Underground laboratories are a relatively new kind of research facility, developed 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 program 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 proposed 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.
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 significantly 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 astrophysics 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 expanding 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. Nonetheless, 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 sufficiently rare occurrences—approximately two per century within our galaxy—that
it is possible none will occur during the long lifetime of the experiment. However, 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 composition 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 astrophysical 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 extensions 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 geosciences, and biosciences that could be enabled by an underground research facility, the committee recommends the development of a mechanism to allow scientists
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.