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Summary
Underground laboratories are a relatively new kind of research facility, devel-
oped primarily because they provide the extremely quiet environment needed
to study rare events such as proton decay and the faint signals associated with
neutrinos—ghostly particles with very little mass and no net charge that only
weakly engage with most “normal” matter. As weak or rare as those signals are,
their study will have profound implications; breakthroughs in any of the leading
physics experiments that study these signals will be the foundations upon which a
significant portion of the physics community builds for decades to come.
Because of the importance of these studies, a number of underground research
facilities have been built around the world, including a modest facility in the United
States. Led by the National Science Foundation (NSF) and working in conjunction
with the Department of Energy (DOE), the research communities that engage in
underground science in the United States developed an integrated research pro-
gram centered around a major underground facility to be located in South Dakota:
the Deep Underground Science and Engineering Laboratory (DUSEL). As part of
the process of developing DUSEL and the program associated with it, NSF and
DOE jointly commissioned this study. The principal charge to the committee was
to independently assess the physics questions that could be addressed with the pro-
posed program, how such a program would impact the stewardship of the research
communities involved, and whether there was a need to develop such a program
in the United States, given similar science programs elsewhere. The committee
also was charged with assessing the potential impact of this facility on research in
nonphysics fields and on broader interests such as education and public outreach.
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In response to this charge, the committee concludes that three of the proposed
physics experiments—(1) a direct detection dark matter experiment on a scale of
one to tens of tons, (2) a long-baseline neutrino oscillation experiment, and (3)
a ton-scale, neutrinoless double-beta decay experiment—are of paramount and
comparable scientific importance. Each of these experiments addresses at least
one crucial question upon which the tenets of our understanding of the Universe
depend. A direct detection dark matter experiment (1) would seek to learn the
nature of the mysterious dark matter that makes up approximately 80 percent
of the material Universe, a subject of enormous significance to astrophysics and
particle physics. A long-baseline neutrino oscillation experiment (2) would signifi-
cantly advance the study of neutrino properties, particularly if it is coupled with
a neutrino beam produced using a new high-intensity proton source at Fermilab.
It would also provide increased sensitivity for the possible detection of proton
decay and neutrinos from supernovas, phenomena whose observation would be
momentous for science. A neutrinoless double-beta decay experiment (3) could
determine whether neutrinos are their own antiparticles, the answer to which will
help us understand how the Universe has evolved. Each of the three experiments
is the central component of an ongoing scientific program and could result in a
breakthrough discovery upon which particle physics, nuclear physics, and astro-
physics will build. The committee concludes that exceptional opportunities will
result from proceeding with plans to build in the United States a world-leading
long-baseline neutrino experiment and developing within the United States both
one direct dark matter detection experiment on the ton to multiton scale and one
neutrinoless double-beta decay experiment on the ton scale for installation at a U.S.
site or, if such a site is not available, at an appropriate overseas facility. Pursuing
this program would not only allow us to address scientific questions of paramount
importance but, as discussed below, would also have a significant positive impact
on the stewardship of the particle and nuclear physics research communities and
would result in the United States assuming a visible leadership role in the expand-
ing field of underground science.
The neutrino oscillation experiment (2) would be a significant improvement
over existing experiments in another respect as well: its sensitivity to the detection
of proton decay, another consequential physics experiment that has been proposed
for DUSEL. The stability of the proton is a crucial issue that will provide a direct
window onto the grand unification of forces and the origin of matter. Nonethe-
less, while the added potential of the experiment would be welcome, the ability to
search for evidence of proton decay should not be the primary factor in selecting
the neutrino detector technology or in siting the experiment.
The neutrino oscillation detector (2) also would contribute to the study of
supernovas, one of the most important astrophysical phenomena. These are suffi-
ciently rare occurrences—approximately two per century within our galaxy—that
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it is possible none will occur during the long lifetime of the experiment. How-
ever, the information gained by studying such an event with the detectors under
consideration for DUSEL would give us enormous insight into events that are
essential in galaxy formation and in the determination of the elemental compo-
sition of solar systems such as ours. The committee concludes that the ability to
study these rare events adds great value to the neutrino oscillation experiment
but should not be a significant consideration in choosing the neutrino detector
technology or siting.
The committee found, moreover, that a fourth physics experiment, a nuclear
astrophysics study to measure low-energy nuclear cross sections relevant to astro-
physical processes, would be scientifically important. These cross sections are quite
small, and efforts to measure them need the protected environment provided by
underground laboratory space to filter out competing signals. Construction of a
small underground accelerator facility would enable these scientifically important
measurements.
The proposed DUSEL facility would provide unique opportunities for fields
outside of physics—the geosciences and subsurface engineering—to explore in situ
the physical and mechanical properties of rock at depths and over areas and times
not currently available to them. Among the proposed experiments are regulated
studies of the influence of fracture systems on rock response to applied loads and
of the interdependence of the thermo-hydromechanical-chemical-biologic aspects
of subsurface systems, and efforts to make rock more “transparent” by developing
imaging techniques that would allow the exploration of subsurface material at a
distance despite its opacity. Enabling the geoscience and subsurface engineering
fields to conduct such studies would be an important step forward for these fields.
The subsurface environment would also give biology researchers an opportunity to
explore life in extreme environments and to learn how biological systems manage
to live in the conditions that exist deep underground.
Co-locating the three main underground physics experiments at a single site
would allow infrastructure, personnel, and expertise to be shared. Co-location
would also contribute to stewardship by fostering synergy among the communities
and by offering an existing infrastructure for future experiments, either exten-
sions of the original research program or new research initiatives. By developing
a facility where these experiments are co-located, the United States would be seen
as a leader in the expanding field of underground science. Lastly, the existence of
such a facility would allow the above-mentioned small underground accelerator
facility for studying processes of nuclear astrophysics to benefit from the shared
infrastructure, personnel, and expertise.
In light of the valuable experiments in subsurface engineering, the geosci-
ences, and biosciences that could be enabled by an underground research facility,
the committee recommends the development of a mechanism to allow scientists
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in fields other than physics to perform research at an underground physics facility
in the United States.
Finally, the report assesses how access to a national facility for underground
research would advance the current set of studies and also provide opportunities
for future studies. The committee concludes that such a facility would be of long-
term benefit to a substantial portion of the physics community and other scientific
communities and that it would guarantee the United States a leadership role in
the expanding global field of underground science generally and on the “intensity
frontier” of the particle physics community in particular.
