Presented at the $16^{\mathrm{th}}$ International Workshop on Top Quark
Physics,
Traverse City, Michigan, USA, 24–29 September, 2023

# Inclusive and differential results of top quark pair production from the
ATLAS and CMS experiments

David Walter on behalf of the ATLAS and CMS Collaborations111Copyright 2023
CERN for the benefit of the ATLAS and CMS Collaborations. Reproduction of this
article or parts of it is allowed as specified in the CC-BY-4.0 license
European Organization for Nuclear Research (CERN), Espl. des Particules 1,
1211 Meyrin, Switzerland

###### Abstract

This report summarizes recent results of inclusive and differential
$\mathrm{t}\bar{\mathrm{t}}$ cross section measurements from the ATLAS and CMS
Collaborations at the LHC. Measurements at $\sqrt{s}=7,\ 8,\ 13$, and
$13.6{\,\mathrm{TeV}}$ are compared to state-of-the-art theory predictions,
using different PDF sets, matrix element calculations, or parton shower
models. No significant disagreement of a single inclusive measurement is
found, with an overall trend towards lower values. For the differential
measurements, no theory model is able to describe the data across all bins.

##  1 Introduction

The top quark is the heaviest particle of the standard model (SM), has a
variety of unique features such as the strongest coupling to the Higgs boson
among all known particles, and is thus of particular interest. In proton-
proton collisions (pp) at the CERN LHC, it is primarily produced via the
strong interaction in top quark-antiquark ($\mathrm{t}\bar{\mathrm{t}}$)
pairs. At the highest center of mass energy of
$\sqrt{s}=13.6{\,\mathrm{TeV}}$, $\mathrm{t}\bar{\mathrm{t}}$ events are
produced in large quantities ($\approx 100$ millions at ATLAS [1] and CMS [2]
during 2022 and 2023). About 90% of the $\mathrm{t}\bar{\mathrm{t}}$ pairs are
produced in gluons fusion, while about 10% are produced from quark-antiquark
initial states. As the top quark decays in almost 100% to a b quark and a W
boson, $\mathrm{t}\bar{\mathrm{t}}$ events are categorized by the subsequent
decay of the two W bosons into all-hadronic (46%), single leptonic (15% for
each lepton channel), and dileptonic with the same flavored (1%) or different
flavored (2%) leptons.

Predictions of the inclusive $\mathrm{t}\bar{\mathrm{t}}$ cross section
($\sigma_{\mathrm{t}\bar{\mathrm{t}}}^{\mathrm{theo}}$) up to next-to-next-to-
leading order (NNLO) in perturbative quantum chromodynamics (QCD) exist,
including soft gluon resumation up to next-to-next-to leading logarithm (NNLL)
[3]. Due to the high energy in $\mathrm{t}\bar{\mathrm{t}}$ events and the
fact that the process can be measured with high precision, similar to the
precision of the theory calculations, measurements allow stringent QCD tests.
Results are further used in the extraction of the strong coupling constant
$\alpha_{\mathrm{S}}$ [4, 5], and in global fits to constrain the proton PDF,
particularly at large momentum fractions for the gluon and the gluon to quark
ratio [5]. From its coupling to the Higgs boson, the top quark mass is of
particular interest also for the electroweak theory (EW) and can also be
extracted from differential cross sections [4, 5].

##  2 Inclusive $\mathrm{t}\bar{\mathrm{t}}$ cross section measurements at
$\sqrt{s}=7{\,\mathrm{TeV}}$

During Run 1 of the LHC in the years 2011 and 2012, various analyses of
$\mathrm{t}\bar{\mathrm{t}}$ events have been performed at $\sqrt{s}=7$ and
$8{\,\mathrm{TeV}}$ and ATLAS and CMS, such as the inclusive
$\mathrm{t}\bar{\mathrm{t}}$ cross section measurements and a combination of
results from the two experiments in the dileptonic channel yielding a
precision of $\delta\sigma_{\mathrm{t}\bar{\mathrm{t}}}=2.6\%$ published in
2022 [4]. ATLAS has recently released a new
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ measurement at
$\sqrt{s}=7{\,\mathrm{TeV}}$ using the full data set from 2011 of
$4.6{\,\mathrm{fb}^{-1}}$ [6]. The analysis targets the single lepton final
state and selects events with exactly one muon or electron, a large amount of
missing energy, and three jets from which 0, 1, or 2 are required to be
identified as coming from a B hadron (b jet). Significant backgrounds arise
from events with W or Z bosons in association with jets, from single top quark
production, or processes without a prompt lepton (nonprompt background). The
nonprompt background is estimated from data via the matrix method, also known
as fake rate method. The probability that a nonprompt lepton is misidentified
as a prompt one is first measured in an independent data set and then applied
to events in data in a region enriched in nonprompt background. The obtained
contribution is used as an estimate for this type of background in the signal
region. Other backgrounds with prompt leptons are estimated using Monte Carlo
(MC) simulation. The signal is separated from the different backgrounds using
a multivariate analysis based on support vector machines (SVNs). SVNs are
chosen because of their firm mathematical foundation, simple geometric
interpretation, and robustness against overtraining. If a minimum of their
loss function is found it is guaranteed the global minimum. Three SVNs are
trained using 21 variables to separate $\mathrm{t}\bar{\mathrm{t}}$, W/Z in
association with b quark, and other backgrounds, in a three-dimensional space.
From this distribution, events are categorized in four regions and binned in
one-dimensional variables. A bin-by-bin Poisson-based likelihood function is
minimized, and systematic uncertainties in the measurement are profiled. The
obtained cross section of $\sigma_{\mathrm{t}\bar{\mathrm{t}}}=168.5\pm
0.7(\mathrm{stat})^{+6.2}_{-5.9}(\mathrm{syst})^{+3.4}_{-3.2}(\mathrm{lumi})\,\mathrm{pb}$
has a relative uncertainty of 4% and is significantly improved with respect to
the 12% precision from the previous measurement from ATLAS at
$7{\,\mathrm{TeV}}$ in this channel. The result is consistent with the SM
prediction of
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}^{\mathrm{theo}}=177^{+10}_{-11}\,\mathrm{pb}$,
and has a tension of two standard deviations with the ATLAS measurement in the
dileptonic channel.

##  3 Inclusive $\mathrm{t}\bar{\mathrm{t}}$ cross section measurements at
$\sqrt{s}=13{\,\mathrm{TeV}}$

The biggest data sample at the LHC was collected at $13{\,\mathrm{TeV}}$
between 2016–2018 and has since been studied in great detail. A new analysis
from ATLAS in the dilepton channel using the full data set of
$140{\,\mathrm{fb}^{-1}}$ documents the most precise measurement of
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ so far [7]. Events containing an
opposite signed (OS) $\mathrm{e}^{\pm}\mu^{\mp}$ pair and either one or two b
jets are selected. Nonprompt background contribution is estimated from data
using events with the same sign (SS) $\mathrm{e}^{\pm}\mu^{\pm}$ dilepton pair
using the OS to SS ratio from simulation. Events from $\mathrm{Z}\to\tau\tau$
with subsequent decays of the $\tau$ leptons into an electron and a muon are
modeled by MC. The normalization is rescaled based on a measurement of
$\mathrm{Z}\to\mathrm{ee}/\mu\mu$ events produced in association with one or
two b jets. Other backgrounds, such as single top W associated (tW)
production, diboson, and $\mathrm{t}\bar{\mathrm{t}}$ production in
association with a boson, are estimated from simulation. Lepton efficiencies
are corrected from scale factors measured in $\mathrm{Z}\to\mathrm{ee}/\mu\mu$
events. To mitigate the systematic effect of the difference in the event
topology in $\mathrm{t}\bar{\mathrm{t}}$ events, an additional lepton
isolation scale factor is measured from events where one of the leptons fails
the isolation criteria. The cross section in fiducial phase space is obtained
simultaneously with the b tagging efficiency from the number of events with
one or two b jets. Extrapolated to the full phase space, this results in

$\sigma_{\mathrm{t}\bar{\mathrm{t}}}=829\pm 1(\mathrm{stat})\pm
13(\mathrm{syst})\pm 8(\mathrm{lumi})\pm 2(\mathrm{beam})\,\mathrm{pb}$ (1)

which is, with a precision of 1.8%, the most precise
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ measurement to date and in agreement
with the SM prediction of
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}^{\mathrm{theo}}=832^{+20}_{-29}(\mathrm{scale})^{+23}_{-23}(m_{\mathrm{top}})^{+35}_{-35}(\mathrm{PDF}+\alpha_{\mathrm{S}})\,\mathrm{pb}$.
The unprecedented precision was achieved also because of the accurate
luminosity determination by ATLAS with an uncertainty of 0.83% [8]. Other
important sources of uncertainty are related to the modeling of the top quark
$p_{\mathrm{T}}$ and the tW background, and the lepton selection.

##  4 Inclusive $\mathrm{t}\bar{\mathrm{t}}$ cross section measurements at
$\sqrt{s}=13.6{\,\mathrm{TeV}}$

The first analyses performed on the new Run 3 data at $13.6{\,\mathrm{TeV}}$
from 2022 were $\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ measurements performed by
the ATLAS and CMS experiments [9, 10]. Inclusive $\mathrm{t}\bar{\mathrm{t}}$
cross section measurements are in particular well suited for early data due to
the high rate of $\mathrm{t}\bar{\mathrm{t}}$ events and their distinctive
signature. Given that they decay in a variety of different particles, it also
presents a good opportunity for data validation. Going from 13 to
13.6${\,\mathrm{TeV}}$, the $\mathrm{t}\bar{\mathrm{t}}$ cross section
increases by about 10%. With a more precise expected precision, the
measurement also represents the first meaningful test of the SM at the new
precision frontier.

The CMS analysis targets final events in the single and dileptonic final
states, including the selection of jets and b jets with various
multiplicities. This allows to obtain a sufficiently large data sample already
from the first 1${\,\mathrm{fb}^{-1}}$ of collected data in August 2022. It
also allows the use of information from the data as much as possible, to
constrain experimental uncertainties on the less studied early data.
Background processes with nonprompt leptons in the single lepton channel are
estimated using the ABCD method, by constructing three regions orthogonal to
the signal region. The probability of a nonprompt lepton to be misidentified
as a prompt one is measured in events with exactly one jet and applied in
regions with a similar selection than the signal region, but with requiring
the lepton to fail the isolation requirement. After a preliminary result was
shown in last year’s top conference, about one month after data taking, the
final result has been published with a few refinements. For example, a new jet
reconstruction algorithm based on the pileup per particle probability
identification (PUPPI) is applied. A likelihood fit binned in the lepton
flavor and multiplicity, and the multiplicity of jets and b jets yields a
result of $\sigma_{\mathrm{t}\bar{\mathrm{t}}}=881\pm
23(\mathrm{stat}+\mathrm{syst})\pm 20(\mathrm{lumi})\,\mathrm{pb}$ with a
precision of 3.5%, in agreement with the SM prediction of
$924^{+32}_{-40}\,\mathrm{pb}$. The total uncertainty also profits from a
large reduction of the luminosity uncertainty where CMS makes use of Z boson
rate measurements to validate the linearity and stability of conventional
methods [11]. This method has also been studied in more detail with a full
uncertainty assessment based on 2017 data [12] and is a valuable new input in
precision luminosity determination in the future.

A different strategy was chosen by the ATLAS Collaboration, which targets
events in the more clean dileptonic channel. This required a large amount of
data which was initially 11.3${\,\mathrm{fb}^{-1}}$ and updated with the full
data from 2022 of 29${\,\mathrm{fb}^{-1}}$. The final result also features the
use of the jet vertex tagger to reject jets from pileup. Similar to the
13${\,\mathrm{TeV}}$ analysis by ATLAS [7], events with one or two b jets are
categorized and $\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ is measured
simultaneously with the b tagging efficiency. The obtained cross section of
$\sigma_{\mathrm{t}\bar{\mathrm{t}}}=850\pm 3(\mathrm{stat})\pm
18(\mathrm{syst})\pm 20(\mathrm{lumi})\,\mathrm{pb}$ shows a small trend
towards a smaller value than predicted. In the analysis, a simultaneous
measurement of $Z\to\mathrm{ee}/\mu\mu$ is performed and the
$\mathrm{t}\bar{\mathrm{t}}$ to Z cross section ratio is obtained in a
consistent treatment of systematic uncertainty where the effect from
luminosity largely cancels. The ratio is in particular interesting to test
different PDF sets, where in general good agreement is observed.

In comparison, despite having two largely different approaches, the two
experiments end up with a very similar uncertainty where the luminosity has
the largest impact, followed by the uncertainty on the leptons and signal
modeling. For CMS, also the uncertainty on b jets is significant, as expected
from including the single lepton channel where the b jet requirement is needed
to separate the backgrounds. In both analyses, likelihood fits are performed,
while ATLAS has smaller prefit uncertainties and constraints, CMS starts with
larger prefit uncertainties that get pulled and constrained by the data.

##  5 Measurements of jet substructure variables in
$\mathrm{t}\bar{\mathrm{t}}$ events at $\sqrt{s}=13{\,\mathrm{TeV}}$

The understanding of the jet substructure, in particular for b jets and large
radius (large-R) jets from hadronic decaying top quarks, is crucial for
analyses targeting final states with top quarks. Information can be used in
tuning studies to improve the modeling of MC simulation which in turn can lead
to a reduction of the related systematic uncertainties in measurements. Jet
identification algorithms based on MVA techniques can be further developed by
exploiting observed features. ATLAS published a new result with differential
cross section measurements of several variables related to the substructure of
events with large-R jets. Variables related to the energy flow in jets, such
as the Les Houches angularity which is a measure of the broadness and shows a
separation between jets from quarks and gluons are studied. Other variables
are related to the 3 prong nature of large-R jets from top quarks, such as the
N-jettiness. Single lepton (all hadronic) $\mathrm{t}\bar{\mathrm{t}}$ events
are selected that have at one (two) large-R jets with
$p_{\mathrm{T}}>350{\,\mathrm{GeV}}$ (and
$p_{\mathrm{T}}>500{\,\mathrm{GeV}}$). The matrix (ABCD) method is exploited
to estimate the nonprompt (multijet) backgrounds in the single lepton (all
hadronic) channel. For selected events, a data excess of 15 to 20% is
observed, compatible with the known mismodeling of
$\mathrm{t}\bar{\mathrm{t}}$ events in the regime with large top quark
$p_{\mathrm{T}}$ [13]. The differential cross sections are extracted at
particle level using an iterative Bayesian unfolding, their uncertainty is
dominated by parton shower modeling, and experimental jet energy scale and
resolution. Additionally to the one-dimensional variables, two-dimensional
distributions are presented to study the modeling at different values of
$m^{\mathrm{top}}$ and $p_{\mathrm{T}}^{\mathrm{top}}$. A quantitative and
qualitative assessment for different models is presented using $\chi^{2}$
tests. Variables found to be sensitive to the three-prong structure of the
large-R jet from the top quark indicate tension between the data and some of
the models. For instance, a reduction of the final state radiation (FSR) scale
is favored.

##  6 Differential $\mathrm{t}\bar{\mathrm{t}}$ cross section measurements at
$\sqrt{s}=13{\,\mathrm{TeV}}$

Both ATLAS and CMS have recently analyzed single and multi-differential
$\mathrm{t}\bar{\mathrm{t}}$ cross section measurements in the dileptonic
channel using the full Run 2 data of 140 and 138$\,\mathrm{fb}$, respectively
[7, 14]. The ATLAS measurement is documented together with the previously
discussed inclusive cross section measurements at 13${\,\mathrm{TeV}}$ and the
same strategy is used. Instead of extracting the inclusive event numbers, the
event numbers are extracted in bins of the observable to be measured and then
unfolded to particle level using a bin-by-bin procedure. Eight variables of
the lepton kinematics are obtained and compared to predictions using different
matrix element generators and parton shower models. At CMS, also events with
the same flavor dilepton pairs are included. Two different versions of event
reconstruction are performed to obtain the kinematics of the two top quarks in
the event, the so-called full and loose kinematic reconstructions. The full
kinematic reconstruction uses hereby six constraints (two W masses, 2 top
quark masses, and the conservation of $p_{\mathrm{T}}$ in the event), while
the top quark mass constraint is released in the loose version. This allows to
measure $m^{\mathrm{t}\bar{\mathrm{t}}}$ in an unbiased way. In general, the
uncertainty was reduced by a factor of around two with respect to a previous
result. The dominating effects are from jet energy scale, backgrounds, and the
lepton selection. The unfolding is performed to parton and particle level
using a $\chi^{2}$ fit. Results are compared to NLO event generators, beyond
NLO predictions, and different PDF sets. While the known feature of
overestimation at higher $p_{\mathrm{T}}^{\mathrm{top}}$ is visible for NLO
predictions, it can be seen that beyond NLO predictions, in particular,
approximate $\mathrm{N}^{3}$LO have a better modeling of
$p_{\mathrm{T}}^{\mathrm{top}}$. Further insight into this issue is provided
from two-dimensional distributions. For example, it is shown that the
$p_{\mathrm{T}}^{\mathrm{top}}$ mismodeling is enhanced at large values of
$m^{\mathrm{t}\bar{\mathrm{t}}}$ and less strong for events with two or more
jets. The analysis also presents three-dimensional distributions which are
promising inputs for future PDF fits. In summary, both, the CMS and ATLAS
differential measurements show that none of the models used for comparison can
describe the data in all bins. While the modeling improves with higher order
predictions, some bins remain to disagree.

##  7 Summary

With the first $\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ measurements at
13.6${\,\mathrm{TeV}}$, the $\mathrm{t}\bar{\mathrm{t}}$ cross section is now
measured at six different centers of mass energies. A summary of inclusive
$\mathrm{t}\bar{\mathrm{t}}$ cross-section measurements at different centers
of mass energies is presented in Fig. 1. The $\mathrm{t}\bar{\mathrm{t}}$
measurements are on the forefront of precision measurements at the LHC and the
first measurement with a precision below 2% has been achieved. While each
inclusive cross section measurement has in general a good agreement with the
prediction, there is a slight systematic trend towards an overprediction
visible. Double and multi-differential cross section measurements have been
performed and give more insight into different modeling aspects. It is
observed that none of the generators can describe the data in all bins, while
the disagreement increases in extreme phase space regions, and when looking in
higher dimensional distributions. Higher-order predictions lead to a better
agreement, but still fail to describe the data in some bins. It remains to be
seen from more data, more precise measurements, and a better understanding of
the theory predictions, if a real deviation from the standard model is
present.

Figure 1: The $\sigma_{\mathrm{t}\bar{\mathrm{t}}}$ at varying center-of-mass
energies at the LHC and the Tevatron. The experimental measurements are
compared to theoretical predictions at NNLO in QCD with NNLL resummation. The
theoretical predictions include uncertainties from renormalization and
factorization scales, PDFs, and $\alpha_{\mathrm{S}}$. Both the measurements
and theoretical calculations are reported for a top quark mass (mtop) of
172.5${\,\mathrm{GeV}}$. To enhance clarity, measurements taken at the same
center-of-mass energy have been slightly offset [15].

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figuret contents=  Submitted to the Proceedings of the US Community Study on
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scale=1,placement=top,opacity=1,color=black,position=3.25in,1.2in

# Snowmass 2021 CMB-S4 White Paper

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Valentino,Snowmass 2021 CMB-S4 White Paper Marion Dierickx,Snowmass 2021
CMB-S4 White Paper Brad Dober,Snowmass 2021 CMB-S4 White Paper Randy
Doriese,Snowmass 2021 CMB-S4 White Paper Shannon Duff,Snowmass 2021 CMB-S4
White Paper Daniel Dutcher,Snowmass 2021 CMB-S4 White Paper Cora
Dvorkin,Snowmass 2021 CMB-S4 White Paper Rolando Dünner,Snowmass 2021 CMB-S4
White Paper Tarraneh Eftekhari,Snowmass 2021 CMB-S4 White Paper Joseph
Eimer,Snowmass 2021 CMB-S4 White Paper Hamza El Bouhargani,Snowmass 2021
CMB-S4 White Paper Tucker Elleflot,Snowmass 2021 CMB-S4 White Paper Nick
Emerson,Snowmass 2021 CMB-S4 White Paper Josquin Errard,Snowmass 2021 CMB-S4
White Paper Thomas Essinger-Hileman,Snowmass 2021 CMB-S4 White Paper Giulio
Fabbian,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Valentina Fanfani,Snowmass 2021 CMB-S4 White Paper Alessandro Fasano,Snowmass
2021 CMB-S4 White Paper Chang Feng,Snowmass 2021 CMB-S4 White Paper Simone
Ferraro,Snowmass 2021 CMB-S4 White Paper Jeffrey P. Filippini,Snowmass 2021
CMB-S4 White Paper Raphael Flauger,Snowmass 2021 CMB-S4 White Paper Brenna
Flaugher,Snowmass 2021 CMB-S4 White Paper Aurelien A. Fraisse,Snowmass 2021
CMB-S4 White Paper Josef Frisch,Snowmass 2021 CMB-S4 White Paper Andrei
Frolov,Snowmass 2021 CMB-S4 White Paper Nicholas Galitzki,Snowmass 2021 CMB-S4
White Paper Patricio A. Gallardo,Snowmass 2021 CMB-S4 White Paper Silvia
Galli,Snowmass 2021 CMB-S4 White Paper Ken Ganga,Snowmass 2021 CMB-S4 White
Paper Martina Gerbino,Snowmass 2021 CMB-S4 White Paper Christos
Giannakopoulos,Snowmass 2021 CMB-S4 White Paper Murdock Gilchriese,Snowmass
2021 CMB-S4 White Paper Vera Gluscevic,Snowmass 2021 CMB-S4 White Paper Neil
Goeckner-Wald,Snowmass 2021 CMB-S4 White Paper David Goldfinger,Snowmass 2021
CMB-S4 White Paper Daniel Green,Snowmass 2021 CMB-S4 White Paper Paul
Grimes,Snowmass 2021 CMB-S4 White Paper Daniel Grin,Snowmass 2021 CMB-S4 White
Paper Evan Grohs,Snowmass 2021 CMB-S4 White Paper Riccardo Gualtieri,Snowmass
2021 CMB-S4 White Paper Vic Guarino,Snowmass 2021 CMB-S4 White Paper Jon E.
Gudmundsson,Snowmass 2021 CMB-S4 White Paper Ian Gullett,Snowmass 2021 CMB-S4
White Paper Sam Guns,Snowmass 2021 CMB-S4 White Paper Salman Habib,Snowmass
2021 CMB-S4 White Paper Gunther Haller,Snowmass 2021 CMB-S4 White Paper Mark
Halpern,Snowmass 2021 CMB-S4 White Paper Nils W. Halverson,Snowmass 2021
CMB-S4 White Paper Shaul Hanany,Snowmass 2021 CMB-S4 White Paper Emma
Hand,Snowmass 2021 CMB-S4 White Paper Kathleen Harrington,Snowmass 2021 CMB-S4
White Paper Masaya Hasegawa,Snowmass 2021 CMB-S4 White Paper Matthew
Hasselfield,Snowmass 2021 CMB-S4 White Paper Masashi Hazumi,Snowmass 2021
CMB-S4 White Paper Katrin Heitmann,Snowmass 2021 CMB-S4 White Paper Shawn
Henderson,Snowmass 2021 CMB-S4 White Paper Brandon Hensley,Snowmass 2021
CMB-S4 White Paper Ryan Herbst,Snowmass 2021 CMB-S4 White Paper Carlos
Hervias-Caimapo,Snowmass 2021 CMB-S4 White Paper J. Colin Hill,Snowmass 2021
CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Richard Hills,Snowmass
2021 CMB-S4 White Paper Eric Hivon,Snowmass 2021 CMB-S4 White Paper,Snowmass
2021 CMB-S4 White Paper Renée Hložek,Snowmass 2021 CMB-S4 White Paper,Snowmass
2021 CMB-S4 White Paper Anna Ho,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021
CMB-S4 White Paper Gil Holder,Snowmass 2021 CMB-S4 White Paper Matt
Hollister,Snowmass 2021 CMB-S4 White Paper William Holzapfel,Snowmass 2021
CMB-S4 White Paper John Hood,Snowmass 2021 CMB-S4 White Paper Selim
Hotinli,Snowmass 2021 CMB-S4 White Paper Alec Hryciuk,Snowmass 2021 CMB-S4
White Paper Johannes Hubmayr,Snowmass 2021 CMB-S4 White Paper Kevin M.
Huffenberger,Snowmass 2021 CMB-S4 White Paper Howard Hui,Snowmass 2021 CMB-S4
White Paper Roberto Ibáñez,Snowmass 2021 CMB-S4 White Paper Ayodeji
Ibitoye,Snowmass 2021 CMB-S4 White Paper Margaret Ikape,Snowmass 2021 CMB-S4
White Paper Kent Irwin,Snowmass 2021 CMB-S4 White Paper Cooper
Jacobus,Snowmass 2021 CMB-S4 White Paper Oliver Jeong,Snowmass 2021 CMB-S4
White Paper Bradley R. Johnson,Snowmass 2021 CMB-S4 White Paper Doug
Johnstone,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
William C. Jones,Snowmass 2021 CMB-S4 White Paper John Joseph,Snowmass 2021
CMB-S4 White Paper Baptiste Jost,Snowmass 2021 CMB-S4 White Paper,Snowmass
2021 CMB-S4 White Paper Jae Hwan Kang,Snowmass 2021 CMB-S4 White Paper Ari
Kaplan,Snowmass 2021 CMB-S4 White Paper Kirit S. Karkare,Snowmass 2021 CMB-S4
White Paper,Snowmass 2021 CMB-S4 White Paper Nobuhiko Katayama,Snowmass 2021
CMB-S4 White Paper Reijo Keskitalo,Snowmass 2021 CMB-S4 White Paper,Snowmass
2021 CMB-S4 White Paper Cesiley King,Snowmass 2021 CMB-S4 White Paper Theodore
Kisner,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Matthias Klein,Snowmass 2021 CMB-S4 White Paper Lloyd Knox,Snowmass 2021
CMB-S4 White Paper Brian J. Koopman,Snowmass 2021 CMB-S4 White Paper Arthur
Kosowsky,Snowmass 2021 CMB-S4 White Paper John Kovac,Snowmass 2021 CMB-S4
White Paper,Snowmass 2021 CMB-S4 White Paper Ely D. Kovetz,Snowmass 2021
CMB-S4 White Paper Alex Krolewski,Snowmass 2021 CMB-S4 White Paper Donna
Kubik,Snowmass 2021 CMB-S4 White Paper Steve Kuhlmann,Snowmass 2021 CMB-S4
White Paper Chao-Lin Kuo,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4
White Paper Akito Kusaka,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4
White Paper Anne Lähteenmäki,Snowmass 2021 CMB-S4 White Paper Kenny
Lau,Snowmass 2021 CMB-S4 White Paper Charles R. Lawrence,Snowmass 2021 CMB-S4
White Paper Adrian T. Lee,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021
CMB-S4 White Paper Louis Legrand,Snowmass 2021 CMB-S4 White Paper Matthaeus
Leitner,Snowmass 2021 CMB-S4 White Paper Clément Leloup,Snowmass 2021 CMB-S4
White Paper,Snowmass 2021 CMB-S4 White Paper Antony Lewis,Snowmass 2021 CMB-S4
White Paper Dale Li,Snowmass 2021 CMB-S4 White Paper Eric Linder,Snowmass 2021
CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Ioannis Liodakis,Snowmass
2021 CMB-S4 White Paper Jia Liu,Snowmass 2021 CMB-S4 White Paper Kevin
Long,Snowmass 2021 CMB-S4 White Paper Thibaut Louis,Snowmass 2021 CMB-S4 White
Paper Marilena Loverde,Snowmass 2021 CMB-S4 White Paper Lindsay Lowry,Snowmass
2021 CMB-S4 White Paper Chunyu Lu,Snowmass 2021 CMB-S4 White Paper Phil
Lubin,Snowmass 2021 CMB-S4 White Paper Yin-Zhe Ma,Snowmass 2021 CMB-S4 White
Paper Thomas Maccarone,Snowmass 2021 CMB-S4 White Paper Mathew S.
Madhavacheril,Snowmass 2021 CMB-S4 White Paper Felipe Maldonado,Snowmass 2021
CMB-S4 White Paper Adam Mantz,Snowmass 2021 CMB-S4 White Paper Gabriela
Marques,Snowmass 2021 CMB-S4 White Paper Frederick Matsuda,Snowmass 2021
CMB-S4 White Paper Philip Mauskopf,Snowmass 2021 CMB-S4 White Paper Jared
May,Snowmass 2021 CMB-S4 White Paper Heather McCarrick,Snowmass 2021 CMB-S4
White Paper Ken McCracken,Snowmass 2021 CMB-S4 White Paper Jeffrey
McMahon,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper P.
Daniel Meerburg,Snowmass 2021 CMB-S4 White Paper Jean-Baptiste Melin,Snowmass
2021 CMB-S4 White Paper Felipe Menanteau,Snowmass 2021 CMB-S4 White Paper Joel
Meyers,Snowmass 2021 CMB-S4 White Paper Marius Millea,Snowmass 2021 CMB-S4
White Paper Vivian Miranda,Snowmass 2021 CMB-S4 White Paper Don
Mitchell,Snowmass 2021 CMB-S4 White Paper Joseph Mohr,Snowmass 2021 CMB-S4
White Paper Lorenzo Moncelsi,Snowmass 2021 CMB-S4 White Paper Maria Elena
Monzani,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Magdy Moshed,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Tony Mroczkowski,Snowmass 2021 CMB-S4 White Paper Suvodip Mukherjee,Snowmass
2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Moritz
Münchmeyer,Snowmass 2021 CMB-S4 White Paper Daisuke Nagai,Snowmass 2021 CMB-S4
White Paper Chandan Nagarajappa,Snowmass 2021 CMB-S4 White Paper Johanna
Nagy,Snowmass 2021 CMB-S4 White Paper Toshiya Namikawa,Snowmass 2021 CMB-S4
White Paper Federico Nati,Snowmass 2021 CMB-S4 White Paper Tyler
Natoli,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Simran Nerval,Snowmass 2021 CMB-S4 White Paper Laura Newburgh,Snowmass 2021
CMB-S4 White Paper Hogan Nguyen,Snowmass 2021 CMB-S4 White Paper Erik
Nichols,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Andrina Nicola,Snowmass 2021 CMB-S4 White Paper Michael D. Niemack,Snowmass
2021 CMB-S4 White Paper Brian Nord,Snowmass 2021 CMB-S4 White Paper Tim
Norton,Snowmass 2021 CMB-S4 White Paper Valentine Novosad,Snowmass 2021 CMB-S4
White Paper Roger O’Brient,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021
CMB-S4 White Paper Yuuki Omori,Snowmass 2021 CMB-S4 White Paper Giorgio
Orlando ,Snowmass 2021 CMB-S4 White Paper Benjamin Osherson,Snowmass 2021
CMB-S4 White Paper Rachel Osten,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021
CMB-S4 White Paper Stephen Padin,Snowmass 2021 CMB-S4 White Paper Scott
Paine,Snowmass 2021 CMB-S4 White Paper Bruce Partridge,Snowmass 2021 CMB-S4
White Paper Sanjaykumar Patil,Snowmass 2021 CMB-S4 White Paper Don
Petravick,Snowmass 2021 CMB-S4 White Paper Matthew Petroff,Snowmass 2021
CMB-S4 White Paper Elena Pierpaoli,Snowmass 2021 CMB-S4 White Paper Mauricio
Pilleux,Snowmass 2021 CMB-S4 White Paper Levon Pogosian,Snowmass 2021 CMB-S4
White Paper Karthik Prabhu,Snowmass 2021 CMB-S4 White Paper Clement
Pryke,Snowmass 2021 CMB-S4 White Paper Giuseppe Puglisi,Snowmass 2021 CMB-S4
White Paper,Snowmass 2021 CMB-S4 White Paper Benjamin Racine,Snowmass 2021
CMB-S4 White Paper Srinivasan Raghunathan,Snowmass 2021 CMB-S4 White Paper
Alexandra Rahlin,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White
Paper Marco Raveri,Snowmass 2021 CMB-S4 White Paper Ben Reese,Snowmass 2021
CMB-S4 White Paper Christian L. Reichardt,Snowmass 2021 CMB-S4 White Paper
Mathieu Remazeilles,Snowmass 2021 CMB-S4 White Paper Arianna Rizzieri,Snowmass
2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Graca Rocha,Snowmass
2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Natalie A.
Roe,Snowmass 2021 CMB-S4 White Paper Kaja Rotermund,Snowmass 2021 CMB-S4 White
Paper Anirban Roy,Snowmass 2021 CMB-S4 White Paper John E. Ruhl,Snowmass 2021
CMB-S4 White Paper Joe Saba,Snowmass 2021 CMB-S4 White Paper Noah
Sailer,Snowmass 2021 CMB-S4 White Paper Maria Salatino,Snowmass 2021 CMB-S4
White Paper Benjamin Saliwanchik,Snowmass 2021 CMB-S4 White Paper Leonid
Sapozhnikov,Snowmass 2021 CMB-S4 White Paper Mayuri Sathyanarayana
Rao,Snowmass 2021 CMB-S4 White Paper Lauren Saunders,Snowmass 2021 CMB-S4
White Paper Emmanuel Schaan,Snowmass 2021 CMB-S4 White Paper Alessandro
Schillaci,Snowmass 2021 CMB-S4 White Paper Benjamin Schmitt,Snowmass 2021
CMB-S4 White Paper Douglas Scott,Snowmass 2021 CMB-S4 White Paper Neelima
Sehgal,Snowmass 2021 CMB-S4 White Paper Sarah Shandera,Snowmass 2021 CMB-S4
White Paper Blake D. Sherwin,Snowmass 2021 CMB-S4 White Paper Erik
Shirokoff,Snowmass 2021 CMB-S4 White Paper Corwin Shiu,Snowmass 2021 CMB-S4
White Paper Sara M. Simon,Snowmass 2021 CMB-S4 White Paper Baibhav
Singari,Snowmass 2021 CMB-S4 White Paper Anže Slosar,Snowmass 2021 CMB-S4
White Paper David Spergel,Snowmass 2021 CMB-S4 White Paper Tyler St.
Germaine,Snowmass 2021 CMB-S4 White Paper Suzanne T. Staggs,Snowmass 2021
CMB-S4 White Paper Antony A. Stark,Snowmass 2021 CMB-S4 White Paper Glenn D.
Starkman,Snowmass 2021 CMB-S4 White Paper Bryan Steinbach,Snowmass 2021 CMB-S4
White Paper Radek Stompor,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021
CMB-S4 White Paper Chris Stoughton,Snowmass 2021 CMB-S4 White Paper Aritoki
Suzuki,Snowmass 2021 CMB-S4 White Paper Osamu Tajima,Snowmass 2021 CMB-S4
White Paper Chris Tandoi,Snowmass 2021 CMB-S4 White Paper Grant P.
Teply,Snowmass 2021 CMB-S4 White Paper Gregg Thayer,Snowmass 2021 CMB-S4 White
Paper,Snowmass 2021 CMB-S4 White Paper Keith Thompson,Snowmass 2021 CMB-S4
White Paper Ben Thorne,Snowmass 2021 CMB-S4 White Paper Peter Timbie,Snowmass
2021 CMB-S4 White Paper Maurizio Tomasi,Snowmass 2021 CMB-S4 White
Paper,Snowmass 2021 CMB-S4 White Paper Cynthia Trendafilova,Snowmass 2021
CMB-S4 White Paper Matthieu Tristram,Snowmass 2021 CMB-S4 White Paper Carole
Tucker,Snowmass 2021 CMB-S4 White Paper Gregory Tucker,Snowmass 2021 CMB-S4
White Paper Caterina Umiltà,Snowmass 2021 CMB-S4 White Paper Alexander van
Engelen,Snowmass 2021 CMB-S4 White Paper Joshiwa van Marrewijk,Snowmass 2021
CMB-S4 White Paper Eve M. Vavagiakis,Snowmass 2021 CMB-S4 White Paper Clara
Vergès,Snowmass 2021 CMB-S4 White Paper Joaquin D. Vieira,Snowmass 2021 CMB-S4
White Paper Abigail G. Vieregg,Snowmass 2021 CMB-S4 White Paper Kasey
Wagoner,Snowmass 2021 CMB-S4 White Paper Benjamin Wallisch,Snowmass 2021
CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Gensheng Wang,Snowmass
2021 CMB-S4 White Paper Guo-Jian Wang,Snowmass 2021 CMB-S4 White Paper Scott
Watson,Snowmass 2021 CMB-S4 White Paper Duncan Watts,Snowmass 2021 CMB-S4
White Paper Chris Weaver,Snowmass 2021 CMB-S4 White Paper Lukas Wenzl,Snowmass
2021 CMB-S4 White Paper Ben Westbrook,Snowmass 2021 CMB-S4 White Paper Martin
White,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper Nathan
Whitehorn,Snowmass 2021 CMB-S4 White Paper Andrew Wiedlea,Snowmass 2021 CMB-S4
White Paper Paul Williams,Snowmass 2021 CMB-S4 White Paper Robert
Wilson,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper
Harrison Winch,Snowmass 2021 CMB-S4 White Paper Edward J. Wollack,Snowmass
2021 CMB-S4 White Paper W. L. Kimmy Wu,Snowmass 2021 CMB-S4 White
Paper,Snowmass 2021 CMB-S4 White Paper Zhilei Xu,Snowmass 2021 CMB-S4 White
Paper Volodymyr G. Yefremenko,Snowmass 2021 CMB-S4 White Paper Cyndia
Yu,Snowmass 2021 CMB-S4 White Paper,Snowmass 2021 CMB-S4 White Paper David
Zegeye,Snowmass 2021 CMB-S4 White Paper Jeff Zivick,Snowmass 2021 CMB-S4 White
Paper Andrea ZoncaSnowmass 2021 CMB-S4 White Paper

Snowmass 2021 CMB-S4 White PaperUniversity of California, Irvine, CA 92697,
USA

Snowmass 2021 CMB-S4 White PaperSchool of Physics, The University of
Melbourne, Parkville, VIC 3010, Australia

Snowmass 2021 CMB-S4 White PaperJohns Hopkins University, Baltimore, MD 21218,
USA

Snowmass 2021 CMB-S4 White PaperIllinois Center for Advanced Studies of the
Universe & Department of Physics, University of Illinois at Urbana-Champaign,
Urbana, IL 61801, USA

Snowmass 2021 CMB-S4 White PaperSLAC National Accelerator Laboratory, Menlo
Park, CA 94025, USA

Snowmass 2021 CMB-S4 White PaperClemson University, Clemson, SC 29634, USA

Snowmass 2021 CMB-S4 White PaperStanford University, Stanford, CA 94305, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Oxford, Oxford OX1 3RH, UK

Snowmass 2021 CMB-S4 White PaperUniversity of California, Berkeley, CA 94720,
USA

Snowmass 2021 CMB-S4 White PaperLawrence Berkeley National Laboratory,
Berkeley, CA 94720, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics & Astronomy, Rice
University, Houston, Texas 77005, USA

Snowmass 2021 CMB-S4 White PaperUniversity of British Columbia, Vancouver, BC
V6T 1Z1, Canada

Snowmass 2021 CMB-S4 White PaperFermi National Accelerator Laboratory,
Batavia, IL 60510, USA

Snowmass 2021 CMB-S4 White PaperUniversity of California, San Diego, La Jolla,
CA 92093, USA

Snowmass 2021 CMB-S4 White PaperHarvard-Smithsonian Center for Astrophysics,
Cambridge, MA 02138, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, Yale University, New
Haven, CT 06520, USA

Snowmass 2021 CMB-S4 White PaperSISSA - International School for Advanced
Studies, Via Bonomea 265, 34136 Trieste, Italy

Snowmass 2021 CMB-S4 White PaperIFPU - Institute for Fundamental Physics of
the Universe, Via Beirut 2, 34014 Trieste, Italy

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, Harvard University,
Cambridge, MA 02138, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics and Astronomy,
University of New Mexico, Albuquerque, NM 87131, USA

Snowmass 2021 CMB-S4 White PaperArgonne National Laboratory, Lemont, IL 60439,
USA

Snowmass 2021 CMB-S4 White PaperSchool of Physics and Astronomy, Cardiff
University, The Parade, Cardiff, CF24 3AA, UK

Snowmass 2021 CMB-S4 White PaperLaboratoire Astroparticule et Cosmologie
(APC), CNRS/IN2P3, Université Paris Diderot, 75205 Paris Cedex 13, France

Snowmass 2021 CMB-S4 White PaperCalifornia Institute of Technology, Pasadena,
CA 91125, USA

Snowmass 2021 CMB-S4 White PaperCornell University, Ithaca, NY 14853, USA

Snowmass 2021 CMB-S4 White PaperNational Institute of Standards and
Technology, Boulder, CO 80305, USA

Snowmass 2021 CMB-S4 White PaperDépartement de Physique Théorique et CAP,
Université de Genève, CH-1211 Genève 4, Switzerland

Snowmass 2021 CMB-S4 White PaperInstitut d’Astrophysique de Paris (IAP), CNRS
& Sorbonne University, Paris, France

Snowmass 2021 CMB-S4 White PaperKavli Institute for Cosmological Physics,
University of Chicago, Chicago, IL 60637, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Chicago, Chicago, IL 60637, USA

Snowmass 2021 CMB-S4 White PaperAix Marseille Univ, CNRS, CNES, LAM,
Marseille, France

Snowmass 2021 CMB-S4 White PaperCentre Pierre Binétruy, Berkeley, CA 94704,
USA

Snowmass 2021 CMB-S4 White PaperUniversity of Cincinnati, Cincinnati, OH
45221, USA

Snowmass 2021 CMB-S4 White PaperAssociated Universities Incorporated,
Washington, DC 20036, USA

Snowmass 2021 CMB-S4 White PaperJet Propulsion Laboratory, California
Institute of Technology, Pasadena, CA 91011, USA

Snowmass 2021 CMB-S4 White PaperLudwig-Maximilians-Universität, 81679 Munich,
Germany

Snowmass 2021 CMB-S4 White PaperUniversity of Texas at Austin, Austin, TX
78712, USA

Snowmass 2021 CMB-S4 White PaperCanadian Institute for Theoretical
Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada

Snowmass 2021 CMB-S4 White PaperDipartimento di Fisica e Scienze della Terra,
Università di Ferrara, Polo Scientifico e Tecnologico - Edificio C Via Saragat
1, 44122 Ferrara, Italy

Snowmass 2021 CMB-S4 White PaperINFN - National Institute for Nuclear Physics,
Via Valerio 2, I-34127 Trieste, Italy

Snowmass 2021 CMB-S4 White PaperJodrell Bank Center for Astrophysics, School
of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Snowmass 2021 CMB-S4 White PaperArizona State University, Tempe, AZ 85287, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics & Astronomy, Dartmouth
College, Hanover, NH 03755, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Applied Mathematics and
Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK

Snowmass 2021 CMB-S4 White PaperDavid A. Dunlap Department of Astronomy and
Astrophysics, University of Toronto, ON, M5S 3H4, Canada

Snowmass 2021 CMB-S4 White PaperEric Chauvin Consulting Engineer

Snowmass 2021 CMB-S4 White PaperAstrophysics and Cosmology Research Unit,
School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000,
South Africa

Snowmass 2021 CMB-S4 White PaperUniversity of Tokyo, 277-8583 Kashiwa, Japan

Snowmass 2021 CMB-S4 White PaperKavli Institute for Particle Astrophysics and
Cosmology, Stanford, CA 94305, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, University of Milano -
Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy

Snowmass 2021 CMB-S4 White PaperCenter for Computational Astrophysics,
Flatiron Institute, New York, NY 10010, USA

Snowmass 2021 CMB-S4 White PaperHigh Energy Accelerator Research Organization
(KEK), Tsukuba, Ibaraki 305-0801, Japan

Snowmass 2021 CMB-S4 White PaperDepartment of Physics and Astronomy,
University of Pennsylvania, Philadelphia, PA 19104, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Sheffield, Sheffield S3 7RH, UK

Snowmass 2021 CMB-S4 White PaperPrinceton University, Princeton, NJ 08544, USA

Snowmass 2021 CMB-S4 White PaperPontificia Universidad Católica de Chile,
Santiago, Región Metropolitana, Chile

Snowmass 2021 CMB-S4 White PaperNorthwestern University, Evanston, IL 60208,
USA

Snowmass 2021 CMB-S4 White PaperDepartment of Astronomy/Steward Observatory,
University of Arizona, Tucson, AZ 85721, USA

Snowmass 2021 CMB-S4 White PaperGoddard Space Flight Center, Greenbelt, MD
20771, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, Simon Fraser
University, Burnaby, BC V5A 1S6, Canada

Snowmass 2021 CMB-S4 White PaperUniversity of Southern California, CA 90089,
USA

Snowmass 2021 CMB-S4 White PaperHaverford College, Haverford, PA 19041, USA

Snowmass 2021 CMB-S4 White PaperPhysics Department, North Carolina State
University, Raleigh, NC 27607, USA

Snowmass 2021 CMB-S4 White PaperOskar Klein Centre for Cosmoparticle Physics,
Stockholm University, AlbaNova, Stockholm SE-106 91, Sweden

Snowmass 2021 CMB-S4 White PaperCase Western Reserve University, Cleveland, OH
44106, USA

Snowmass 2021 CMB-S4 White PaperCenter for Astrophysics and Space Astronomy,
Department of Astrophysical and Planetary Science, University of Colorado,
Boulder, CO 80309, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Minnesota, Minneapolis, MN
55455, USA

Snowmass 2021 CMB-S4 White PaperFlorida State University, Tallahassee, FL
32306, USA

Snowmass 2021 CMB-S4 White PaperColumbia University, New York, NY 10027, USA

Snowmass 2021 CMB-S4 White PaperCavendish Laboratory, University of Cambridge,
Cambridge, CB3 0HE, UK

Snowmass 2021 CMB-S4 White PaperSorbonne Université, Université Paris Diderot,
CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE,
F-75252 Paris, France

Snowmass 2021 CMB-S4 White PaperDunlap Institute for Astronomy and
Astrophysics, University of Toronto, ON M5S 3H4, Canada

Snowmass 2021 CMB-S4 White PaperMiller Institute for Basic Research in
Science, 468 Donner Lab, Berkeley, CA 94720, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Virginia, Charlottesville, VA
22903, USA

Snowmass 2021 CMB-S4 White PaperHerzberg Astronomuy and Astrophyiscs - NRC,
Victoria, Canada

Snowmass 2021 CMB-S4 White PaperUniversity of Victoria, Victoria, BC V8P 5C2,
Canada

Snowmass 2021 CMB-S4 White PaperUniversity of California, Santa Barbara, CA
93106, USA

Snowmass 2021 CMB-S4 White PaperKavli Institute for the Physics and
Mathematics of the Universe, University of Tokyo, Kashiwa, Japan

Snowmass 2021 CMB-S4 White PaperWashington University in St Louis, St. Louis,
MO 63130, USA

Snowmass 2021 CMB-S4 White PaperUniversity of California, Davis, CA 95616, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Pittsburgh and PITT PACC,
Pittsburgh, PA 15260, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, Ben-Gurion University,
Be’er Sheva 84105, Israel

Snowmass 2021 CMB-S4 White PaperAalto University, FIN-00076, Aalto, Finland

Snowmass 2021 CMB-S4 White PaperAstronomy Centre, School of Mathematical and
Physical Sciences, University of Sussex, Brighton BN1 9QH, UK

Snowmass 2021 CMB-S4 White PaperUniversity of Turku, Turku, Finland

Snowmass 2021 CMB-S4 White PaperUniversité Paris-Saclay, CNRS/IN2P3, IJCLab,
91405 Orsay, France

Snowmass 2021 CMB-S4 White PaperUniversity of Washington, Seattle, WA 98195,
USA

Snowmass 2021 CMB-S4 White PaperTexas Tech University, Lubbock, TX 79409, USA

Snowmass 2021 CMB-S4 White PaperPerimeter Institute, Waterloo, ON N2L 2Y5,
Canada

Snowmass 2021 CMB-S4 White PaperISAS JAXA, Sagamihara, Kanagawa 252-5210,
Japan

Snowmass 2021 CMB-S4 White PaperVan Swinderen Institute for Particle Physics
and Gravity, University of Groningen, 9747 AG Groningen, The Netherlands

Snowmass 2021 CMB-S4 White PaperDépartement d’Astrophysique, CEA Saclay
DSM/Irfu, 91191 Gif-sur-Yvette, France

Snowmass 2021 CMB-S4 White PaperSouthern Methodist University, Dallas, TX
75275, USA

Snowmass 2021 CMB-S4 White PaperStony Brook University, Stony Brook, NY 11794,
USA

Snowmass 2021 CMB-S4 White PaperEuropean Southern Observatory, Karl-
Schwarzschild-Str. 2, DE-85748 Garching b. Munchen, Germany

Snowmass 2021 CMB-S4 White PaperAnton Pannekoek Institute for Astronomy,
University of Amsterdam, 1098 XH Amsterdam, The Netherlands

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, University of
Wisconsin-Madison, Madison, WI 53706, USA

Snowmass 2021 CMB-S4 White PaperRemote Science Services, LLC

Snowmass 2021 CMB-S4 White PaperSpace Telescope Science Institute, Baltimore,
MD 21218, USA

Snowmass 2021 CMB-S4 White PaperEONS SpA

Snowmass 2021 CMB-S4 White PaperUniversity of Rome Tor Vergata, 00133 Roma RM,
Italy

Snowmass 2021 CMB-S4 White PaperAix Marseille Univ, CNRS/IN2P3, CPPM,
Marseille, France

Snowmass 2021 CMB-S4 White PaperInstituto de Física de Cantabria, Cantabria,
Spain

Snowmass 2021 CMB-S4 White PaperBrookhaven National Laboratory, Upton, NY
11973, USA

Snowmass 2021 CMB-S4 White PaperRaman Research Institute, Bengaluru, Karnataka
560080, India

Snowmass 2021 CMB-S4 White PaperThe Pennsylvania State University, University
Park, PA 16802, USA

Snowmass 2021 CMB-S4 White PaperDepartment of Physics, Kyoto University, Kyoto
606-8502, Japan

Snowmass 2021 CMB-S4 White PaperDipartimento di Fisica “Aldo Pontremoli”,
Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy

Snowmass 2021 CMB-S4 White PaperBrown University, Providence, RI 02912, USA

Snowmass 2021 CMB-S4 White PaperInstitute for Advanced Study, Princeton, NJ
08540, USA

Snowmass 2021 CMB-S4 White PaperSyracuse University, Syracuse, NY 13244, USA

Snowmass 2021 CMB-S4 White PaperUniversity of Oslo, Oslo, WPQC XM, Norway

Snowmass 2021 CMB-S4 White PaperMichigan State University, East Lansing, MI
48824, USA

Snowmass 2021 CMB-S4 White PaperSmithsonian Astrophysical Observatory,
Cambridge, MA 02138, USA

Snowmass 2021 CMB-S4 White PaperBell Laboratories, Murray Hill, NJ 07974, USA

Snowmass 2021 CMB-S4 White PaperMassachusetts Institute of Technology,
Cambridge, MA 02139, USA

## CMB-S4 Overview and Context

The cosmic microwave background ‘Stage 4’ CMB-S4 project is the next
generation ground-based cosmic microwave background experiment, designed to
cross critical thresholds in our understanding of the origin and evolution of
the Universe, from the highest energies at the dawn of time through the growth
of structure to the present day. The CMB-S4 science case is spectacular: the
search for primordial gravitational waves as predicted from inflation and the
imprint of relic particles including neutrinos, unique insights into dark
energy and tests of gravity on large scales, elucidating the role of baryonic
feedback on galaxy formation and evolution, opening up a window on the
transient Universe at millimeter wavelengths, and even the exploration of the
outer Solar System. The CMB-S4 sensitivity to primordial gravitational waves
will probe physics at the highest energy scales and cross a major
theoretically motivated threshold in constraints on inflation. The CMB-S4
search for new light relic particles will shed light on the early Universe
10,000 times farther back than current experiments can reach. Finally, the
CMB-S4 Legacy Survey, covering 60% of the sky with unprecedented sensitivity
and angular resolution from centimeter- to millimeter-wave observing bands,
will have a profound and lasting impact on Astronomy and Astrophysics and
provide a powerful complement to surveys at other wavelengths, such as the
Vera Rubin Observatory (VRO) Legacy Survey of Space and Time and those
conducted by the Nancy Grace Roman Space Telescope, Euclid, and others yet to
be imagined. We emphasize that these critical thresholds cannot be reached
without the level of community and agency investment and commitment required
by CMB-S4. In particular, the CMB-S4 science goals are out of the reach of any
projected precursor experiment by a significant margin.

The formal CMB-S4 Collaboration was established in 2018 with the ratification
of the bylaws and election of the various officers including the collaboration
Executive Team and Governing Board. As of March 2022 the Collaboration has 320
members, 76 of whom hold positions within the organizational structure. These
members represent 114 institutions in 19 countries on 6 continents, including
27 US states.

CMB-S4 is a joint NSF and DOE project, with the construction phase expected to
be funded as an NSF MREFC project and a DOE HEP MIE project. DOE Critical
Decision 0 (CD-0) was approved in July 2019. An integrated project office has
been constituted and tasked with advancing the CMB-S4 project in the NSF MREFC
Preliminary Design Phase and toward DOE Critical Decision CD-1. Support for
the Integrated Project Office is being provided in part by DOE through their
lead lab LBNL, and by NSF through an MSRI-R1 award to the University of
Chicago, as the lead NSF institution.

CMB-S4 was recommended by the 2014 Particle Physics Project Prioritization
Panel (P5) report Building for Discovery: Strategic Plan for U.S. Particle
Physics in the Global Context and by the 2015 National Academies report A
Strategic Vision for NSF Investments in Antarctic and Southern Ocean Research.
The community further developed the science case in the 2016 CMB-S4 Science
Book [1] and surveyed the status of the technology in the 2017 CMB-S4
Technology Book [2]. This work formed the foundation for the joint NSF-DOE
Concept Definition Task Force (CDT), a subpanel of the Astronomy and
Astrophysics Advisory Committee (AAAC), a FACA committee advising DOE, NASA,
and NSF. The CDT report was enthusiastically accepted by the AAAC in October
2017.

Building on the CDT report, the CMB-S4 Collaboration and the pre-Project
Development Group, composed of experienced project leaders drawn primarily
from the national laboratories, produced the comprehensive document, The
CMB-S4 Science Case, Reference Design, and Project Plan [3], which we refer to
here as the Decadal Survey Report (DSR). The DSR has been updated to reflect
the advanced design in the The CMB-S4 Preliminary Baseline Design Report,
hereafter the PBDR, which is scheduled to be posted soon. The material
presented in this white paper has been extracted from these major overview
reports, as well as many detailed project documents. These reports and
numerous other reports, collaboration bylaws, workshop and working group wiki
pages, email lists, and much more may be found at the website
https://CMB-S4.org. We also refer the reader to the dedicated Snowmass White
Paper on the CMB experimental program [4].

In November 2021, CMB-S4 was strongly recommended (with no caveats) by the
2020 Decadal Survey report Pathways to Discovery in Astronomy and Astrophysics
for the 2020s. The Technical, Risk, and Cost Evaluation (TRACE) that the
survey had performed by the Aerospace Corporation provided a cost estimate
similar to ours and evaluated the project risk as medium-low. The Decadal
Survey recommendation reads “The National Science Foundation and the
Department of Energy should jointly pursue the design and implementation of
the next generation ground-based cosmic microwave background experiment
(CMB-S4).”

To achieve its transformational science goals, CMB-S4 requires an enormous
increase in sensitivity over all current CMB experiments combined, and roughly
an order-of-magnitude increase over any projected precursor experiment. A
significant and unique feature of CMB-S4 from the outset has been the use of
multiple sites, specifically combining the two best currently developed sites
on Earth for millimeter-wave observing: the high Atacama Plateau in Chile and
the geographical South Pole. The design of CMB-S4 exploits key features of
these two sites, namely the ability to drill deep on a single small patch of
the sky through an extraordinarily stable atmosphere from the South Pole, and
the ability to survey up to 80% of the sky from the exceptionally high and dry
Atacama site.

Current experimental efforts at these two sites are already being consolidated
into two major precursor observatories to CMB-S4, the Simons Observatory (SO)
and the South Pole Observatory (SPO), members of whose teams also make up a
large part of the CMB-S4 collaboration. The timing of both of these
observatories is well-aligned with CMB-S4, enabling them to act as valuable
pathfinders for CMB-S4 by providing technical and scientific data that have
informed and will continue to inform our design and operations. To this end,
both have also provided Letters of Intent to share their technical and cost
data with CMB-S4. Nonetheless, while both will make significant advances in
key CMB science goals, they will still fall well short of the thresholds
targeted by CMB-S4. For example, to match the sensitivity to primordial
gravitational waves provided by the ultra-deep CMB-S4 survey, SPO would have
to integrate for nearly 50 years; it would take SO a similar amount of time to
match the sensitivity to light relics provided by the CMB-S4 deep and wide
survey.

From space, the Japanese LiteBIRD CMB satellite mission was selected by JAXA
for launch late in the decade for a 3-year mission, concurrent with CMB-S4
operations. With its lower resolution but wider frequency coverage, LiteBIRD’s
science goals are distinct from but highly complementary to CMB-S4’s, and we
are already discussing the parameters of a possible Memorandum of
Understanding to enable both experiments to enhance their reach using elements
of the other’s data.

In short, CMB-S4 will enable transformational science that cannot be achieved
otherwise, the CMB-S4 concept has clear community and agency support, and the
CMB-S4 collaboration and project are moving forward.

## Key Science Goals and Objectives

The rich and diverse set of CMB-S4 scientific goals are organized into four
themes:

  1. 1.

primordial gravitational waves and inflation;

  2. 2.

the dark Universe;

  3. 3.

mapping matter in the cosmos;

  4. 4.

the time-variable millimeter-wave sky.

The first two science themes relate to fundamental physics, and are of
particular interest to the particle physics community. The other two themes
relate to the broader scientific opportunities made possible by a millimeter-
wave survey of unprecedented depth and breadth. Here we briefly review the key
high-level goals and refer the reader to the more detailed science case in the
DSR, PBDR, and in the Decadal Survey and Snowmas2021 science white papers
referenced.

### Primordial gravitational waves and inflation.

We have a historic opportunity to open up a window to the primordial Universe
[5]. If the predictions of some of the leading models for the origin of the
hot big bang are borne out, CMB-S4 will detect the signature of primordial
gravitational waves in the polarization pattern of the CMB [6]. This detection
would provide the first evidence for the quantization of gravity, reveal new
physics at the energy scale of grand unified theories, and yield insight into
the symmetries of nature.

The current leading scenario for the origin of structure in our Universe is
cosmic inflation, a period of accelerated expansion prior to the hot big bang,
as discussed in the dedicated Snowmass 2021 White Papers [7, 8], also see [9,
10]. During this epoch, quantum fluctuations were imprinted on all spatial
scales in the Universe. These fluctuations seeded the density perturbations
that developed into all the structure in the Universe today. While there are
still viable alternative models for the early history of the Universe, the
simplest models of inflation are exceptionally successful in describing the
data.

Tantalizingly, the observed scale dependence of the amplitude of density
perturbations has quantitative implications for the amplitude of primordial
gravitational waves, commonly parameterized by $r$, the ratio of fluctuation
power in gravitational waves to that in density perturbations. All inflation
models that naturally explain the observed deviation from scale invariance and
that also have a characteristic scale equal to or larger than the Planck scale
predict $r\gtrsim 0.001$. A well-motivated sub-class within this set of models
is detectable by CMB-S4 at 5$\sigma$. The observed departure from scale
invariance is a potentially important clue that strongly motivates exploring
down to $r=10^{-3}$. With an order of magnitude more detectors than precursor
observations, and exquisite control of systematic errors, CMB-S4 will improve
upon limits from pre-CMB-S4 observations by a factor of five to reach this
target, allowing us to either detect primordial gravitational waves or rule
out large classes of inflationary models and dramatically impact how we think
about the theory.

### The dark Universe.

In the standard cosmological model, about 95% of the energy density of the
Universe is in dark matter and dark energy. As discussed in the dedicated
Snowmass 2021 White Papers [11, 12, 13], with CMB-S4 we can address numerous
questions about these dark ingredients, such as: How is matter distributed on
large scales? Does the dark matter have non-gravitational interactions with
baryons? Are there additional unseen components beyond dark matter and dark
energy?

Light relic particles are one very well-motivated possibility for additional
energy density, as additional light particles appear frequently and numerously
in extensions to the standard model of particle physics [14, 15]. For large
regions of the unexplored parameter space in these models, the light particles
are thermalized in the early Universe. The Planck satellite has sensitivity to
light particles that fell out of thermal equilibrium after the first 50 micro-
seconds of the Universe. With CMB-S4 we can push back this frontier by over a
factor of 10,000, to the first fractions of a nanosecond.

The contribution of light relics to the energy density, often parameterized as
the “effective number of neutrino species,” $N_{\mathrm{eff}}$, leads to
observable consequences in the CMB temperature and polarization anisotropy.
Current data are only sensitive enough to detect additional relics that froze
out after the quark-hadron transition, so CMB-S4’s ability to probe times well
before that transition is a major advance. Specifically CMB-S4 will constrain
$\Delta N_{\mathrm{eff}}<0.06$ at 95% C.L., achieving sensitivity to Weyl
fermion and vector particles that froze out at temperatures a few hundred
times higher than that of the QCD phase transition.

CMB-S4 will also enable a broader exploration of the dark Universe in
combination with other probes, often significantly enhancing them by breaking
their intrinsic degeneracies. It will improve or detect various possibilities
for the properties of dark matter beyond the simplest cold dark matter models
[16, 17]. It will add to dark energy constraints through precision
measurements of the primordial power spectrum, and through precision
measurements of the lensing convergence power spectrum, through the CMB-
lensing-derived mass calibration of galaxy clusters [18], and through CMB
lensing tomography [19].

### Mapping matter in the cosmos.

Observations indicate there is roughly five times more dark matter than
baryonic matter and that most of the baryonic matter is in the form of hot
ionized gas rather than cold gas or stars. CMB-S4 will be able to map out
normal and dark matter separately by measuring the fluctuations in the total
mass density (using gravitational lensing) and the ionized gas density (using
Compton scattering).

Observations of gravitational lensing of the CMB are key to many CMB-S4
science goals. CMB-S4 lensing data will lead to a precise two-dimensional map
of the total matter distribution. The statistical properties of this mass map
will provide important constraints on dark energy [19], modified gravity [19],
and the neutrino masses [20]. When combined with CMB-S4-derived or external
catalogs of galaxies or galaxy clusters, this mass map can be used to “weigh”
the galaxy or cluster samples. With galaxies, this can be done in a redshift-
dependent or tomographic manner out to redshifts as high as $z\sim 5$, making
possible new precision tests of cosmology and gravity. With robust CMB-
lensing-based cluster masses at high redshift, the abundance of galaxy
clusters can be used as an additional probe of dark energy and neutrino
masses.

Most of the baryons in the late Universe are believed to be in a diffuse
ionized plasma that is difficult to observe [21, 22, 23]. CMB-S4 will measure
the effect of Compton scattering by this gas (the Sunyaev-Zeldovich or SZ
effects), both the spectral distortion from hot electrons (thermal SZ or tSZ)
and a general redshift or blueshift of the scattered photons due to coherent
bulk flows along the line of sight (kinematic SZ or kSZ). The nature of this
scattering makes the SZ effects independent of redshift. With a deep and wide
survey covering a large amount of volume and an ultra-deep survey imaging
lower-mass clusters, CMB-S4 will be an effective probe of the crucial regime
of $z\gtrsim 2$, when galaxy clusters were vigorously accreting new hot gas
while at the same time forming the bulk of their stars [24]. The CMB-S4
catalog will contain an order of magnitude more clusters at $z>2$ than will be
discovered with Stage 3 CMB experiments [18, 25]. CMB-S4 will also measure the
diffuse tSZ signal on the sky and make a temperature-weighted map of ionized
gas that can be used to measure the average thermal pressure profiles around
galaxies and groups of galaxies. CMB-S4 will also make maps of the kSZ effect,
which will be combined with data from other surveys to make maps of the
projected electron density around samples of objects. Applications of these
maps include measuring ionized gas as a function of radius, directly
constraining the impact of feedback from active galactic nuclei and supernovae
on the intergalactic medium [26] and constraining theories of modified gravity
using the bulk flow amplitude as a function of separation. Even without
overlapping galaxy catalogs, the kSZ signal can be used to probe the epoch of
reionization, in ways that are highly complementary to the measurements of the
neutral gas that can be obtained with redshifted Ly-$\alpha$ and 21-cm studies
[27, 28, 29, 30].

### The time-variable millimeter-wave sky.

There have been relatively few studies of the variable sky at millimeter
wavelengths, with systematic surveys using CMB data only very recently being
undertaken [31, 32, 33, 34]). A deep, wide, millimeter-wave survey with time-
domain capability will provide key insights into transient or burst events,
moving sources such as Solar-System objects, and variable sources such as
stars and active galactic nuclei (AGN).

Targeted follow-up observations of gamma-ray bursts, core-collapse supernovae,
tidal disruption events, classical novae, X-ray binaries, and stellar flares
have found that there are many transient events with measured fluxes that
would make them detectable by CMB-S4. A systematic survey of the mm-wave sky
with a cadence of a day or two over a large fraction of the sky, combined with
an ultra-deep daily survey of a few percent of the sky, would be an excellent
complement to other transient surveys, filling a gap between radio and optical
searches [31]. Gamma-ray burst afterglows are particularly interesting targets
as they peak at millimeter wavelengths and there is a possibility of capturing
mm-wave afterglows that have no corresponding gamma-ray trigger, either from
the geometry of relativistic beaming and/or from sources at very high redshift
[31]. Both are predicted theoretically, but have never been detected.

CMB-S4 will play an active role in multi-messenger astronomy, providing a long
baseline with high-cadence sampling in both intensity and linear polarization
over a wide sky area. For example, the IceCube event IC170922A is believed to
be associated with a flaring gamma-ray state of the blazar TXS 0506+056. In
December 2014, however, the same source appears to have had a neutrino
luminosity at least 10 times larger with no associated gamma emission—and no
data existed at other wavelengths. Having high-cadence wide-field non-gamma-
ray data will be critical to understand sources like this one. Any similar
source is likely to be included in CMB-S4’s near-daily, high-signal-to-noise
monitoring of the blazar population. The wide-area nature of the survey will
also make it straightforward to search for gravitational wave sources,
particularly for sources that happen to be poorly localized and are
challenging for other instruments.

## Technical Overview

The CMB-S4 Collaboration and Project have developed a Preliminary Baseline
Design that meets the measurement requirements and therefore can deliver the
CMB-S4 science goals. The main components of the Preliminary Baseline Design
are described in detail in the PBDR and summarized here. The major components
are as follows:

  * •

An ultra-deep survey covering 3% of the sky (more if a gravitational wave
signal is detected) to be conducted over seven years using: fourteen 0.55-m
refractor small-aperture telescopes (SATs) at 155 GHz and below and four
0.44-m SATs at 230/280 GHz, with dichroic, horn-coupled superconducting
transition-edge-sensor (TES) detectors in each SAT, measuring two of the eight
targeted frequency bands between 25 and 280 GHz; and one 5-m class “delensing”
large-aperture telescope (LAT), equipped with detectors distributed over seven
bands from 20 to 280 GHz. Measurements at degree angular scales and larger
made using refractor telescopes with roughly 0.5-m apertures have been
demonstrated to deliver high-fidelity, low-contamination polarization
measurements at these scales. The combination of the SATs with the 5-m LAT
therefore provides low-resolution $B$-mode measurements with excellent control
of systematic contamination, as well as the high-resolution measurements
required for delensing. The ultra-deep survey SATs and 5-m LAT are to be
located at the South Pole to allow targeted observations of the single small-
area field, with provisions to relocate a fraction of the SATs in Chile if,
for example, a high level of $r$ is detected.

The total detector count for the 18 SATs is 147,936, with the majority of the
detectors allocated to the 85 to 155 GHz bands. The total number of science-
grade 150-mm detector wafers required for 18 SATs is 216\. The delensing LAT
will have a total TES detector count of 129,024, with the majority of the
detectors allocated to the 90 to 150 GHz bands. The total number of science-
grade 150-mm diameter detector wafers required for this single LAT is 85.

  * •

A deep and wide survey covering approximately 60% of the sky to be conducted
over seven years using two 6-m LATs located in Chile, each equipped with
137,996 TES detectors distributed over eight frequency bands spanning 25 to
280 GHz. The total number of science-grade 150-mm diameter detector wafers
required for these two LATs is 170.

In the context of their legacy value to the wider community, we refer to the
deep/wide and ultra-deep high-resolution surveys together as the CMB-S4 Legacy
Survey. The total detector count for CMB-S4 is 552,952, requiring 471 science
grade wafers. This is an enormous increase over the detector count of all
Stage-3 experiments combined. Such a dramatic increase in scale is required to
meet the CMB-S4 science goals.

## Technical Readiness

The CMB-S4 reference design uses existing, well-demonstrated technology that
has been developed and demonstrated by the CMB experimental groups over the
last decade, scaled up to unprecedented levels. The design and implementation
plan addresses the considerable technical challenges presented by the required
scaling up of the instrumentation and by the scope and complexity of the data
analysis and interpretation. Features of the design and plan include: scaled-
up superconducting TES detector arrays with well-understood and robust
material properties and processing techniques; high-throughput mm-wave
telescopes and optics with unprecedented precision and rejection of systematic
contamination; full internal characterization of astronomical foreground
emission; large cosmological simulations and improved theoretical modeling;
and computational methods for extracting minute correlations in massive,
multi-frequency data sets, which include noise and a host of known and unknown
signals.

A CMB-S4 Risk and Opportunity Management Plan describes the continuous risk
and opportunity management process implemented by the project, consistent with
DOE O413.3B, “Project Management for the Acquisition of Capital Assets,” and
the NSF 21-107, “Research Infrastructure Guide.” The plan establishes the
methods of assessing CMB-S4 project risk and opportunities for all subsystems
as well as the system as a whole. The CMB-S4 risk register has 211 risks
identified. There are 26 risks currently assessed as High. The project is
working on mitigations to ensure that these risks are lowered to reasonable
levels on a timescale consistent with the overall project schedule.

For example, a current identified High risk is meeting the scaled-up
production throughput and testing timeline of the transition-edge-sensor
detector arrays. This is a major focus of the R&D program supported by the
DOE. The Integrated Project Office formed a CMB-S4 Detector Fabrication Group
(CDFG) in January 2020 to facilitate the collaboration between multiple
fabrication sites and to develop a single detector fabrication plan and
produce prototype detectors that satisfy CMB-S4 acceptance criteria. The CDFG
is envisioned to be active throughout the duration of CMB-S4 detector
fabrication efforts.

## Organization, Partnerships, and Current Status

CMB-S4 is both a scientific collaboration and a DOE/NSF project. While these
are certainly tightly coupled, they do have different roles and
responsibilities; the overall organization of CMB-S4 therefore decouples into
the organization of the collaboration and the project.

The CMB-S4 Project has developed in tandem with the Collaboration, paced by
the funding agencies. In 2019 DOE approved Critical Decision 0, identifying
the project need, and NSF funded a project development proposal led by the
University of Chicago. In 2020 DOE appointed Berkeley Lab as the DOE lead
laboratory. Together Berkeley Lab and U. Chicago lead a single, integrated,
joint project with 121 members, 53 of whom are also collaboration members. The
project office has developed a wide range of project organization and
documentation, including the Work Breakdown Structure, Risk and Opportunity
Register, detailed Cost Book and Technically-Limited Schedule. The top level
WBS Structure is summarized in Table 1.

Table 1: CMB-S4 WBS Structure WBS |  WBS Title |  WBS Description
---|---|---
1.01 |  Project Office |  Labor, travel, and materials necessary to plan, track, organize, manage, maintain communications, conduct reviews, and perform necessary safety, risk, and QA tasks during all phases of the project. Overall project Systems Engineering is a subsection of this WBS element. However, subsystem-related management and support activities for planning, estimating, tracking, and reporting as well as their specific EH&S and QA tasks are included in each of the subsystems.
1.03 |  Detectors |  Labor, materials, and equipment associated with the design, fabrication and testing of the detector wafers. R&D activities to support development of a Conceptual Design pre CD-1 for DOE.
1.04 |  Readout |  Labor, materials, and equipment associated with the design, fabrication and testing of the detector readout system. R&D activities to support development of a Conceptual Design pre CD-1 for DOE.
1.05 |  Module Assembly and Testing |  Labor, materials, and equipment associated with the design, parts fabrication, assembly and testing of the detector modules. R&D activities to support development of a Conceptual Design pre CD-1 for DOE.
1.06 |  Large Aperture Telescopes |  Labor, materials, and equipment associated with the design, prototyping, materials selection, construction and certification for the Large Aperture Telescope System. Integration and commissioning in North America.
1.07 |  Small Aperture Telescopes |  Labor, materials, and equipment associated with the design, prototyping, materials selection, construction and certification for the Small Aperture Telescope System. R&D activities to support development of a Conceptual Design pre CD-1 for DOE. Integration and commissioning in North America.
1.08 |  Data Acquisition and Control |  Labor, materials, and equipment associated with the design, construction, certification, and delivery of the control systems for the observatories and data acquisition. R&D activities to support development of a Conceptual Design pre CD-1 for DOE.
1.09 |  Data Management |  Labor, materials, and equipment associated with the design, construction, certification, and delivery of the data management system. R&D activities to support development of a Conceptual Design pre CD-1 for DOE.
1.10 |  Chile Infrastructure, Integration and Commissioning |  Labor, travel, and materials necessary to plan, track, manage, maintain communications, conduct reviews, and perform necessary safety monitoring on site including oversight of all shipping of CMB-S4 components to Chile and oversight of construction activities on site. On-site Integration and Commissioning of the CMB-S4 telescopes and infrastructure in Chile.
1.11 |  South Pole Infrastructure, Integration and Commissioning |  Labor, travel, and materials necessary to plan, track, manage, maintain communications, conduct reviews, and perform necessary safety monitoring on site including oversight of all shipping of CMB-S4 components to the South Pole and oversight of construction activities on site. On-site Integration and Commissioning of the CMB-S4 telescopes and infrastructure at the South Pole.

The organizational chart of the Integrated Project Office is shown in Fig. 1.
A key feature of the organization is the role of collaboration members in the
project office, in particular as leaders of the Level 2 systems. The Level 2
managers are supported by engineering and project-management leaders. The
NSF/DOE scope distribution promotes the engagement and participation of
universities and national laboratories. Graduate students, postdocs,
professional technicians and engineers are expected to be involved in all
aspects of the project.

The Integrated Project Office is responsible for forming partnerships with key
stakeholder institutions, including DOE National Laboratories, universities,
and potential collaborating projects such as the Simons Observatory, South
Pole Observatory, and the CCAT-prime project. Partnerships are also expected
to include foreign institutions participating in the CMB-S4 Science
Collaboration and contributing to the CMB-S4 Project.

Figure 1: Organizational Chart of the Integrated Project Office. The figure
includes the expected distribution of project scope by funding agency (NSF =
blue, DOE = green). We are actively pursuing partners who could make
significant scope contributions in areas aligned with their expertise.

The CMB-S4 project is expected to include significant contributions from
collaborating institutions supported by funding agencies other than NSF and
DOE. These “in-kind” contributions will be defined as deliverables to the
project. Major contributions from partners will need to be negotiated and
incorporated in the project design and Work Breakdown Structure within the
next one to two years to avoid adding schedule and cost risk.

## Schedule

The CMB-S4 Project schedule estimate is developed from the work scope defined
in the WBS and decomposing the work packages into detailed activities,
estimating the duration of each activity, sequencing the activities in time,
and establishing necessary predecessors and successors. The data are captured
in a formal Project Management Control System (PMCS), which encompasses the
costs, schedules, work scope, and other activity attributes that define how
the project will be executed. Throughout the life of the project, control and
analysis of the schedule requires the use of progress reporting, schedule
change control systems, such as the use of project baseline change requests,
earned value performance management, and variance analysis to determine if
additional action is required to get the schedule back in line with the plan.

The CMB-S4 project schedule has 8995 activities, 14,348 relationships, 21
Level 1/2, 110 Level 3, 179 Level 4, and 2179 Level 5 milestones. This
preliminary schedule has been developed as a technically-limited schedule and
will be updated once funding profile guidance from the DOE/NSF is available.
Our current technically limited schedule is shown in Figure 2.

Seven years of operations are needed to achieve the CMB-S4 science goals.

Figure 2: Major activities by WBS for a technically-limited schedule for the
CMB-S4 project. The timeline will depend on funding profile from DOE and NSF.
To address the highest technical, cost, and schedule risks, the Detector,
Readout, and Modules (DRM) R&D, prototyping, and pre-production are
prioritized to allow production to begin as early as possible. In parallel,
telescope design and fabrication are synchronized with DRM delivery for
assembly and test in the US before shipping to observation sites. Integration
and commissioning will be completed at the installed locations.

## Cost

The CMB-S4 Project cost estimate has been developed from applying resource
allocations to specific activities captured in the WBS, establishing budgets
at the start of the project and to support budget status during the project.
The Cost Estimating Plan (CEP) focuses on the methodology to develop the
activity-based estimates and facilitates the financial management and control
of the Project as described in the Project Controls System Management Plan.

For task-based estimating, formal procedures are used to define tasks and
assign them to the lowest-level elements of the WBS. The ensemble of tasks
represents all the required resources, activities, and components of the
entire project. Each of the tasks is included in the Integrated Project
Schedule (IPS) and estimated by the teams using accepted techniques. The
estimates are documented by a Basis of Estimate (BOE).

The cost estimates and schedule have been prepared by the IPO Project
Managers, project controls specialists and the L2 Subsystem Control Account
Managers and Scientists experienced in the specialized fields required to
accomplish the CMB-S4 Project. Vendor estimates and quotations, engineering
calculations, drawings, and other pertinent data, which are used to support
the cost estimate, are collected and organized into a BOE. The details of the
cost estimating process and assumptions are contained in the CMB-S4 Cost
Estimating Plan.

The CMB-S4 point estimate project cost in January 2022 was $636M for DOE Total
Estimated Cost (TEC) funds plus NSF MREFC, fully loaded and escalated to the
year of expenditure, and for a technically limited schedule without
contingency. The Monte Carlo simulation contingency analysis of the CMB-S4
resource-loaded, risk-applied schedule yielded a budget contingency of $266M.
The cost are split roughly 50/50 between DOE and NSF scope. DOE Other Project
Costs (OPC) and NSF pre-MREFC funds total approximately $61M. The IPO is
continuing to improve the quality of the cost, schedule and funding profile of
the CMB-S4 project in preparation for agency reviews.

In-kind contributions from private and international partners are being
pursued and, if realized, will reduce the total cost to NSF and DOE.

## Summary

The science case for CMB-S4 remains as exciting as ever, as evidenced by DOE
CD-0 “Mission Need” approval and by the strong endorsement by the 2020 Decadal
Survey of Astronomy and Astrophysics. In fact, the science case has become
stronger as scientists spanning the particle physics and cosmology communities
have explored the potential of the transformational measurements CMB-S4 will
make to achieve its threshold-crossing science goals. The technology is well
understood and field tested. The CMB-S4 Collaboration and Integrated Project
Office are established and highly functioning. The project is well developed,
has been extensively reviewed, and is on path to achieve DOE CD-1 and NSF PDR
on the timescale of a year, depending on funding.

## References

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