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



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