#  Snowmass Neutrino Frontier:
NF04 Topical Group Report
Neutrinos from natural sources  Submitted to the Proceedings of the US
Community Study on the Future of Particle Physics (Snowmass 2021)

Y. Koshio  Okayama University, Okayama, Japan  G.D. Orebi Gann  University of
California, Berkeley, Berkeley, CA, USA and Lawrence Berkeley National
Laboratory, Berkeley, CA, USA  E. O’Sullivan  Uppsala University, Uppsala,
Sweden  I. Tamborra  Niels Bohr Institute, University of Copenhagen,
Copenhagen, Denmark

###### Contents

  1. 1 Executive Summary
    1. 1.1 Physics of neutrinos from natural sources
    2. 1.2 Enabling technologies
    3. 1.3 Synergies
  2. 2 Early universe neutrinos
    1. 2.1 Motivation and current understanding
      1. 2.1.1 Cosmic neutrino background
      2. 2.1.2 Neutrino properties through cosmological data
    2. 2.2 Future prospects
      1. 2.2.1 Prospects for theoretical developments
      2. 2.2.2 PTOLEMY
  3. 3 Solar Neutrinos
    1. 3.1 Historical context
    2. 3.2 Physics opportunities
    3. 3.3 Recent technological developments
    4. 3.4 Prospects for future measurements
      1. 3.4.1 Water Cherenkov detectors
      2. 3.4.2 Liquid scintillator detectors
      3. 3.4.3 Hybrid optical detectors
      4. 3.4.4 Noble liquid and solid state detectors
      5. 3.4.5 Other concepts
  4. 4 Geoneutrinos
    1. 4.1 Motivation
    2. 4.2 Current understanding: Borexino and KamLAND
    3. 4.3 Prospects for future measurements
      1. 4.3.1 SNO+
      2. 4.3.2 JUNO
      3. 4.3.3 Ocean Bottom Detector (OBD)
      4. 4.3.4 Theia
      5. 4.3.5 Other underground LS detectors
      6. 4.3.6 The search for 40K geoneutrinos
      7. 4.3.7 LiquidO
      8. 4.3.8 Coherent neutrino-nucleus scattering
      9. 4.3.9 6Li-doped scintillator
  5. 5 Supernova Burst Neutrinos and Diffuse Supernova Neutrino Background
    1. 5.1 Motivation and current understanding
      1. 5.1.1 Neutrino flavor mixing
      2. 5.1.2 Neutrinos as probes of the supernova physics
      3. 5.1.3 Supernova pointing and early warning through neutrinos
      4. 5.1.4 Neutrinos as probes of physics beyond the Standard Model
      5. 5.1.5 Current experimental landscape
    2. 5.2 Future Prospects
      1. 5.2.1 Prospects for theoretical developments
      2. 5.2.2 Prospects for measurements beyond electron anti-neutrino
      3. 5.2.3 Expanding the detection horizon for core-collapse supernovae
      4. 5.2.4 Early warning for multi-messenger physics
      5. 5.2.5 Measuring the diffuse supernova neutrino background
  6. 6 Atmospheric Neutrinos
    1. 6.1 Motivation and current understanding
    2. 6.2 Prospects for future measurements
      1. 6.2.1 Prediction of atmospheric neutrino flux
      2. 6.2.2 Neutrino oscillation measurements
  7. 7 High energy astrophysical neutrinos
    1. 7.1 Motivation and current understanding
      1. 7.1.1 Sources of high energy astrophysical neutrinos
      2. 7.1.2 Experimental landscape
      3. 7.1.3 High energy astrophysical neutrinos as probes for particle physics
      4. 7.1.4 High energy astrophysical neutrinos as probes for astrophysical environments
    2. 7.2 Future Prospects
      1. 7.2.1 Future Experiments and technologies
      2. 7.2.2 High energy astrophysical neutrinos as probes for particle physics
      3. 7.2.3 High energy astrophysical neutrinos as probes for astrophysical environments
  8. 8 Conclusion
  9. A Letters of Interest submitted to NF04
  10. B White Papers submitted to NF04

##  1 Executive Summary

###  1.1 Physics of neutrinos from natural sources

This report covers a broad range of neutrino sources, from low-energy
neutrinos from the early universe to ultra high-energy sources. We divide this
report by source, and discuss the motivations for pursuing searches in each
case, the current state of the field, and the prospects for future theoretical
and experimental developments. We consider neutrinos produced in the early
universe; solar neutrinos; geoneutrinos; supernova neutrinos, including the
diffuse supernova neutrino background (DSNB); neutrinos produced in the
atmosphere; and high-energy astrophysical neutrinos.

Strong support for this program is essential to preserve and grow our ability
to use neutrinos to probe the universe. These areas of study offer unique
insights and opportunities, including the ability to probe the interaction of
neutrinos with matter; constraints on the nature and mass of the neutrino that
are complementary to, and provide invaluable input to, neutrinoless double
beta decay and direct mass searches; an understanding of the formation,
evolution, and eventual demise of stars; and flavor physics.

The detection of the cosmic neutrino background will be crucial to unveil the
Dirac or Majorana nature of neutrinos, as well as obtain information on their
absolute mass. In this endeavor, PTOLEMY will play a crucial role. Upcoming
cosmological surveys will be instrumental to probe the early universe physics
and infer the eventual existence of new physics. To this purpose, it remains
essential to progress in our understanding of neutrino flavor mixing in the
early universe.

Future precision measurements of solar neutrinos offer the potential to
resolve uncertainties in the metal content of our Sun, to inform our
understanding of stellar evolution, to probe matter effects in a unique
environment, to search for non-standard interactions and new physics effects,
and to perform precision tests of the three-flavor neutrino mixing paradigm. A
suite of next-generation detectors is under development, which leverage novel
technology to improve background discrimination, in order to enhance
sensitivity to these low-energy neutrinos. A similar suite of detectors is
also sensitive to measure antineutrinos produced within the earth – so-called
geoneutrinos. New measurements of the geoneutrino flux will inform models for
heat production within the Earth. Of particular interest are measurements of
the U/Th ratio; a measurement of the flux produced from the mantle, which can
be inferred using a suite of results from different geographical locations, or
measured directly with an ocean-bottom detector; and the potential to detector
the K antineutrino flux.

The next galactic supernova is expected to be detected in photons, neutrinos,
and gravitational waves and will carry a formidable amount of information on
the physics of the collapse of massive stars. This multi-messenger detection
will be crucial to test our conjectures on the supernova mechanism. To this
purpose, progress is being made in order to use neutrinos to alert astronomers
and gravitational wave physicists of the core collapse as soon as possible
through the SNEWS network. Neutrinos will carry clear signatures of the nature
of the stellar collapse, e.g. in the instance of black hole formation. Despite
swift progress in the field in the last decade, the impact of neutrino flavor
mixing in the dense supernova core remains to be assessed and it is one of the
main goals for the next decade, due to its implications on the supernova
hydrodynamics, nucleosynthesis and detectable neutrino signal. In order to
maximize the physics we will extract from this once-in-a-generation event, it
is imperative to have neutrino experiments with high uptime and using a
variety of detection technologies in order to enhance the statistics of the
measurement, but also to ensure measurements of all neutrino flavours. It is
expected that the next supernova burst will test the existence of new physics
or place stringent constraints on it; however, to this purpose a consistent
modeling of new physics in the source is mandatory. The upcoming DSNB
detection will push low-energy neutrino astronomy to extra-galactic scales.
Various upcoming experiments aim to enable new physics avenues through the
DSNB detection, which will provide complementary insight on the supernova
population as well as on new or exotic physics.

Research into atmospheric neutrinos has had great success in the last two
decades, most notably including the first discovery of neutrino oscillations.
In atmospheric neutrino observations by next-generation detectors, planned for
operation in the near future, it is expected that the neutrino oscillation
parameters, especially $\theta_{23}$ octant, mass ordering, and $\delta_{CP}$,
will be determined with high accuracy. These detectors also offer sensitivity
to physics beyond the standard model, such as sterile neutrinos, Lorentz
invariance violation, non-standard interactions, and CPT violation, which have
not currently been observed. Precise measurements of the atmospheric neutrino
flux are also important topics for non-oscillation physics such as nucleon
decay, since the atmospheric neutrino signal forms a dominant background to
such searches. To this end, reductions in the uncertainty of hadron production
by primary proton interactions forms an important component of the program. In
addition, there is interest in searches for prompt neutrinos from atmospheric
charm production by cosmic rays.

The advent of real-time astronomy through high-energy neutrinos has already
led to a handful of likely observations of cosmic sources connected to high
energy neutrino production. The expected growing numbers of such detections as
well as the larger statistics enabled by upcoming neutrino telescopes will
allow to probe particle acceleration in the sources as well as the mechanisms
powering cosmic accelerators. In the next decade, we expect the emergence of
large neutrino telescopes which will use radio detection to capture neutrinos
up to ZeV in energy. This will enable discoveries of cosmic acceleration of
neutrinos at the highest energies. In addition, information on the neutrino
properties and new physics can be established at these extreme energies for
the first time.

###  1.2 Enabling technologies

Neutrinos from natural sources typically fall into a few energy regimes, and
we observe synergies in the experimental techniques and detector requirements
within those regimes.

Low-energy neutrinos, in the MeV to 10s of MeV range, have been successfully
interrogated using a range of techniques, including radiochemical experiments,
water Cherenkov detectors, organic liquid scintillator detectors, noble liquid
detectors, and solid state detectors. Future progress will hinge on critical
developments, including fast and spectrally-sensitive photon detection, novel
scintillating materials, new purification techniques, and low-background
detectors. These detector developments are closely coupled with improvements
in readout electronics that can handle the dynamic range of physics addressed
in these detectors, and sophisticated analysis techniques.

At the high energy end of the neutrino spectrum, the main challenge lies with
the low flux of signal. The required large instrumented volumes drive the need
for powerful yet inexpensive sensors. The field has seen great success with
the use of optical sensors deployed in large neutrino telescopes. Looking
towards the future, the development of optical sensors that use a collection
of "multi-PMTs", smaller photosensors in a single unit that allow for more
information about the directionality of the incoming photons, seems promising.
In order to instrument the volumes required for the highest energy
astrophysical neutrinos beyond the PeV-scale, where the flux is even lower,
the development and deployment of inexpensive radio receivers, both in-ice as
well as balloon-borne or space-based missions to enhance the observable
volume, are required.

###  1.3 Synergies

All areas within this topical group rely heavily on continued theoretical
development, and we wish to emphasise the need for continued, ongoing support
in this area. There is also strong synergy with the computing frontier, as
theoretical simulations of astrophysical objects and their related
microphysics move towards a new level of complexity and multi-dimensional
modeling; in parallel, synergies with the computing frontier will be essential
as experiments scale up and data production moves into PBs, along with the
need for ever-more sophisticated analysis techniques to enhance sensitivity
and fully leverage detector capabilities. As neutrinos play an essential role
in multi-messenger astronomy, synergies are also foreseen with the cosmic
frontier and with the astronomy community in order to take full advantage of
upcoming multi-messenger discoveries.

##  2 Early universe neutrinos

###  2.1 Motivation and current understanding

The Early Universe constitutes an ideal laboratory to investigate fundamental
physics as well as the eventual existence of New Physics.

####  2.1.1 Cosmic neutrino background

The cosmic background of neutrinos (CNB) is a relic of the Universe when it
was about $1$ s old [1, 2, 3]. It consists of about $112$ cm-3 neutrinos and
antineutrinos of all flavors. The CNB has not been detected yet, however
ongoing efforts aim to achieve its detection threshold. The CNB detection will
shed light on the Dirac or Majorana nature of neutrinos, the absolute scale of
neutrino masses, as well as open promising avenues in the field of dark-matter
astronomy.

####  2.1.2 Neutrino properties through cosmological data

Upcoming cosmological data could be especially relevant to investigate the yet
unknown neutrino properties [2]. In fact, while we routinely measure the mass-
squared differences of neutrinos from flavor oscillation experiments, the
neutrino mass ordering is still uncertain, but it is expected to be measured
through neutrino experiments within the next decade. On the other hand, the
absolute mass of neutrinos is still unknown. The cosmological measurement of
the latter will complement ongoing searches, e.g. the ones carried out by the
KATRIN Collaboration which aims to measure the electron endpoint spectrum in
tritium beta decay [4, 5, 6]. The eventual existence of a tension between
cosmological and experimental measurements could provide new insights on
particle physics and astrophysics.

The radiation density in the early universe consists of the contribution of
neutrinos and photons, after the decoupling of electrons and positrons [3].
Before neutrinos become non-relativistic, the radiation density is
parametrized in terms of the effective number of thermally excited neutrino
degrees of freedom, $N_{\rm eff}$. The expected value of the effective number
of radiation species is $N_{\rm eff}=3.045$, where the deviation from $3$
takes into account the residual neutrino heating by electron and positron
annihilations, in addition to other small corrections. It is important to
measure $N_{\rm eff}$ precisely as it could provide indications on the
eventual existence of New Physics, such as extra sterile neutrino families or
non-standard interactions of neutrinos. Despite the existence of possible ways
to evade the $N_{\rm eff}$ constraints for New Physics scenarios, another
challenge concerning sterile neutrinos revolves around the fact that the mass
scale of these particles is unknown and we ignore their production mechanism
in the early universe, despite the fact that they could have a major impact on
the cosmological observables.

###  2.2 Future prospects

####  2.2.1 Prospects for theoretical developments

The measurement of the absolute neutrino mass is one of the main goals of
Stage-4 CMB experiments [7, 2, 8, 9] that are expected to pin down the
neutrino masses with an uncertainty of up to $15$ meV. Stage-4 CMB experiments
also promise to achieve a precision of about $1\%$ in the determination of
$N_{\rm eff}$ [10]. The measurement of the primordial helium and deuterium
abundances will also be essential and is planned to be achieved through the
advent of 30 m-class telescopes [11].

Such upcoming measurements will need to be contrasted with simulations of the
evolution of the early universe at the time of neutrino decoupling in order to
constrain any New Physics scenarios. However, one of the theoretical
challenges is to provide a self-consistent solution of the neutrino quantum
kinetic equations including scattering as well as entropy flow. These kinds of
calculations are currently not available as they are beyond the state-of-the-
art computationally, however they are crucial to disentangle subtle effects
due to New Physics from standard scenarios [12, 13, 14, 15, 16, 17, 18]. An
important and unsolved issue concerns the modeling of neutrino flavor mixing
in the context of the early universe [19, 20, 21, 22, 23, 24]. In particular,
because of the large density of neutrinos, the coherent forward scattering of
neutrinos onto each other makes the flavor evolution non-linear, leading to
counter-intuitive effects in the flavor evolution. As future cosmological
surveys become available, they will enable the measurement of cosmological
variables with high precision, hence the exact role played by neutrino mixing
needs to be clarified. In addition, dark sector scenarios may mimic the
phenomenology of the Standard Model, with subtle implications for the physics
of the early universe yet to be grasped.

####  2.2.2 PTOLEMY

PTOLEMY is a landmark project with the goal of detecting the CNB [25]. In
order to detect neutrinos, PTOLEMY relies on surfaces of atomic tritium weakly
bound to monolayer graphene. Such surfaces are oriented perpendicularly with
respect to a high magnetic field, hence electrons are transported through
drift processes. Measurements with an RF antenna (relying on cyclotron
radiation emission spectroscopy) provide an initial estimate of the momentum
components of electrons near the endpoint. Then the high-precision and
dynamically configured electromagnetic filter takes care of exponentially
draining the kinetic energy of electrons. When it reaches a few eV, the
residual kinetic energy of electrons is measured by a calorimeter. The
combination of measurements with high resolution and the 2D topology aims to
guarantee high precision in the neutrino capture signal.

The experimental realization was proposed in 2013 with a prototype currently
under development at Gran Sasso National Laboratories. PTOLEMY embraces a
diverse program from neutrino masses to sterile neutrinos searches to future
dark matter detection concepts. It provides an opportunity to open new
insights on neutrino properties, on the physics of the early universe as well
as concerning the development of new technologies.

##  3 Solar Neutrinos

Strong support for an ongoing future program in solar neutrinos is critical to
ensure that we fully explore the new physics of neutrino mass and
oscillations, and that we understand in detail the behavior of our nearest
star.

The only significant observed matter effect to date comes from measurements of
solar neutrinos. That observation not only provides the most precise
measurement of the mixing angle $\theta_{12}$, which impacts expected rates of
neutrinoless double beta decay, but provides excellent sensitivity to new
physics in the transition region between matter-enhanced and vacuum
oscillations.

Observations of solar neutrinos provide the only significant observed matter
effect to-date, which critically impacts our understanding of neutrino
properties and behavior, as well as offering a precise handle on mixing
parameters that affect the allowed phase space for processes such as
neutrinoless double beta decay. This field also offers unique opportunities to
probe the solar core, with potential insights into stellar evolution, as well
as neutrino interactions and properties (sterile neutrinos, NSIs).

###  3.1 Historical context

Solar neutrinos have a decades-long history of paradigm-shifting results,
along with two Nobel prizes, including early demonstrations of neutrino flavor
change, and evidence for non-zero neutrino mass. Neutrinos are produced by
several reactions in the core of the Sun, each giving rise to a characteristic
spectrum. These neutrinos are associated with two fusion cycles: the _pp_
chain, which is the primary source of solar power, and the CNO cycle, which
contributes about 1% of solar power, but offers insights into heavier element
production, and the formation of heavier stars.

All neutrinos produced during solar fusion are created in the electron
flavour. As they propagate from their production point, they undergo flavour
conversion due to the changing matter density of the Sun. This has provided
one of the first opportunities to probe the interaction of neutrinos with
matter.

A broad range of experiments have interrogated this source, ranging from a
suite of radiochemical experiments that leveraged interactions on either
chlorine or gallium – Homestake, GALLEX, GNO, and SAGE [26, 27, 28, 29, 30,
31, 32, 33] – to real-time observations with large-scale optical neutrino
detectors leveraging water or liquid scintillator as targets, including
Kamiokande, Super-Kamiokande (Super-K), the Sudbury Neutrino Observatory
(SNO), KamLAND, and Borexino. A full chronology of the exciting history of
this field can be found in [34]. Here, we focus on the most recent results,
and current open questions.

The Super-Kamiokande (Super-K) experiment made a high statistics measurement
of the flux of 8B neutrinos via elastic scattering (ES) [35], which is
sensitive primarily to $\nu_{e}$, but with some admixture of $\nu_{x}$. This
result was combined with the charged current (CC) measurement from the Sudbury
Neutrino Observatory (SNO), an interaction that is sensitive only to $\nu_{e}$
at relevant energies, allowing a measurement of the pure $\nu_{e}$ flux [36].
Disagreement between the two at the 3$\sigma$ level was evidence for some non-
electron component in the solar neutrino flux. This was confirmed by SNO’s
neutral current (NC) measurement, an interaction that is equally sensitive to
all three flavours, and which confirmed at 5$\sigma$ that the $\nu_{e}$
produced in the Sun’s core were changing flavour prior to detection on Earth
[37]. However, it was the result from the KamLAND liquid scintillator (LS)
detector that confirmed that this flavour change was in fact due to
oscillation, by observing the characteristic oscillation pattern in the
spectrum of reactor neutrinos [38].

In more recent years, Borexino has dominated the field of low-energy solar
neutrinos, producing a full suite of measurements of neutrinos from almost
every branch, including: first direct detection of _pp_ [39] and _pep_
neutrinos [40], the best precision on the 7Be flux [41, 42], as well as 8B
flux and spectrum measurements. A comprehensive spectroscopic study of the pp-
chain of solar neutrinos followed [43, 44, 45]. Perhaps the crowning
achievement to date is the first detection of neutrinos from the sub-dominant
CNO cycle [46, 47].

###  3.2 Physics opportunities

A number of open questions remain in the field. Ongoing experimental programs
can offer some insights but, in many cases, new detectors will be needed to
continue to push the bounds of our knowledge in this field.

##### CNO neutrinos

A precision measurement of CNO neutrinos is one of the most exciting open
questions in solar neutrinos. These neutrinos offer a handle on the metal
content of the Sun, which could help to resolve recent discrepancies. The
exact metal content (the metallicity) of a star’s core affects the rate of the
CNO cycle. This, in turn, influences the temperature and density profile —
and, thus, the evolution — of the star, as well as the opacity of its outer
layers.

The metallicity and opacity of the Sun affect the speed of sound waves
propagating through its volume. Historically, predictions from the Standard
Solar Model (SSM) agreed beautifully with results from helioseismology
regarding the speed of sound in the Sun. However, more-recent, more
sophisticated spectroscopic measurements of photospheric absorption lines
produced results that were significantly lower in opacity than previously
thought, leading to discrepancies with the helioseismological data [48, 49].
Precise measurements of CNO neutrinos offer the only independent handle by
which to investigate this difference. Borexino has made the first detection of
these neutrinos, but the uncertainty is as yet too large to offer insight on
stellar models. New data is needed, which will require ultra-low backgrounds,
low threshold, and excellent resolution in order to improve upon current
knowledge.

A recent review [34] considered the prospects for future precision
measurements of CNO neutrinos, and found hybrid (Cherenkov + scintillation)
detectors (Sec. 3.4.3) to be the optimal approach for percent-level precision,
due to the combination of low radioactive background, particle identification,
and directional information available in these detectors.

##### MSW transition

The current state of understanding of the pp-chain neutrinos can be found in
[34], showing recent results from each experiment; combined fits also exist
that further constrain the parameter space [50]. Although data exists in the
transition region between vacuum- and matter-dominated oscillation, the
precision is not yet sufficient to either confirm the MSW oscillation
scenario, or to offer sensitivity to possible NSI models. Additional data in
this sensitive region would offer strong constraints on non-standard models,
and further insights into the interaction of neutrinos with matter.

Constraints in the transition region are currently dominated by spectral
measurements of 8B neutrinos from the water Cherenkov experiments, given the
current scale of uncertainty on LS-based measurements in this region. Super-K
offers unparalleled size and access to low-energy neutrinos via the ES
measurement, which is highly complementary to the spectral sensitivity from
SNO’s CC events, albeit with lower statistics, and the flavor-blind NC
measurement. Both Super-K and SNO have contributed to the understanding of the
8B spectral shape, pushing the energy threshold to increasingly low energies
in order to start to probe the transition region [51, 52, 53]. Borexino’s
program has contributed many data points across the full span of the spectrum,
and the radiochemical experiments contribute primarily to the vacuum-dominated
region. However, more data is needed in the transition region in order to
probe this sensitive regime, to seek to confirm the MSW prediction, and to
search for sterile neutrinos, NSIs, and other non-standard effects.

##### Day/night effect

Both SNO and Super-K have sought evidence of the predicted day/night effect
[54, 55], which would lead to some regeneration of $\nu_{e}$ as neutrinos pass
through the earth at night, due to matter effects. Super-K have measured a
nearly 3$\sigma$ indication of this effect. Borexino have also searched for
this effect, and placed strong constraints. Greater statistics are required to
confirm the precise magnitude and nature of this effect.

##### Oscillation parameters

Reactor measurements of $\Delta m^{2}_{21}$ from KamLAND data provide a
terrestrial comparison to solar neutrino results, with some small tension
persisting in the value of $\Delta m^{2}_{21}$ [56]. New data is needed to
understand whether this discrepancy will persist, and potentially lead to new
understanding. Improved precision on measurements of $\theta_{12}$ will
support precision tests of the 3-flavor mixing model, and inform the parameter
space for neutrinoless double beta decay searches.

##### pp neutrinos

A precise measurement of _pp_ neutrinos offers insight into solar luminosity,
and the ability to probe the luminosity constraint. A leading consideration
for improved precision in the _pp_ measurement is the 14C background, inherent
in any organic scintillator.

##### hep neutrinos

A search for _hep_ neutrinos is extremely challenging due to the very low
predicted flux. The best limits on this flux come from SNO, and are currently
a few times the SSM prediction [57]. These neutrinos are the highest energy of
the solar neutrino branches, and are produced furthest from the core [34]. A
measurement of this flux could offer insights into the solar density in the
outer core.

###  3.3 Recent technological developments

Although a separate topical group (NF10) exists for neutrino detection
technology, here we summarise some recent developments that could
revolutionize the field of solar neutrinos.

Early water Cherenkov detectors were critical in identifying the solar
neutrino signal, thanks to their directional sensitivity, which allows
pointing back to the source. Scintillation experiments in contrast offer high
light yields, with associated low thresholds, critical for observation of CNO
neutrinos, and excellent resolution. Particle-depending quenching also offers
particle identification (PID) via pulse shape discrimination. An experiment
that can combine these features would have unprecedented sensitivity to solar
neutrinos, allowing enhanced background rejection with good resolution, at low
threshold.

Although existing LS experiments were not designed with this capability in
mind, significant progress has been made towards realizing directional
sensitivity in these detectors. The Borexino Collaboration continues to break
ground with their recent result that demonstrated statistical direction
reconstruction using early-time PMT hits [58]. SNO+ has indications of event-
level directional sensitivity in LS data, enhanced by the current low fraction
of PPO, which results in a slower scintillation time profile, enhancing the
clarity of the prompt Cherenkov component.

Significant work is ongoing within the community to develop an experiment that
would be a truly “hybrid” detector – designed to leverage both Cherenkov and
scintillation light together. The potential of this technology for solar
neutrinos is discussed in Section 3.4.3. The relevant technology, which
includes novel scintillators, fast photon detectors, and spectral sorting, is
discussed in the report from NF10.

Another novel technology that is under development is opaque liquid
scintillators. These intentionally shorten the scattering length of the LS in
order to confine light near the interaction point, thus providing excellent
vertex resolution and information regarding event topology. These are
discussed in Section 3.4.2 and, again, the technology development is described
in more detail in the report from NF10.

New developments in photon detection techniques could also prove critical to
this field. Fast photon detectors, such as Large Area Picosecond Photon
Detectors (LAPPDs) and spectral sorting techniques, such as dichroicons, offer
the potential for significantly enhanced precision for low-energy neutrino
detection. A solution to large area, low noise, high quantum efficiency, cost
effective optical detectors would be game changing across the field.

###  3.4 Prospects for future measurements

####  3.4.1 Water Cherenkov detectors

Super Kamiokande

Super-K has benefited from a number of upgrades over its lifetime. The most
recent of these is the addition of gadolinium, a project known as SK-Gd, which
will enhance neutron capture efficiency [59, 60]. This can improve the
separation of solar neutrinos from radioactive background from cosmic ray
induced spallation, improving Super-K’s sensitivity to the day/night effect
and the 8B spectral shape.

Hyper Kamiokande

The sheer size of the Hyper-Kamiokande (Hyper-K) experiment will make it a
powerful tool for solar neutrino measurement. The ability to reconstruct the
charged particle’s direction also offers a robust signal/background
discriminant. Construction and operation is expected to begin this decade
[61]. At over 250 ktonne total mass, with 40% coverage, Hyper-K has the
potential to contribute a great deal to the picture of high-energy solar
neutrinos, in particular.

Hyper-K will detect ES of 8B neutrinos, offering sensitivity to the spectral
shape and the day/night effect. The former in particular is a statistically-
limited measurement, making Hyper-K the perfect tool to provide improved
precision. Within 10 years of operation, Hyper-K will be able to measure the
day/night asymmetry to better than 4 (8) $\sigma$ at the values currently
predicted by reactor (solar) experiments.

Sensitivity to non-standard interactions via a spectral measurement of 8B in
the vacuum-matter transition region will depend on the energy threshold
achieved. At 3.5 (4.5) MeV, 5 (3) $\sigma$ sensitivity is predicted to the so-
called “MSW upturn” predicted by the standard picture of oscillations.
Deviation from that predicted shape could suggest the presence of NSIs or
sterile neutrinos.

The high statistics available to Hyper-K will make it possible to observe
short time-period variations in the solar neutrino flux, resulting in
something akin to real-time monitoring of the solar core, and may also
facilitate the first significant detection of _hep_ neutrinos.

####  3.4.2 Liquid scintillator detectors

SNO+

SNO+ is a currently operating, kton-scale LS detector. Located in SNOLAB in
Ontario, Canada, SNO+ benefits from one of the deepest sites available for
low-background studies, at 6 km water equivalent. This results in incredibly
low cosmogenic backgrounds, in particular the 11C that can be a limiting
factor in precision low-energy solar neutrino measurements. The primary goal
for SNO+ is a search for neutrinoless double beta decay (NLDBD) via loading of
the LS with tellurium, but during the current pure LS operations, SNO+ will
have sensitivity to solar neutrinos, in particular the 8B neutrinos [62].
Preliminary LS data from SNO+ shows levels of radon daughters that, although
sufficient for the NLDBD target, may limit the low-energy solar neutrino
program. Precision measurements of this regime would require significant
reduction of these backgrounds, similar to that achieved during the first
period of Borexino operations [41]. This could potentially be addressed by
methods that would include recirculating through the SNO+ scintillator
processing systems, built with several purification capabilities, along with
other background reduction techniques.

Early data from the initial water phase of SNO+ have already demonstrated low
levels of cosmogenic and external background, allowing a measurement of the 8B
spectrum in water [63]. Sensitivity to CNO neutrinos can be estimated under
certain assumptions for background reduction. With background reduction of
approximately a factor of 10 for the U- and Th-chains and 1000 for 210Bi,
negligible 40K, and constraining the _pep_ flux based on the _pp_ flux, as was
done by Borexino in their discovery paper, SNO+ could achieve better than 15%
precision on the CNO flux, which could be sufficient to resolve the solar
metalicity question.

SNO+ will also offer improved precision in $\Delta m^{2}_{21}$. Sensitivity to
this parameter using both solar neutrinos and reactor neutrinos will provide
additional data to resolve current (small) discrepancies in measurements from
solar and terrestrial sources.

JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) is under construction in
China, with a 20-kton target mass and a goal of 3% energy resolution at 1 MeV
[64, 65]. Although the primary goal is a precision reactor neutrino
measurement, the 700-m overburden and planned radiopurity mean JUNO will have
a rich physics program, including certain solar neutrino measurements. The
relatively shallow site will limit the low-energy solar neutrino program, but
the large size and precision resolution will offer excellent sensitivity to
both 7Be and 8B neutrinos, allowing some insights into solar metalicity. At
the target background levels, a threshold of 2 MeV could be achieved for 8B
neutrinos. This is substantially lower than what is possible in a Cherenkov
detector, and 1 MeV lower than achieved by Borexino for a measurement of the
8B spectral shape [66]. This spectral measurement gives JUNO sensitivity to
non-standard interactions, and 2 (3) $\sigma$ sensitivity to the day/night
effect at current reactor- (solar-) favoured parameter values [67]. With the
ability to measure $\Delta m^{2}_{21}$ to percent-level precision from reactor
neutrinos, and to approximately 20% using solar neutrinos, JUNO will provide a
uniquely precise cross check on the consistency of data from these two
sources.

LiquidO The use of opaque scintillator allows confinement of the light from a
neutrino interaction close to the original interaction point [68]. Combined
with fibre instrumentation, this results in an event imaging capability that
can enhance background rejection even at low energies. The clean signatures of
different particle types allow for separation of electrons from gammas,
offering excellent rejection of radioactive background for solar neutrino
detection. The possibility of doping also opens up the option of charged
current detection, for example on indium, allowing for excellent spectral
sensitivity [69].

####  3.4.3 Hybrid optical detectors

So-called “hybrid” optical neutrino detectors seek to leverage the combined
benefits of Cherenkov and scintillation light in a single detector.

Slow fluors By deploying certain fluors in combination with LS, the time
profile of the scintillation can be delayed [70, 71], thus enhancing the
ability to separately identify Cherenkov photons, and to reconstruct direction
for low-energy interactions. This improvement must be balanced against
potential degradation in vertex reconstruction due to reduced precision in
photon time-of-flight information. However, optimisation can lead to 10%
sensitivity to the CNO neutrino flux from a few kton-yr exposure in a high
light yield LS detector.

Theia

Theia is a proposed large hybrid optical neutrino detector, capable of
leveraging both Cherenkov and scintillation light in a single detector. Theia
is a realisation of the Advanced Scintillation Detector Concept first proposed
in [72]. This concept offers a number of benefits, including directional
sensitivity at low threshold, combined with the good vertex and energy
resolution of a scintillator detector, as well as further handles on particle
and event identification – in addition to the PSD-based PID traditionally used
in LS detectors – from the Cherenkov / scintillation ratio. Theia would
combine a novel LS target - such as water-based LS (WbLS) or a slow
scintillator, with fast, high efficiency photon detectors and potential
spectral sorting using technology such as dichroicons [73] to optimize this
hybrid detection capability. The proposed site at the Sanford Underground
Research Facility (SURF) laboratory in SD, USA would offer a high-energy
neutrino programme as part of the Long Baseline Neutrino Facility, as well as
a broad program of low-energy physics. Although pure LS offers improved
radiopurity, directional sensitivity would at least in part offset the
increased levels of contamination inherent in the water component of a WbLS
target. Theia offers better than 10 (1)% precision on the CNO solar neutrino
flux with a WbLS (pure LS) target [74, 75, 76], allowing a high-confidence
resolution of the metallicity problem, as well as sensitivity to the shape of
the low-energy 8B spectrum, providing sensitivity to non-standard effects in
the vacuum-matter transition region. Possible isotope loading is also being
explored to offer a CC interaction, which would provide a high fidelity
measure of the underlying neutrino spectrum, potentially offering enhanced
sensitivity both to CNO neutrinos, and to the 8B spectral shape.

The power of hybrid detection means that even a relatively small, few-hundred
ton fiducial detector can achieve enticing reach if filled with a high light
yield, ultra clean scintillator, as explored in [76]. With current technology:
linear alkyl benzene (LAB) with 2 g/L of PPO, and instrumented with standard
photomultiplier tubes, at 1.6-ns transit time spread, such a detector could
achieve 14% precision on the CNO neutrino flux, dropping below 10% if a
constraint is imposed on the pep flux based on knowledge of the pp flux, as
done by Borexino in their discovery paper [46]. With fast photon detectors,
such as LAPPDs [77], the precision is below 5%.

Other underground LS detectors A few-kton hybrid LS detector is under
consideration for deployment at the China Jinping Underground Laboratory
(CJPL). The 2.4-km rock overburden results in a cosmic-ray muon flux almost as
low as that at SNOLAB [78]. Detector sensitivity depends on the final
configuration, with options from 1- to 4-ktonne in fiducial mass, and 200 to
1000 photoelectron/MeV light collection under consideration [79]. The optimal
detector would achieve percent-level measurements of _pp_ , 7Be, and 8B
neutrinos, and a few-percent on the _pep_ flux. A CNO measurement could reach
better than 15% precision for the larger, high-resolution detector
configurations. The experiment would have good sensitivity to the 8B spectral
shape. A 100-ton prototype is planned for the middle of this decade.

A 20-m right cylindrical detector has been proposed in YemiLab, in Korea, and
a funding proposal is currently under consideration, with a White Paper under
preparation. Such a detector would consider of over 2 kton of LS, with large-
area PMTs for photon detection. This detector would interrogate a number of
low-energy sources, including solar neutrinos.

A new underground laboratory is being developed in the Agua Negra tunnel:
ANDES (Agua Negra Deep Experiment Site) between Argentina and Chile. This
would be the first deep underground laboratory in the southern hemisphere, and
could provide significant opportunities for deep underground experiments, such
as precision solar neutrino searches.

####  3.4.4 Noble liquid and solid state detectors

Nuclear recoil detectors Experiments designed to search for coherent neutrino-
nucleus elastic scattering, known as CE$\nu$NS [80, 81], can offer sensitivity
to higher-energy solar neutrinos via this channel. The cross section for
CE$\nu$NS interactions is favourable due to an A2 dependence; however, the
nuclear scatters typically fall below 10 keV, requiring detectors with
excellent resolution and very low threshold. CE$\nu$NS offers precise flux
measurements for the higher energy fluxes, and potential spectral sensitivity
across the range from vacuum- to matter-dominated oscillation, giving
sensitivity to possible active-to-sterile mixing [82].

Technologies include scintillating crystals, germanium semiconductors, and
noble liquid detectors. Some of these offer parallel sensitivity to an ES
signal as well. The COHERENT suite of detectors will include several of these,
and will make measurements of CC and NC cross sections of relevance for future
CE$\nu$NS detection of solar neutrinos. A more detailed understanding of these
interactions will be important for interpretation of data from these future
experiments.

Ultra low-threshold detectors such as SuperCDMS could potentially detect the
low-energy branches, such as _pp_ neutrinos, if thresholds in the few-eV range
can be achieved [83, 84]. However, equaling the precision of existing
measurements from Borexino would require ton- to multi-ton-scale exposure
[85].

CYGNUS is a proposed gaseous He / SF6 TPC, which would have directional
sensitivity to nuclear recoils. A 1000 m3-scale experiment could detect 10s of
solar neutrinos, with a large detector being needed for precision
measurements.

Noble liquid bubble chambers offer low threshold sensitivity to nuclear
recoils and, to-date, relative insensitivity to electron recoils, providing
good background discrimination even at low, sub-keV energies. This technology
could be used for CE$\nu$NS detection of solar neutrinos down to a threshold
of 1.4 MeV. A 10-kg experiment will be deployed at SNOLAB as a demonstration.
A future, ton-scale or larger detector could have sensitivity to detect up to
40 NC events from 8B interactions per ton-yr.

Liquid xenon A Generation-3 liquid xenon (LXe) dark matter (DM) detector, such
as DARWIN or a future PandaX upgrade, will have sensitivity to solar neutrinos
via both nuclear and electron scattering. Such a detector combines an
extremely low threshold and good radiopurity, with excellent PSD for
discrimination of electron and nuclear interactions. This offers a broad
program of solar neutrino physics: precision _pp_ flux measurement via ES;
1.4% precision on a measurement of $\sin^{2}\theta_{W}$; flux and spectral
measurement of 8B neutrinos, for sensitivity to NSI and sterile neutrinos; a
measurement of the CNO neutrino flux, if the $\nu\nu\beta\beta$ background can
be reduced by 3 orders of magnitude; and sensitivity to _hep_ neutrinos via
CE$\nu$NS, if resolution can be improved in the 1–3 keVnr range.

The PandaX observatory currently consists of a 4-ton LXe TPC, located in the
CJPL. With anticipated low background in the range from keV to 10 MeV, a
future upgrade with a 30–100 ton target mass could perform a precision
measurement of the _pp_ solar neutrino spectrum, allowing a sensitive search
for an anomalous neutrino magnetic moment. This detector could also observe
coherent nuclear scattering of 8B neutrinos, offering sensitivity to NSIs.

DUNE, and other liquid argon TPCs The Deep Underground Neutrino Experiment
(DUNE) will deploy a number of liquid argon (LAr) TPC modules at the Sanford
Underground Research Facility, to view the neutrino beam from Fermilab. While
the primary goal is to search for CP violation, this detector may also offer
sensitivity to higher-energy solar neutrinos. Although it may be limited by
intrinsic radioactivity in the detector, DUNE could offer improved
measurements of $\Delta m^{2}_{21}$ as well as measurements of the 8B and
_hep_ fluxes.

A DUNE-like module with a focus on achieving low background, via underground
argon, enhanced radiopurity requirements, additional shielding, and improved
coverage and readout for enhanced in-situ background discrimination – would
expand the solar neutrino potential to include _hep_ neutrinos, and lower-
energy fluxes such as _pp_ neutrinos [86]. Sensitivity to solar oscillation
parameters and the shape of the survival probability, via spectral
measurement, would allow searches for NSIs, as well as to address the current
mild tension between solar and terrestrial measurements of oscillation
parameters. Some studies consider the use of the Cherenkov signal in LAr as a
handle for background discrimination, which could enhance detection of the
solar ES signal. Sufficient reduction of 42Ar might even make a measurement of
CNO feasible in such a detector [87].

The Global Argon Dark Matter Collaboration (GADMC) plans to deploy purified
LAr in a range of TPCs for DM detection. Solar neutrino sensitivity is offered
via both CE$\nu$NS and CC interaction on argon. A 1500 ton-yr exposure would
allow collection of several thousand CNO neutrino interactions, along with
6000 _pep_ events and 16000 from 7Be. Flux measurements will require
improvements in radon reduction by 3 orders of magnitude beyond that achieved
in smaller experiments [88]. CE$\nu$NS measurements of 8B and, if use of
underground argon allows the threshold to be pushed below 200 eVnr, _pep_ and
CNO neutrinos will also be accessible.

Selena

Designed as a search for neutrinoless double beta decay, the Selena experiment
could also offer sensitivity to solar neutrinos via electron neutrino capture
on 82Se [89]. Selena will detect ionization on a pixelated array read out via
CMOS. Track reconstruction allows for particle identification, and spatio-
temporal correlation allows for identification of coincidence decays even over
extremely long half lives. This strategy targets background-free measurements
of solar neutrinos. The 172 keV threshold for electron neutrino capture offers
sensitivity to all branches of solar neutrinos. The resulting signal offers a
three-fold coincidence that results in negligible accidental background and,
to-date, no identified background isotope that can mimic the particular decay
sequence. A 100 ton-year exposure would provide 1% precision on _pp_
neutrinos, 8% for _pep_ , and 10% on the CNO neutrino flux. Measurement of the
energy of 7Be neutrinos, and of the predicted shift from the energy of the
equivalent laboratory decay, offers sensitivity to the solar core temperature
at the 25% level.

####  3.4.5 Other concepts

High energy solar neutrinos High-energy neutrinos can be produced from dark
matter (DM) annihilation, and from interaction of cosmic rays with the solar
atmosphere. To-date, both searches have yielded null results, but include
highly-competitive limits on the spin-dependent DM-nucleon cross sections for
DM with mass above $\approx$100 GeV. These searches are statistically limited,
and larger next-generation detectors, including Baikal-GVD, Hyper-Kamiokande,
IceCube Gen2, and KM3Net can add to the picture. First detection of solar
atmospheric neutrinos is within the reach of next-generation detectors, and
would allow neutrino telescopes to explore new baselines, and matter
densities. High-energy neutrinos can also be produced in solar flares, but
have yet to be detected.

The idea of deploying a neutrino detector on a spacecraft offers scope for a
smaller detector at shorter baselines, and the ability to vary the point of
observation. Such a project would be costly, and would need to travel
extremely close to the Sun to achieve sufficient detection rates.

##  4 Geoneutrinos

The reader is referred to [90] and [91] for a comprehensive description of the
geological questions and associated experimental program in this area. The
following section attempts to provide an overview of the current status of the
field, with a focus on prospects in the next decade.

###  4.1 Motivation

Radioactive decay of naturally occurring uranium, thorium and potassium in the
Earth accounts for over 99% of radiogenic heat production. These decays
produce a flux of antineutrinos, so-called “geoneutrinos”, that is
proportional to the heating power. Measurements of geoneutrinos offer us the
ability to assay the earth, to probe radiogenic heat production, and test
models of the Earth’s composition.

Questions that can be addressed using geoneutrinos include:

  * •

Understanding the contribution of radiogenic (U, Th) heat production to heat
flow and energetics in the deep Earth.

  * •

Understanding the degree to which mantle convection is driven by radiogenic
heat.

  * •

Testing models of the crust composition.

  * •

Potentially, probing the distribution of reservoirs in the mantle, the nature
of the core-mantle boundary, and the existence of radiogenic elements in the
core.

Measurements of geoneutrinos can shed light on these questions by measuring
the total geoneutrino flux, and the U/Th ratio, inferred from a spectral fit.
Geographical variations in the flux and U/Th ratio are also of great interest,
since these can be used to constrain the relative contributions from the crust
and mantle independently from model constraints.

###  4.2 Current understanding: Borexino and KamLAND

Two experiments to date have successfully measured the geoneutrino flux: the
KamLAND experiment in Japan, and Borexino in Italy. Both experiments detected
the inverse beta decay (IBD) of a electron antineutrino on a proton, yielding
a positron and neutron. The coincidence signature allows for suppression of
many sources of background, and the high light yield of the chocen liquid
scintillator (LS) technology allows for a spectral fit, which can be used to
extract limits on the U/Th ratio.

Results from these experiments have demonstrated the technique, and yielded
results consistent with model predictions, albeit with large uncertainties.
The precision is not yet sufficient to determine the U/Th ratio. Additional,
and more precise measurements will be important to facilitate extraction of
geological results in the future.

KamLAND was built to detect antineutrinos produced from nuclear reactors, and
has a long history of successful antineutrino physics measurements.
Backgrounds to a geoneutrino search are dominated by reactor antineutrinos,
due in part to the overlap in the energy spectra of the two signals. The
comprehensive unbinned maximum likelihood analysis also takes into account
cosmogenic backgrounds, fast neutrons, atmospheric neutrinos, ($\alpha$,n)
reactions, and accidental coincidences [92, 93]. The oscillated electron
antineutrino flux from the sum of 238U and 232Th measured at the Earth’s
surface was found to be $3.6^{+0.6}_{-0.6}\times 10^{6}$cm2s-1. The number of
geoneutrinos observed was $168.8^{+26.3}_{-26.5}$. This 15.6% uncertainty is a
result of fixing the Th/U mass ratio in the fit. The absence of geoneutrinos
is rejected at over 8$\sigma$. Full details of the analysis, and results for
the case of an unconstrained Th/U ratio can be found in [90].

The Borexino experiment has executed an extensive program of low-energy solar
neutrino measurements, thanks in part to its excellent levels of radio purity,
and well understood detector response. These features also support an
antineutrino program, and Borexino has successfully measured the geoneutrino
flux at LNGS in Italy. As for KamLAND, the dominant background is from reactor
antineutrinos, the flux of which is fit for in the analysis and found to be
consistent with expectation. Over 3262.74 days of data, a signal of
$52.6^{+9.6}_{-9.0}$ geoneutrinos was observed from the sum of 238U and 232Th.
The resulting precision is approximately 18%, with a constrained Th/U mass
ratio.

These results can also be interpreted in the context of various geological
models. By constraining the crust contribution, the null hypothesis of
geoneutrinos from the mantle can be excluded at 99% C.L. The resulting signal
can be converted into a radiogenic heat of $24.6^{+11.1}_{-10.4}$ TW from 238U
and 232Th in the mantle. Combining this with certain values from model
predictions, the total radiogenic heat of the Earth can be estimated, and
compared to a range of model predictions. Although consistent with all models
to within 3$\sigma$, a preference is found for models with relatively high
radiogenic power, corresponding to a cool initial environment at Earth’s early
formation stages, and small values of heat from secular cooling. By fitting
the data with a constraint on the reactor antineutrino background, the
existence of a hypothetical georeactor at the center of the Earth having power
greater than 2.4 TW at 95% C.L. can be excluded. A comprehensive description
of this analysis can be found in [94], and a summary in [95].

###  4.3 Prospects for future measurements

Future measurements of geoneutrinos will seek to increase statistics, add data
points at new geographical locations, and improve precision in order to
enhance sensitivity to the Th/U ratio via a spectral analysis. [90] provides a
comprehensive overview of several forthcoming projects. The following
summarises what each project could achieve for geoneutrino measurement.

It is important to emphasise that significant power comes from combined
analyses of data from experiments at different geographical locations [96].
Each experiment needs the local crust geology to be characterised, such that a
combined analysis yields the common mantle component, or becomes sensitive to
potential lateral inhomogeneity in the mantle component. It will be important
to support effort towards such analyses in order to maximally benefit from
forthcoming experiments.

####  4.3.1 SNO+

SNO+ is an operational LS experiment in Sudbury, Canada. At 780 tons of pure
LS, SNO+ can expect to observe $25--30$ geoneutrino events per year, with a
relatively low reactor background: approximately 1:1 in the region of
interest. The local geology has been extensively characterised, and the region
is predicted to have a higher natural flux than at either KamLAND or Borexino,
making SNO+ a useful component in a future global analysis.

####  4.3.2 JUNO

The JUNO experiment is under construction in China, with a planned 20-kton LS
volume and excellent energy resolution, of 3% at 1 MeV, in order to target a
hierarchy measurement from a precision measurement of the oscillated reactor
neutrino spectrum. The predicted geoneutrino signal is approximately 400
events / yr, with a reactor background of approximately 8 times that in the
relevant region of interest. This should allow for a 5% precision measurement
of the geoneutrino flux with 10 years of data, giving approximately 30%
uncertainties on the Th/U ratio [97].

####  4.3.3 Ocean Bottom Detector (OBD)

The crustal component of the geoneutrino flux dominates all measurements to
date. The oceanic crust is thin, with low Th and U abundances, allowing
greater sensitivity to the mantle contribution. A detector in the ocean could
meet many of the needs of a geoneutrino program. Such a detector could be
moved to multiple locations, and would observe a signal dominated by the
mantle contribution. With appropriate detector technology, this detector could
measure the total radiogenic heat in the Earth, the Th/U ratio, distinguish
the mantle contribution, and potentially resolve flux differences at different
locations. Originally conceived of as the Hanohano experiment, an OBD could be
a powerful instrument for physics, offering a broad program of low-background
astro-particle physics complementary to the geoneutrino measurement [98]. A
prototype liquid scintillator detector has been funded, as part of a broader
program to develop and deploy a multi-kton detector, and efforts are ongoing
to deploy this detector on the sea floor. Monte Carlo studies show that a
1.5-kton scale detector could achieve better than 3$\sigma$ on a mantle
geoneutrino measurement with 3 years of data [99].

####  4.3.4 Theia

Theia is a proposed large-scale (10s of kton) “hybrid” optical neutrino
detector, capable of leveraging both Cherenkov and scintillation light
simultaneously [75]. The addition of the scintillation component improves
energy and vertex reconstruction and, critically for a geoneutrino
measurement, allows a lower threshold than a traditional water Cherenkov
detector. However, the ability to detect the Cherenkov signal offers some
handle on direction reconstruction, and additional particle identification –
beyond the PSD-based PID traditionally used in LS detectors – from the
Cherenkov / scintillation ratio. These features can improve background
rejection. Theia would offer a very high statistics measurement in the
continental US, with the potential to extract the Th/U ratio from a spectral
fit.

####  4.3.5 Other underground LS detectors

The Jinping underground laboratory in China offers an extremely deep
environment for low-background and rare event searches. A few-kton liquid
scintillator experiment is being designed for this space, which could offer
good sensitivity to the geoneutrino flux. Although to-date there is no US
involvement in this effort, it can contribute to the global picture of
geoneutrino measurements, and US efforts can benefit from this
complementarity.

A future detector at YemiLab in Korea, or in the proposed ANDES site in South
America, could also offer the potential for measurements of geoneutrinos.

####  4.3.6 The search for 40K geoneutrinos

Below the threshold for IBD detection, 40K geoneutrinos are expected to
contribute significantly to the overall flux, but offer a unique challenge for
detection. One approach involves ES detection in a Cherenkov-sensitive LS
detector (a so-called hybrid detector), such as those discussed above – Theia,
JUNO, Jinping, or another kton-scale or larger detector [100].

####  4.3.7 LiquidO

The LiquidO concept involves a highly instrumented volume of opaque
scintillator, designed to confine light close to the interaction point, thus
preserving information about the vertex. This imaging capability offers a
degree of PID that enhances background rejection even at low energies. The
possibility of doping also opens up new windows, such as a charged current
detection of geoneutrinos, with a lower threshold than IBD, potentially
allowing detection of the 40K signal.

####  4.3.8 Coherent neutrino-nucleus scattering

Coherent elastic neutrino-nucleus scattering allows sensitivity to low-energy
neutrinos. A diverse array of detector technology can be employed for this
purpose, from scintillating crystals, to germanium semiconductors, to noble
liquid detectors. This technique may offer sensitivity to the geoneutrino
flux, although such detectors would need to combat the large background from
solar and reactor neutrinos, and the energy of recoils from the geoneutrino
signal would be extremely low [101]. Nuclear recoil detectors with directional
sensitivity, such as CYGNUS, could offer an additional handle for this [102].

####  4.3.9 6Li-doped scintillator

6Li doping of organic scintillators offers many benefits for antineutrino
detection, including excellent PSD for n/g discrimination, large neutron
capture cross section, and high light yield. Improved precision in the IBD
detection from such techniques could enhance directional sensitivity, which
would potentially allow for separation of different components of the
antineutrino signal.

##  5 Supernova Burst Neutrinos and Diffuse Supernova Neutrino Background

###  5.1 Motivation and current understanding

Core-collapse supernovae stem from collapsing massive stars, during the
collapse about $10^{58}$ neutrinos are emitted for $\mathcal{O}(10)$ s with
average energy of $\mathcal{O}(10)$ MeV [1, 103, 104, 105]. Being produced so
abundantly, neutrinos play a crucial role in supernovae, transporting energy
and lepton number and giving insight into the extreme environment within the
interior of the progenitor star.

The majority of supernovae are assumed to explode because neutrinos revive the
stalled shock wave to finally trigger the explosion. While major progress has
been made in the context of hydrodynamical simulations of the core collapse
[106, 104, 105], flavor mixing is neglected in the modeling of neutrino
transport in hydrodynamical simulations. In addition, magneto hydrodynamical
simulations are yet to include sophisticated neutrino transport.

The supernova neutrino signal contains an imprint of both the supernova
environment, with extreme conditions that we cannot recreate in the
laboratory, and the properties of the neutrinos themselves. Neutrinos free-
stream from the supernova, acting as an early warning for electromagnetic
telescopes with advanced notice between minutes and days [107]. This early
warning will enable observations of the first light from the shock breakout,
carrying important progenitor and explosion properties, such as the radius and
surface composition, as well as the mass-loss history [108].

The detection horizon for the burst of neutrinos from a core-collapse
supernova with current detectors is limited to our own galaxy and the
surrounding satellite galaxies. Hyper-Kamiokande will extend the horizon out
to Andromeda, our neighboring galaxy. Because we are limited to nearby stars,
the rate of core-collapse supernovae from which we can expect to detect
neutrinos is around a few per century [109]. It is therefore imperative to
have a continuous observing campaign using neutrino detectors with high uptime
in order to capture this once-in-a-lifetime event.

####  5.1.1 Neutrino flavor mixing

In order to extract the physics from the supernova signal, theoretical
progress is needed. Despite huge advancements occurring in the past decade
concerning the development of multi-dimensional hydrodynamical simulations
with sophisticated transport of neutrinos [103, 105, 106, 104], our
understanding of neutrino flavor conversions in supernovae remains preliminary
[103, 110, 19, 111]. In fact, because of the large density of particles, the
coherent forward scattering of neutrinos onto each other is responsible for
making the flavor evolution non-linear; moreover, the latter is crucially
affected by the neutrino angular divergence. A relatively recent development
concerns the occurrence of flavor mixing triggered by the pairwise scattering
of neutrinos in the proximity of the weak decoupling region in the supernova
core [19, 111, 112]. Because of the numerical complications intrinsic to the
modeling of this non-linear phenomenon, a full modeling of flavor mixing is
lacking; hence, the consequences of neutrino mixing on the observable neutrino
signal remain to be clarified as well as its effect on the explosion dynamics
itself.

####  5.1.2 Neutrinos as probes of the supernova physics

Despite the uncertainties related to the neutrino physics, the neutrino signal
could be a crucial probe of the supernova physics taking place just before the
explosion [113, 114, 115, 116]. Neutrinos are also emitted in the latest
stages preceding the stellar collapse [117, 118]; their detection for nearby
core collapses will be crucial to inform astronomers about the upcoming
supernova burst and to provide insight on the pre-collapse phase. In addition,
the neutrino signal can be adopted to optimize the time window for the search
of gravitational waves [119, 120] and will be a crucial observable in the case
of black hole forming collapses [121]. The long term neutrino emission during
the cooling phase may help to extract information on the nuclear equation of
state and neutron star formation as well as the nucleosynthesis [122, 123,
124, 125].

####  5.1.3 Supernova pointing and early warning through neutrinos

Neutrinos from a supernova will arrive on Earth minutes to days before the
photons, and therefore will provide a valuable early warning for multi-
messenger followup. The direction of the supernova can be extracted using the
electron elastic scattering detection channel, which can preserve the
direction of the incoming neutrino. Super-Kamiokande has the best prospect for
this measurement due to its large volume and because detection via the
Cherenkov light preserves direction. The expected resolution in Super-
Kamiokande is between $\sim$ 3 and 6 degrees for a supernova at 10 kpc. As
more detectors come online in the near future, using triangulation to compare
the arrival times of the signal in detectors around the globe can yield
directional constraints. In [126, 127], the arrival of the early signal to
triangulate the direction was found to constrain the direction to a few
percent of the sky in the most favorable case. Fitting the shape of the
neutrino curve can further improve the uncertainty of arrival time in
detectors [128].

Measuring neutrinos preceding collapse can give advance warning to neutrino
and gravitational wave detectors that the burst of supernova neutrinos is
imminent. These neutrinos are lower in energy and therefore liquid
scintillator experiments are ideally suited for this measurement. Sensitivity
to these presupernova neutrinos was first demonstrated by Kamland [129]. The
recent addition of Gadolinium to Super-Kamiokande has also allowed for a
reduced energy threshold and a corresponding sensitivity to the presupernova
signal [130]. Future large-scale dark matter experiments are also expected to
have sensitivity [131].

####  5.1.4 Neutrinos as probes of physics beyond the Standard Model

In the light of current uncertainties on the physics of neutrino flavor
conversion and related degeneracies with the supernova properties (e.g., its
mass, nuclear equation of state, etc.), the neutrino signal from the next
nearby supernova burst is unlikely to provide smoking gun signatures on the
mass ordering of neutrinos and mixing parameters. However, New Physics may
largely modify the detectable neutrino signal as well as the supernova physics
itself. Of special interest in this context are New Physics scenarios directly
linked to neutrinos or leading to lepton number violation [132, 133, 134, 135,
136, 137, 138, 139, 140].

####  5.1.5 Current experimental landscape

Current neutrino detectors will be able to detect a burst of neutrinos from a
core-collapse supernova if sufficiently nearby, in our galaxy or the
surrounding Magellanic clouds. The time-dependent rate of supernova neutrinos
will allow us to produce a neutrino lightcurve, on which will be imprinted the
details of the explosion and the dense environment. The statistics for the
lightcurve will be driven by water and in-ice Cherenkov detectors, which
primarily detect supernova neutrinos via inverse beta decay. The large
instrumented volume of IceCube results in the best statistics to measure the
neutrino lightcurve for a supernova in our galaxy [141]. Super-Kamiokande,
will measure supernova neutrinos on an event-by-event bases allowing not only
a study of the neutrino lightcurve but also the time-evolution of the neutrino
energy [142].

Flavor-independent measurements of supernova neutrinos are of great importance
to disentangle the complicated flavor-mixing effects arising in supernovae.
Liquid scintillator detectors such as SNO+ or JUNO may have sensitivity to the
neutral current interaction on Carbon. Detectors using heavy nuclei such as
lead, including the currently-operating HALO experiment, would expect a modest
number of interactions via neutral current interactions. A particularly
interesting development in the detection context concerns the employment of
coherent neutrino nucleus scattering [143]. In recent years, there has been
growing attention on the possibility of detecting supernova neutrinos through
direct detection dark matter experiments [144] as well as proposals to build
neutrino telescopes exploiting this detection channel [145].

###  5.2 Future Prospects

####  5.2.1 Prospects for theoretical developments

As existing and upcoming neutrino observatories prepare for the next supernova
burst, the theory community should progress in the modeling to be ready to
take full advantage of the precious information carried by the supernova
explosion. The fact that neutrinos through their flavor mixing could radically
affect the transport of energy and lepton number in the supernova core implies
that further attention should be devoted to improve our understanding of the
behavior of neutrino mixing in dense media and especially of fast pairwise
conversion. The physics of flavor mixing in the proximity of supernova shocks
remains to be clarified [146] as well as the role of turbulent fluctuations
[147]. In addition to an improved understanding of neutrino physics, the self-
consistent modeling of New Physics scenarios and an improved understanding of
the impact of such scenarios on the core-collapse physics will be crucial to
place robust constraints in the near future. One of the goals on the
theoretical front would be an end-to-end consistent simulation of the burst
physics and related detectable signatures. A consistent modeling of the
neutrino signal in the neutrino telescopes and a good understanding of the
detector background are also necessary.

Within the multi-messenger scenario, neutrinos are one of messengers. In order
to maximize the amount of information that can be extracted from the other
messengers, it is important to pin down all the uncertainties on the expected
neutrino signal. In particular, the determination of the angular location of
the supernova burst in the sky and the employment of neutrinos for timing are
also affected by existing uncertainties on flavor mixing [127].

As we approach the DSNB detection, improved theoretical models and better
understanding of the detector backgrounds are absolutely needed [148]. In
order to maximally exploit the DSNB detection, it is crucial to progress on
the theoretical modeling of the supernova population and neutrino physics
[149, 150, 151, 152]; in particular, large theoretical uncertainties are
linked to the supernova rate and the role of binaries in the evolution of
massive stars.

####  5.2.2 Prospects for measurements beyond electron anti-neutrino

Current detectors mainly measure core-collapse supernova neutrinos via the
inverse beta decay channels, and are therefore mainly sensitive to electron
anti-neutrinos. Detectors which can measure the neutral current channel will
play an important role in making a measurement independent of oscillation
effects, but will be low statistics. Next-generation dark matter detectors
will view neutral current interactions with high enough statistics to extract
oscillation-independent effects.

DUNE will measure the electron neutrino channel with high statistics and low
backgrounds. This will enable a measurement of the neutronization peak, a
feature only visible in electron neutrinos, which carries information about
neutrino ordering and can be used to calibrate the distance to the supernova.
The THEIA detector, which is proposed to sit at the same site as DUNE, has the
potential to act as a supernova trigger for DUNE thanks to the fast timing and
lower threshold, which gives greater burst sensitivity, thus extending the
DUNE detection horizon.

####  5.2.3 Expanding the detection horizon for core-collapse supernovae

The expected rate for a supernova to occur within our current detectable
volume is only a few per century. The next big frontier in supernova neutrino
detection will be to expand the detection horizon to include more galaxies. As
the neutrino event rate scales with the inverse squared of the distance, this
is a significant challenge. Hyper-Kamiokande will be large and essentially
background-free, and so has good prospects to detect supernova bursts out to
Andromeda.

####  5.2.4 Early warning for multi-messenger physics

The SuperNova Early Warning System (SNEWS) is a global network of neutrino
observatories formed in 2000 to report the detection of core-collapse
supernovae across multiple detectors. Neutrinos arrive at Earth prior to the
optical signal from the core-collapse explosion. Thus, neutrinos provide the
possibility to alert observatories of an imminent supernova explosion even
before it is visible in the sky; this is the challenge and purpose of the
SNEWS network. It includes all sensitive neutrino detectors including Super-K
(Japan), IceCube (South Pole), KamLAND (Japan), KM3NeT (Mediterranean), and
HALO (Canada). SNEWS2.0 is a follow-up effort that will play a crucial role in
maximizing the potential for multi-messenger observations in the next core-
collapse supernovae. SNEWS2.0 will improve the alert system by including more
information about the supernova, such as the direction, as well as an alert
for pre-supernova neutrino emission. Future neutrino detectors Hyper-K, JUNO,
SNO+, NOvA, Baksan, Darwin, Darkside, DUNE, nEXO, XENONnT and LZ plan to
connect to SNEWS2.0.

####  5.2.5 Measuring the diffuse supernova neutrino background

The ongoing enrichment of the Super-Kamiokande detector with Gadolinium and
the upcoming JUNO detector provide very promising prospects for discovery
[103, 153, 154]. Other proposed large-scale liquid scintillator or hybrid
detectors, such as THEIA, would also be sensitive to the DSNB signal [155]. In
addition, the employment of coherent neutrino nucleus scattering detectors
could provide upper limits on the non-electron flavors [156]. Hyper-Kamiokande
will move beyond discovery, towards precision measurements of the DSNB. This
can allow for a measurement of the average neutrino energy, as well as the
fraction of failed supernovae that end as black holes [157].

##  6 Atmospheric Neutrinos

###  6.1 Motivation and current understanding

Atmospheric neutrinos are produced by collisions of primary cosmic rays with
nuclei in the atmosphere, and by the decay of hadrons produced by these
collisions. The breakthrough discovery in atmospheric neutrinos was neutrino
oscillation by Super-Kamiokande in 1998[158].

Atmospheric neutrino detectors that are currently in operation and providing
results for neutrino oscillation are mainly water (including ice) Cherenkov
types such as Super-Kamiokande[159],IceCube[160], and ANTARES[161]. These
observations have led to precise measurements of neutrino oscillation
parameters, especially for $\theta_{23}$ and $\Delta m^{2}_{32}$. Recently,
the precise observations and large statistics of Super-Kamiokande have made it
possible to study all the sub-leading effects of neutrino oscillations. Here,
they show that the Earth-matter effect plays an important role in neutrino
oscillations, which resolves mass ordering, two possible $\theta_{23}$
regions, and $\delta_{CP}$. The results prefers normal mass ordering and 1st
$\theta_{23}$ octant and $\delta_{CP}\sim 3/2\pi$ although not with large
probability around one sigma level.

In addition to that, the significance of the appearance of tau neutrinos due
to atmospheric neutrino oscillations was reported to be $\sim 5$ sigma in
Super-Kamiokande[162] and 3.2 sigma in IceCube[163], and those results were
consistent. These experiments also reported the search for sterile neutrinos,
although no significant signal was found[164, 165, 161]. Such a precise
measurement of neutrino oscillation parameters and search for various
phenomena caused by neutrino oscillation are tasks to be achieved in future
atmospheric neutrino experiments.

Another topic includes the precise measurement of atmospheric neutrino fluxes.
In recent years, several results, both theoretical and observational, have
been reported. Several groups, such as HKKM[166], Bartol[167], and FLUKA
group[168], have made theoretical predictions. The differences in flux
prediction by these models comes from the choices of the hadron interaction
model and the measurement of the primary cosmic-ray spectrum, and is about
10%. On the other hand, atmospheric neutrino flux measurements have been
reported from several experiments, most recently from Super-Kamiokande[169],
IceCube[170, 171, 172], and ANTARES[173]. Their results are consistent with
the predictions with uncertainties. In addition, Super-Kamiokande also reports
the directional dependence of the atmospheric neutrino flux on the rigidity
cutoff and the long-term variability expected due to solar activity, which are
also consistent with the predictions. More accurate atmospheric neutrino flux
predictions and observations are also future tasks.

One more thing to be pointed out is that the atmospheric neutrino events are
background to nucleon decay searches (proton decay, neutron-antineutron
oscillation, etc.), and cosmogenic dark matter searches, thus, understanding
the atmospheric neutrino flux and interaction is important for those searches.

###  6.2 Prospects for future measurements

####  6.2.1 Prediction of atmospheric neutrino flux

Recently, two groups (Nagoya and Bartol) have repeated the HKKM and Bartol
flux calculations. Compared to the original version, the model is easier to
tune because the primary flux and hadron production data have been greatly
improved. The model improvements are motivated by various on-going and future
experiments. In this calculation, the uncertainties of the hadron interaction
is critical. This theoretical group will consider the beam experiment whose
kinematic region is important for atmospheric neutrino production. Various
beam experimental data already exist, but they are insufficient, therefore, it
is important to obtain the results of the currently planned experiments, such
as NA61/SHINE, EMPHATIC, protoDUNE-II and Forward Physics Facility at the LHC,
as soon as possible.

####  6.2.2 Neutrino oscillation measurements

##### Super-Kamiokande and Hyper-Kamiokande

As mentioned in the previous section, atmospheric neutrino measurements are
sensitive to mass ordering, the two possible $\theta_{23}$ regions, and
$\delta_{CP}$ due to the Earth-matter effect. There are several challenges,
such as flux uncertainties, neutrino/anti-neutrino separation, and
reconstruction of multi-particle final states. A new phase of gadolinium-doped
Super-Kamiokande began in 2020, which should help more effective separation of
neutrino/anti-neutrino.

In Hyper-Kamiokande[174], the sensitivity of these parameters becomes quite
good because of their huge statistics. For example, it is expected in ten
years observation to resolve the mass ordering at three sigma level for both
normal and inverted assumptions and when $\sin^{2}\theta_{23}>0.53$, determine
the $\theta_{23}$ octant at two sigma level when $|\theta_{23}-45|>4^{\circ}$,
and make it possible to realize a precise $\delta_{CP}$ measurement when
combined with the beam neutrino measurement. For the appearance of tau
neutrinos, more than 700 events are expected to be detected in 10 years of
operation. This will allow us to obtain a more accurate cross section
measurement of charged-current $\nu_{\tau}$ interaction. Hyper-Kamiokande is
expected to improve the bounds on various non-standard neutrino properties
obtained by Super-Kamiokande, such as the presence of sterile neutrinos, non-
standard neutrino interactions with matter, and Lorentz invariance violation,
by combining data from intermediate detectors several kilometers from the beam
and atmospheric neutrino observations.

##### IceCube

IceCube has the sensitivity to observe atmospheric neutrinos in a very wide
energy range, from the order of GeV to 100 TeV. The sensitivity over a wide
energy range enables not only precise measurement of ‘conventional’
atmospheric neutrinos, but also ’prompt’ neutrinos from the decay of heavy
hadrons which have not yet been discovered. The IceCube Upgrade improves the
resolution of neutrino energy and direction and lowers the energy threshold,
allowing especially for improved neutrino oscillation sensitivity in
atmospheric neutrinos. In particular, the measurement of the matter effect of
atmospheric neutrinos by IceCube Upgrade, combined with the JUNO experiment,
determines the mass ordering with a sensitivity of 5 sigma after 3$\sim$7
years of observation[175]. In addition, the atmospheric neutrino high-energy
region is well suited for searching for various BSM physics, such as sterile
neutrino, Lorentz violations, non-standard interactions, and future IceCube
experiment is expected to provide a lot of knowledge into these physics.

##### DUNE

DUNE can measure atmospheric neutrinos well in the sub-GeV energy region. This
is due to its ability to efficiently tag protons. The expected number of
events in 400 kton-year is $\sim$5000 (for charged-current 0p0$\pi$),
$\sim$9000 (for 1p0$\pi$), and $\sim$250 (for 2p0$\pi$). This will lead, for
example, a certain area containing $\delta_{CP}=\pi$ can be excluded at the
three sigma level for an input value of $\delta_{CP}=3\pi/2$[176]. DUNE will
be also possible to provide a new window of BSM scenario from the atmospheric
neutrino data.

##### Theia

Theia is a proposed multi-kiloton hybrid optical neutrino detector that takes
full advantage of scintillation and Cherenkov light information[177]. Various
physics potentials have been evaluated, as well as the potential to utilize
atmospheric neutrino observation data. Especially, a placement of Theia in the
LBNF beamline could further explore various non-standard neutrino properties
mentioned in the discussion of Hyper-Kamiokande.

##  7 High energy astrophysical neutrinos

###  7.1 Motivation and current understanding

High energy astrophysical neutrinos emerge from Nature’s accelerators:
powerful astrophysical systems that allow particles to be accelerated to
energies significantly higher than achievable with man made accelerators.
These neutrinos can help us to understand the dense environments in which they
are created, and give access to particle physics phenomena at high energies.

The field of experimental high energy neutrino astrophysics has emerged only
over the last decade. In the coming decade, we need to move beyond the
discovery phase to robustly measure and characterize of sources. Moreover, we
must further extend our energy reach in order to access source classes
connected to the highest energy particles in our Universe.

####  7.1.1 Sources of high energy astrophysical neutrinos

High energy neutrinos can be produced when accelerated protons interact with
the surrounding environment such that charged pions are formed, which
subsequently decay into neutrinos [178]. This can either happen through
inelastic hadronuclear reactions (protons interacting with protons or
neutrons, pp or pn), or through photohadronic interactions (protons
interacting with photons, p$\gamma$) [179].

Cosmic accelerators are good candidates for the production of high energy
neutrinos. The dense environment has sufficient ambient material to act as a
target and shocks can accelerate protons to high energies. The IceCube
Neutrino Observatory made the first discovery of high energy astrophysical
neutrinos in 2013 [180]. Since then, several neutrinos events have been
detected in likely coincidence with cosmic accelerators [181, 182, 183, 184,
185, 186]. However, more work remains to be done on the modeling of the
production of high-energy particles within a multi-messenger framework that
takes into account the population properties of these sources.

Other potential sources of high energy astrophysical neutrinos include those
produced in dark matter annihilation [187, 188] and the so-called GZK
neutrinos expected from interactions between cosmic rays and the cosmic
microwave background [189, 190].

####  7.1.2 Experimental landscape

Current experimental efforts to measure and characterize the flux of high
energy astrophysical neutrinos is dominated by neutrino telescopes, which use
optical modules to detect the photons produced in neutrino interactions in ice
or water. Neutrinos detected by these experiments range from GeV to PeV in
energy. The IceCube Neutrino Observatory [191] is the largest operating
detector, instrumenting a cubic kilometer of glacial ice at the geographic
South Pole. Other neutrino telescopes in operation are located in the Northern
hemisphere with the aim to measure neutrinos from the Southern sky. These
include ANTARES [192] in the Mediterranean Sea and NT200+ [193] in Lake
Baikal.

To observe the expected low flux of neutrinos with energies above a few PeV,
new detection techniques are required. Detectors that utilize the Asakaryan
effect, which produces coherent radio emission in ice and rock, are promising.
Due to the long attenuation length of photons in this wavelength, sparse
instrumentation can view the large volumes required to measure the anticipated
low flux of these neutrinos. In-ice experiments such as ARIANNA [194] and ARA
[195] and balloon borne experiments such as ANITA [196] have served as
prototypes of the technique. RNO-G [197] is currently deploying
instrumentation in Greenland and, when completed, will be the largest radio
neutrino detector to date.

####  7.1.3 High energy astrophysical neutrinos as probes for particle physics

Neutrinos from astrophysical accelerators provide a unique opportunity to
probe particle physics at the highest energies. Interactions of these
neutrinos as they pass through the Earth allow us to measure the cross
sections with neutrino energies up to $\sim$ a PeV, compared with below 400
GeV from neutrino beams [198, 199]. Measurements of the flavour ratio of
astrophysical neutrinos has constrained flavour-changing processes, especially
those predicted by new and beyond standard model physics [200, 201, 202]. The
long propagation lengths of astrophysical neutrinos have also allowed
constraints of energy-dependent tests for fundamental neutrino physics [203].
Current indirect detection limits on the cross section for dark matter with
masses above 1 TeV are driven by searches from ANTARES and IceCube [188].

In order to improve our current understanding, we require both higher
statistics of astrophysical neutrinos with energies up to PeV as well as
measurements from the yet-unexplored energy region above a few PeV.

####  7.1.4 High energy astrophysical neutrinos as probes for astrophysical
environments

The search for high energy neutrinos was born from another mystery in
astrophysics: what are the sources of the high energy cosmic rays that bombard
Earth’s atmosphere? High energy neutrinos are inherently tied to hadronic
activity and, unlike charged cosmic rays, neutrinos can travel directly from
their point of creation to our detectors without interacting en route. Sources
of high energy neutrinos should therefore be tied to the sources of cosmic
rays. In 2018, IceCube announced for the first time that an astronomical
source of these neutrinos could be identified when neutrinos from the
direction of blazar TXS 0506 +056 coincided with an enhancement in gammas as
detected in Fermi [181]. A subsequent follow up search from this direction
showed enhanced neutrino emission in 2014 - 2015 that was uncorrelated with
gamma emission [182]. More recently, an IceCube neutrino alert has been linked
to a tidal disruption event or a superluminous supernova [183, 186], and an
analysis of ten years of IceCube data has indicated high energy neutrino
emission from the nearby active galaxy NGC 1068 [184].

The study of the properties of the high energy astrophysical flux also gives
insight to the acceleration properties and material properties of the dense
astrophysical environments in which they are produced [204]. In particular the
flavour composition and the energy shape of the neutrino flux has been well
studied using the astrophysical neutrinos detected in IceCube [205].

###  7.2 Future Prospects

####  7.2.1 Future Experiments and technologies

*In-ice optical and radio

IceCube-Gen2 [206] is an envisioned next-generation neutrino telescope that
would significantly advance the study of high energy astrophysical neutrinos.
A planned optical extension with photosensors that have $\sim$ 3 times the
photo collection efficiency would cover 8 times more instrumented volume
compared to current IceCube. The addition of a $\sim$ 500 km radio array
allows for the energy reach to extend to the EeV scale. IceCube-Gen2 will make
firmer associations between astrophysical neutrinos and their sources, will
improve the characterization of the properties of the astrophysical neutrino
flux, will better understand the propagation of high energy particles in the
Universe, and will probe fundamental physics at high energies. A factor of ten
more astrophysical neutrinos with improved directional reconstruction is
anticipated, allowing IceCube-Gen2 to see sources that are five times fainter
compared with current technology and allowing for fewer false coincidences
with potential sources. The enhanced detector will provide more opportunities
for multi-messenger detection, especially in the electromagnetic regime but
also potentially in connection with gravitational waves [206]. The improved
statistics will allow for characterization of the energy spectrum and flavor
composition over a wide range of energies, which can be used to both probe the
acceleration mechanism of cosmic rays as well as to search for new flavor-
violating physics. The extension to neutrinos energies up to EeV in particular
will allow for the study of neutrinos connected to sources of the highest
energy cosmic rays and to probe the predicted GZK neutrinos.

In the near future, Northern hemisphere telescopes on the scale of IceCube
will come online, including KM3NeT [207] in the Mediterranean Sea, Baikal-GVD
[208] in Lake Baikal, and P-ONE in the Pacific Ocean near the coast of Canada
[209]. Together, these detectors will improve the discovery potential for high
energy neutrinos from the Southern sky by three orders of magnitude [210].

*Surface detection of earth-skimming tau neutrinos

As high energy tau neutrinos pass through matter, charged current interactions
produce a tau particles. The subsequent decays create air showers that can be
detected either optically, using water Cherenkov detectors, or via geomagnetic
radio emission using antennas.

TAMBO (Tau Air-Shower Mountain-Based Observatory) [211] is a proposed detector
to be built in Peru with the aim to measure Earth-skimming astrophysical tau
neutrinos in the 100 TeV to 100 PeV energy range. Tau-neutrino-induced air
showers are detected in cubic-meter-scale water Cherenkov detectors placed
$\sim$ 100 m apart. With the nominal design of 22,000 tanks, it is expected
that the TAMBO will have an effective area $\sim$ 10 times larger than the
current IceCube effective area for tau neutrinos at $\sim$ 3 PeV [CITE HE and
UHE whitepaper]. The southern hemisphere site will allow TAMBO to view the
galactic centre and the strategic location within a valley allows a high
geometric acceptance for air shower production.

Trinity is a proposed system of 18 air-shower Cherenkov telescopes optimized
for detecting Earth-skimming neutrinos with energies between 10 PeV and 1 EeV.
The intended Trinity site is in the Northern hemisphere and so it will view
the northern sky, making it complementary to the astrophysical neutrino
measurements from IceCube. A prototype will be deployed on Frisco Peak, Utah
in 2022.

BEACON (The Beamforming Elevated Array for COsmic Neutrinos) [212] is a
concept that uses radio antennas to measure the air showers from high energy
astrophycial taus neutrinos. It is currently in the demonstration phase, with
a prototype deployed at the White Mountain Research Station of California. The
final design envisions antennas deployed at several sites around the world for
a full sky coverage. Within 3 years of observations with a full-scale
instrument consisting of 1000 stations, BEACON is expected to achieve a
sensitivity comparable to pessimistic models of cosmogenic neutrinos.

GRAND (Giant Radio Array for Neutrino Detection) is a planned large-scale
radio observatory that intends to view earth-skimming tau neutrinos with
ultrahigh energies from 100 PeV to 100 EeV. The final envisioned configuration
consists of $\sim$ 20 geographically separate and independent sub-arrays each
consisting of 104 antennas that view a total of $\sim$ 10000 cubic kilometers.
Due to the large size of the array, sub-degree angular resolution of the
incoming neutrino direction is anticipated. This will enable searches for
point sources of ultrahigh energy astrophysical neutrinos. A prototype of 35
antennas was deployed in 2018 in the Tain Shan mountains in China [213]. Plans
for a larger prototype consisting of 300 antennas is currently underway [214].

*Balloon-borne

PUEO (Payload for Ultrahigh Energy Observations) [215] is the proposed
successor to the ANITA balloon experiment. PUEO will aim to measure
astrophysical neutrinos with energies above 1 EeV and is expected to have more
than an order of magnitude more sensitivity than ANITA below 30 EeV. Like
ANITA, PUEO will view a large volume of Antarctic ice from a long-duration
balloon. Expected improvements for PUEO include more instrumentation as well
as an improved triggering system.

*Space missions

POEMMA (Probe Of Extreme Multi-Messenger Astrophysics) is a NASA space-based
mission designed to measure ultra high energy cosmic rays and cosmic neutrinos
by viewing the optical signal produced in neutrino-induced air showers in the
Earth’s atmosphere. Two cameras will facilitate two complementary detection
techniques: an ultraviolet camera that allows for observations of florescence
and an optical camera for the detection of Cherenkov light. EUSO-SPB2 (The
Extreme Universe Space Observatory aboard a Super Pressure Balloon 2) will
provide useful input to the detection strategy of POEMMA by demonstrating the
detection technique and by measuring key backgrounds. EUSO-SPB2 has been
approved and will launch in Spring of 2023.

*Radar echo technique

The RET (radio echo telescope) [216] is a proposed experiment exploring a new
detection technique for neutrinos above a PeV in energy. RET uses the radar
echo method, where the ionization deposits induced in dense material such as
ice by high energy cascades are detected via the reflection of radio waves.
RET recently demonstrated the detection concept using an electron beam at SLAC
to mimic the ionization produced in the cascade. This method would bridge the
energies visible with optical detectors and proposed UHE detectors such as
radio arrays.

####  7.2.2 High energy astrophysical neutrinos as probes for particle physics

*Cross section measurements

IceCube has measured the neutrino cross section for neutrino energies between
6.3 TeV and 980 TeV [198] with an uncertainty of 30-40%. The measurement
agrees well with standard model predictions, but the enhanced statistics
expected with the next generation of experiments will allow for precision
tests to beyond standard model phenomena such as extra dimensions, leptoquarks
or sphalerons, as well as probe down to the regime where non-perturbative QCD
effects are expected to start becoming important.

Future detectors will have the ability to explore effects related to W-boson
production, including hidden Glashow resonances due to neutrino-nucleus
collisions [217] and processes affecting neutrino-photon interactions [218].

*Dark matter

Weakly interacting massive particles (WIMPs) are primary candidates for the
observed dark matter in our Universe. High energy astrophysical neutrinos can
indirectly probe WIMPs if they decay to neutrinos. Future neutrino telescopes
will probe new phase space of WIMP cross section and mass currently allowed.

*New physics

In general the long path lengths and large energy ranges provided by
astrophysical neutrinos make them good candidates for searching for new
physics phenomena. The higher energy reach of future detectors could provide a
completely new regime in which to search for unexpected physics. For a more
complete description, we refer the reader to [219]. In particular, the search
for neutrino decay, secret or non-standard neutrino interactions, BSM
extensions to fundamental symmetries, quantum gravity and exotic scenarios
such as magnetic monopoles can be probes with the next generation of neutrino
telescopes .

####  7.2.3 High energy astrophysical neutrinos as probes for astrophysical
environments

*Particle acceleration in cosmic sources

With the expected high statistics measurements from future telescopes,
measurements of the detailed properties of accelerated particles are expected.
This is especially interesting as the energies for particle acceleration as
well as the plasma processes in the dense environments of these systems cannot
be probed directly by other means. Precision measurements of the spectral
features will allow to study cosmic-ray interaction processes and maximum
acceleration energies.

*Sources of astrophysical neutrinos

There have been likely associations between high energy neutrinos and blazars,
star burst galaxies, tidal disruption events, and supernovae. The next
generation of neutrino telescopes will make high significance observations of
the brightest sources and allow for characterization of the spectral
properties of the neutrinos as well as the flaring behaviour of the emission,
and the source properties [220, 221, 222, 223, 224, 225, 226, 227, 228].

Future detectors are also expected to resolve the question of the sources of
cosmic rays emitted from our galaxy. IceCube has best sensitivity to the
Northern sky, but near-term and future detectors in the Northern hemisphere as
well as improved detectors such as IceCube-Gen2 can begin to probe high energy
astrophysical neutrinos from the galactic centre.

##  8 Conclusion

A rich landscape of physics can be pursued by utilising the broad range of
neutrino natural sources, offering insights into the natural world, and the
properties of these mysterious particles. One aspect many of these areas have
in common is that, as precision advances and we continue to push the bounds of
our understanding, greater sensitivity is required, and it becomes ever more
challenging to build a case for a dedicated detector. Thus, we see a future in
which synergies between physics topics play an ever increasing role, with
emphasis moving towards large-scale detectors that offer a broad program of
physics, across multiple sources and multiple energy scales. This places
considerable emphasis on novel technology development, as well as
collaboration across traditional topical boundaries, in order to achieve a
successful suite of next-generation experiments.

## Acknowledgements

The authors wish to thank all members of the NF04 Topical Group for their
engagement and involvement in the discussions and workshops that led to this
report, and comments on the report itself. In particular we wish to thank
Maury Goodman, Georgia Karagiorgi, Spencer Klein, Lisa Koerner, and Michael
Smy for their helpful input on the report. We also wish to thank those who
presented overview talks on many of these topics at dedicated and shared
workshops as part of the Snowmass process. We wish to thank all Convenors of
Topical Groups under the Neutrino Frontier for invaluable discussions, and a
most heartfelt thank you to Patrick Huber, Kate Scholberg, and Elizabeth
Worcester for their stalwart guidance throughout this process.

##  Appendix A Letters of Interest submitted to NF04

The tables below list all LOIs submitted to NF04 as part of the Snowmass
process.

Table 1: LOIs submitted to NF04 part i
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CF/SNOWMASS21-CF7_CF6-NF4_NF10_Ignacio_Taboada-092.pdf
CF/SNOWMASS21-CF7_CF6-NF4_NF10_Nepomuk_Otte-202.pdf
IF/SNOWMASS21-IF6_IF8-NF4_NF9-CF1_CF2_Rick_Gaitskell-172.pdf
NF/SNOWMASS21-NF10_NF4-CF1_CF0-IF8_IF0-UF1_UF3-137.pdf
NF/SNOWMASS21-NF10_NF4-CF7_CF0-IF10_IF0_Prohira-109.pdf
NF/SNOWMASS21-NF10_NF4-CF7_CF1-TF11_TF0_Darren_Grant-105.pdf
NF/SNOWMASS21-NF1_NF4-RF4_RF5_Aurisano-154.pdf
NF/SNOWMASS21-NF1_NF4_Pedro_Ochoa-034.pdf
NF/SNOWMASS21-NF1_NF4_SNOplus-185.pdf
NF/SNOWMASS21-NF2_NF4_Sousa_Thakore-134.pdf
NF/SNOWMASS21-NF3_NF4-CF7_CF0-TF11_TF0_Segev_BenZvi-043.pdf
NF/SNOWMASS21-NF3_NF4_Russell_Neilson-017.pdf
NF/SNOWMASS21-NF4_NF0_Matthew_Szydagis-163.pdf
NF/SNOWMASS21-NF4_NF1-RF4_RF0-CF7_CF1_SUPERK-050.pdf
NF/SNOWMASS21-NF4_NF10-CF1_CF0_Alec_Habig-124.pdf
NF/SNOWMASS21-NF4_NF10-CF1_CF7-020.pdf
NF/SNOWMASS21-NF4_NF10-CF7_CF0-TF11_TF0-IF10_IF0_Mauricio_Bustamante-195.pdf
NF/SNOWMASS21-NF4_NF10-CF7_CF1_Mauricio_Bustamante-044.pdf
NF/SNOWMASS21-NF4_NF10-IF6_IF0_Orebi_Gann-089.pdf
NF/SNOWMASS21-NF4_NF10_OceanBottomDetector-201.pdf
NF/SNOWMASS21-NF4_NF10_PTOLEMY-021.pdf
NF/SNOWMASS21-NF4_NF10_Pattavina-076.pdf
NF/SNOWMASS21-NF4_NF10_Steve_Biller-013.pdf
NF/SNOWMASS21-NF4_NF10_Steve_Biller-060.pdf
Table 2: LOIs submitted to NF04 part ii
NF/SNOWMASS21-NF4_NF3-CF1_CF7-TF9_TF11-039.pdf
---
NF/SNOWMASS21-NF4_NF3-CF7_CF6-TF11_TF9_DUNE-054.pdf
NF/SNOWMASS21-NF4_NF3_Matthew_Strait-090.pdf
NF/SNOWMASS21-NF4_NF5-TF11_TF0-158.pdf
NF/SNOWMASS21-NF4_NF6-CF7_CF3-TF9_TF11-IF2_IF10_Wissel-064.pdf
NF/SNOWMASS21-NF5_NF4-084.pdf
NF/SNOWMASS21-NF5_NF4-RF4_RF0_Savarese-127.pdf
NF/SNOWMASS21-NF5_NF4_Baudis-085.pdf
NF/SNOWMASS21-NF6_NF4-IF2_IF8-139.pdf
NF/SNOWMASS21-NF8_NF4-CF1_CF7_Evan_Grohs-174.pdf
NF/SNOWMASS21-NF8_NF4-CF3_CF7-TF9_TF8_Johns-121.pdf
NF/SNOWMASS21-NF9_NF4_Zimmerman,Eric-069.pdf
TF/SNOWMASS21-TF11_TF9-NF4_NF10-IF6_IF0_Alex_Friedland-085.pdf

##  Appendix B White Papers submitted to NF04

The following lists all White Papers submitted directly to NF04 as part of the
Snowmass process.

  * •

J. Aalbers, K. Abe, V. Aerne), F. Agostini, S. Ahmed Maouloud, D.S. Akerib, et
al. ”A Next-Generation Liquid Xenon Observatory for Dark Matter and Neutrino
Physics”, arXiv:2203.02309 [physics.ins-det] (pdf). (also under RF03, CF01,
IF08, UF03)

  * •

Junhui Liao, Yuanning Gao, Zhuo Liang, Zebang Ouyang, Chaohua Peng, Fengshou
Zhang, Lei Zhang, Jian Zheng, Jiangfeng Zhou. ”Introduction to a low-mass dark
matter project, ALETHEIA: A Liquid hElium Time projection cHambEr In dArk
matter“, arXiv:2203.07901 [astro-ph.IM] (pdf). (also under CF01, RF03, IF08,
UF03)

  * •

Markus Ackermann, Sanjib K. Agarwalla, Jaime Alvarez-Muñiz, Carlos A.
Argüelles, et al. ”High-Energy and Ultra-High-Energy Neutrinos”,
arXiv:2203.08096 [hep-ph] (pdf). (also under CF07, TF09, IF10)

The following lists White Papers submitted to the Neutrino Frontier with
interest to multiple topical groups:

  * •

Luis A. Anchordoqui, A. Ariga, T. Ariga, et al. “The Forward Physics Facility:
Sites, Experiments, and Physics Potential”, arXiv:2109.10905 [hep-ph] (pdf).
(also under EF0, RF0, CF0, TF0)

  * •

M. Askins, Z. Bagdasarian, N. Barros, E. W. Beier, et al. “Theia: Summary of
physics program”, arXiv:2202.12839 [hep-ex] (pdf). (also under IF0, UF0)

  * •

Jonathan L. Feng, Felix Kling, Mary Hall Reno, Juan Rojo, Dennis Soldin, et
al. ”The Forward Physics Facility at the High-Luminosity LHC“,
arXiv:2203.05090 [hep-ex] (pdf). (also under EF0, RF0, CF0, TF0, AF0, IF0)

  * •

A. Abed Abud, B. Abi, R. Acciarri, et al. (DUNE Collaboration). ”DUNE Physics
Summary”, arXiv:2203.06100 [hep-ex] (pdf). (also under RF04, CF01)

  * •

John Arrington, Joshua Barrow, Brian Batell, Robert Bernstein, Nikita Blinov,
et al. ”Physics Opportunities for the Fermilab Booster Replacement”,
arXiv:2203.03925 [hep-ph] (pdf). (also under RF0, TF0, AF0)

  * •

G. Bernardi, E. Brost, D. Denisov, G. Landsberg, M. Aleksa, D. d’Enterria, P.
Janot, M.L. Mangano, et al. “The Future Circular Collider: a Summary for the
US 2021 Snowmass Process”, arXiv:2203.06520 [hep-ex] (pdf). (also under EF0,
RF0, TF0, AF0, IF0)

  * •

A. Abed Abud, B. Abi, R. Acciarri, M. A. Acero, M. R. Adames, G. Adamov, et
al. ”A Gaseous Argon-Based Near Detector to Enhance the Physics Capabilities
of DUNE“, arXiv:2203.06281 [hep-ex] (pdf). (also under IF0)

  * •

Dave Casper, Maria Elena Monzani, Benjamin Nachman, Costas Andreopoulos,
Stephen Bailey, Deborah Bard, et al. ”Software and Computing for Small HEP
Experiments”, arXiv:2203.07645 [hep-ex] (pdf). (also under EF0, RF0, CF0,
CompF0)

  * •

Sunanda Banerjee, D. N. Brown, David N. Brown, Paolo Calafiura, Jacob Calcutt,
Philippe Canal, et al. ”Detector and Beamline Simulation for Next-Generation
High Energy Physics Experiments”, arXiv:2203.07614 [hep-ex] (pdf). (also under
EF0, RF0, CF0, IF0, CompF02)

  * •

Jaret Heise. ”The Sanford Underground Research Facility”, arXiv:2203.08293
[hep-ex] (pdf). (also under RF0, CF0, IF0, CompF0, UF0, CommF0)

  * •

A. Alekou, E. Baussan, N. Blaskovic Kraljevic, M. Blennow, M. Bogomilov, E.
Bouquerel, et al. ”The European Spallation Source neutrino Super Beam“,
arXiv:2203.08803 [physics.acc-ph] (pdf). (also under RF0, CF0, AF0, UF0)

  * •

J. R. Alonso, J. M. Conrad, Y. D. Kim, S. H. Seo, M. H. Shaevitz, J. Spitz, D.
Winklehner. ”IsoDAR Overview”, arXiv:2203.08804 [hep-ex] (pdf). (also under
AF0, CommF01)

  * •

Emanuela Barzi, S. James Gates Jr., Roxanne Springer. ”In Search of Excellence
and Equity in Physics“, arXiv:2203.10393 [physics.soc-ph] (pdf). (also under
EF0, RF0, CF0, TF0, AF01, IF0, CompF0, UF0, CommF0)

  * •

Henning O. Back, Walter Bonivento, Mark Boulay, Eric Church, Steven R.
Elliott, et al. ”A Facility for Low-Radioactivity Underground Argon”,
arXiv:2203.09734 [physics.ins-det] (pdf). (also under CF0, IF09, UF04)

  * •

J. M. Campbell, M. Diefenthaler, T. J. Hobbs, S. Höche, J. Isaacson, F. Kling,
et al. ”Event Generators for High-Energy Physics Experiments”,
arXiv:2203.11110 [hep-ph] (pdf). (also under EF0, CF0, TF0, CompF02)

  * •

M. Endo, K. Hamaguchi, M. Ibe, T. Ishibashi, A. Ishikawa, M. Ishino, M.
Ishitsuka, S. Kanemura, M. Kuriki, T. Mori, S. Moriyama, H. Nanjo, W. Ootani,
and Y. Sato. ”Japan’s Strategy for Future Projects in High Energy Physics“,
arXiv:2203.13979 [hep-ex] (pdf). (also under EF0, RF0, CF0, AF0, UF0)

  * •

E. Barzi, B. Barish, W. A. Barletta, I. F. Ginzburg, S. Di Mitri. ”High Energy
& High Luminosity $\gamma$$\gamma$ Colliders”, arXiv:2203.08353 [physics.acc-
ph] (pdf). (also under EF0, RF0, CF0, TF0, AF03)

  * •

Sergey Pereverzev, Gianpaolo Carosi, Viacheslav Li. ”Superconducting Nanowire
Single-Photon Detectors and effect of accumulation and unsteady releases of
excess energy in materials”, arXiv:2204.01919 [quant-ph] (pdf). (also under
CF0, IF06)

## Glossary

cluster on board (COB)

    An ATCA motherboard housing four RCEs
cathode plane assembly (CPA)

    The component of the SP detector module that provides the drift HV cathode
data acquisition (DAQ)

    The data acquisition system accepts data from the detector FE electronics, buffers the data, performs a , builds events from the selected data and delivers the result to the offline
data selector

    The portion of the DAQ front-end fragment that accepts trigger commands and returns the corresponding data fragment. Not to be confused with .
DAQ front-end readout (FER)

    The portion of a DAQ front-end fragment that accepts data from the detector electronics and provides it to the FEC.
DAQ front-end fragment

    The portion of one DAQ partition relating to a single front-end computer (FEC) and corresponding to an integral number of detector units. See also
DAQ partition

    A cohesive and coherent collection of DAQ hardware and software working together to trigger and read out some portion of one detector module; it consists of an integral number of . Multiple DAQ partitions may operate simultaneously, but each instance operates independently
DAQ run

    A period of time over which relevant data taking conditions and DAQ configuration are asserted to be unchanged. Multiple DAQ runs may occur simultaneously when multiple DAQ partitions are active. This term should not be confused with DUNE experiment or beam “runs” which typically span many DAQ runs.
data fragment

    A block of data read out from a single DAQ front-end fragment that span a contiguous period of time as requested by a trigger command
detector module

    The entire DUNE far detector is segmented into four modules, each with a nominal $10\text{\,}\mathrm{kt}$ fiducial mass
detector unit

    A may be partitioned into a number of similar parts. For example the single-phase TPC is made up of APA units
DUNE

    Deep Underground Neutrino Experiment
external trigger logic (ETL)

    Trigger processing that consumes detector module level information and other global sources of trigger input and emits information back to the
external trigger candidate

    Information provided to the about events external to a detector module so that it may be considered in forming
field cage (FC)

    The component of a LArTPC that contains and shapes the applied E field
front-end (FE)

    The front-end refers a point that is “upstream” of the data flow for a particular subsystem. For example the front-end electronics is where the cold electronics meet the sense wires of the TPC and the front-end DAQ is where the DAQ meets the output of the electronics
front-end computer (FEC)

    The portion of one DAQ partition that hosts the , and . It hosts the and corresponding portion of the .
inter-process communication (IPC)

    A system for software elements to exchange information between threads, local processes or across a data network. An IPC system is typically specified in terms of protocols composed of message types and their associated data schema.
large electron multiplier (LEM)

    A micro-pattern detector suitable for use in ultra-pure argon vapor; LEMs consist of copper-clad PCB boards with sub-millimeter-size holes through which electrons undergo amplification
one-pulse-per-second signal (1PPS signal)

    An electrical signal with a fast rise time and that arrives in real time with a precise period of one second
ProtoDUNE

    Either of the two DUNE prototype detectors constructed at CERN and operated in a CERN test beam (expected fall 2018). One prototype implements SP and the other DP technology
run control (RC)

    The system for configuring, starting and terminating the DAQ
single-phase (SP)

    Distinguishes one of the DUNE far detector technologies by the fact that it operates using argon in its liquid phase only
SP module

    single-phase detector module
trigger command message (TCM)

    A message flowing down the trigger hierarchy from global to local context. Also see
technical design report (TDR)

    A formal project document that describes the experiment at a technical level
trigger candidate

    Summary information derived from the full data stream and representing a contribution toward forming a
trigger command

    Information derived from one or more trigger candidates that directs elements of the detector module to read out a portion of the data stream
trigger notification

    Information provided by to external trigger logic (ETL) about its processing
Micro Telecommunications Computing Architecture ($\mu$TCA)

    The computer architecture specification followed by the crates that house charge and light readout electronics in the dual-phase module

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