messenger
era has been heralded by the recent co-detection of gamma rays and
gravitational waves in a binary neutron star merger, the co-detection of gamma
rays and neutrinos in a blazar flare, and several examples of neutrinos
consistent with production in tidal disruption events. Over the next decade,
simultaneous observations with different techniques promise to reveal where
these extreme-energy cosmic messengers come from, and how they came to be.

To summarize, the Cosmic Frontier strategy detailed in this report encompasses
a science-rich program with small, medium, and large experiments which are
planned on both near-term and long-term timescales. The community consensus is
that this coming decade must feature strong support for this program in full
to ensure that HEP will enjoy the new era of discovery and groundbreaking
precision measurements that have been promised for the next two decades, as
well as building even more powerful experiments to reach key science
thresholds.

####  5 Energy Frontier [9]

After decades of pioneering explorations and milestone discoveries, the SM of