Executive Summary

Discoveries involving neutrinos are reshaping the foundations of our understanding of nature. The detection of neutrinos coming from the Sun and from an exploding star, and discoveries from underground experiments of the past decades, were recognized by the 2002 Nobel Prize in physics. More recent underground neutrino experiments have excited the scientific community with definitive observations that neutrinos of different types transform into one another, implying that they have mass.

Indeed, neutrinos have moved onto center stage in astrophysics and in particle physics, and for good reason. The discovery that neutrinos have mass provides us with the first tangible evidence for physics beyond the very successful Standard Model of elementary particles. And the neutrino mass indicated by these experiments leads to the conclusion that neutrinos account for about as much of the mass of the universe as do bright stars. Finally, the discovery that neutrinos have mass supports certain formulations of the long-sought theory that would unify the forces and particles.

These discoveries create a number of new fundamental questions and opportunities to further advance our understanding of the universe and the laws that govern it. They have spurred proposals for new initiatives, including both a project to develop a large neutrino detector under the ice at the South Pole (IceCube) and a proposal to develop a new deep underground laboratory within the United States that can house a broad range of important future experiments. This report was commissioned to review and assess the scientific merit of these two proposals (see



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Executive Summary Discoveries involving neutrinos are reshaping the foundations of our understanding of nature. The detection of neutrinos coming from the Sun and from an exploding star, and discoveries from underground experiments of the past decades, were recognized by the 2002 Nobel Prize in physics. More recent underground neutrino experiments have excited the scientific community with definitive observations that neutrinos of different types transform into one another, implying that they have mass. Indeed, neutrinos have moved onto center stage in astrophysics and in particle physics, and for good reason. The discovery that neutrinos have mass provides us with the first tangible evidence for physics beyond the very successful Standard Model of elementary particles. And the neutrino mass indicated by these experiments leads to the conclusion that neutrinos account for about as much of the mass of the universe as do bright stars. Finally, the discovery that neutrinos have mass supports certain formulations of the long-sought theory that would unify the forces and particles. These discoveries create a number of new fundamental questions and opportunities to further advance our understanding of the universe and the laws that govern it. They have spurred proposals for new initiatives, including both a project to develop a large neutrino detector under the ice at the South Pole (IceCube) and a proposal to develop a new deep underground laboratory within the United States that can house a broad range of important future experiments. This report was commissioned to review and assess the scientific merit of these two proposals (see

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Appendixes A and B for the charge and Appendix C for brief biographies of the committee members). In this report, the science that requires instrumenting a very large volume of ice deep under Earth’s surface with photodetectors is assessed. The goal of such exploratory experiments is to open the neutrino window on the universe and to elucidate the origin and acceleration of nature’s highest-energy particles. High-energy neutrinos provide a unique probe into understanding the acceleration mechanisms from astrophysical objects such as active galactic nuclei and gamma-ray bursts that could produce such particles. Detecting these neutrinos is particularly attractive because they reach Earth without absorption and can therefore give insight into their sources and production mechanisms. The second class of experiments assessed are those that might be placed in a new deep underground laboratory. In recent years, experiments performed below the surface of Earth have received more and more worldwide attention in nuclear physics, particle physics, and cosmic-ray physics, as well as astrophysics and cosmology. Such laboratories, shielded from cosmic rays, allow the study of rare phenomena and provide a window onto the unraveling of some of the most fundamental questions in physics and astrophysics today. The dramatic discoveries of neutrino oscillations (and mass) are a direct result of such experiments, and future deep underground experiments could be key to unraveling some of the most fundamental questions in physics and astronomy. Since the committee finds that the scientific goals of an underground laboratory go well beyond neutrino experiments, it has assessed the scientific potential for such a facility in a broader context. In addition to providing a scientific assessment of IceCube and of a deep underground laboratory, the committee addresses their overlaps and complementarity, as well as how each initiative fits into international plans. Finally, the committee emphasizes that this report is consistent with, and should be viewed within the context of, the broader planning for future projects in physics and astronomy. In particular, the National Research Council report Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century (National Academies Press, Washington, D.C., 2003) identifies a set of important questions at the interface of astronomy and physics, several of which would be addressed by these projects. By their nature, these two projects are interdisciplinary and bridge traditionally separate disciplines. The recent Department of Energy/National Science Foundation long-range plans for nuclear physics and particle physics also endorse these projects. The DOE/NSF plans find IceCube and a deep underground laboratory to be important projects within the context of the scientific goals and priorities of nuclear and particle physics.

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ICECUBE The IceCube experiment planned for the South Pole will use a cubic kilometer of deep ice instrumented with photodetectors as a gigantic high-energy neutrino detector. At this depth the ice is sufficiently transparent to minimize light losses (although some scattering may still occur), and it provides a quiet environment in which to place a large phototube array. Deep underwater experiments with similar goals have also been proposed for the Mediterranean Sea, but at this time they are not as developed as the IceCube concept. Furthermore, the water and ice detectors could have complementary features, both technically and in their sky coverage. An international collaboration has formed to build IceCube, which is a larger version of the pioneering Antarctic Muon and Neutrino Detector Array (AMANDA) experiment that has provided initial results and a great deal of experience working with such techniques at the South Pole. AMANDA successfully demonstrated design implementation, data taking, and neutrino detection. IceCube has been successfully reviewed technically and is ready for construction. It includes some technical improvements over AMANDA that promise to provide a more robust and flexible detector system. IceCube is an exploratory experiment at the forefront of a new area of science. Although it is not possible to predict the neutrino rates for such unknown physics, the best estimates from high-energy gamma-ray sources and cosmic-ray rates suggest that the sensitivity of the proposed cubic-kilometer scale of IceCube is sufficient to observe neutrinos from known astrophysical sources. In addition, it is known from AMANDA and other experiments that cosmic-ray interactions with our atmosphere at energies of a trillion electron volts (TeV) and above produce a copious supply of neutrinos; the study of these interactions will be of significant interest for investigating neutrino behavior at these energies. (The absence of such a point-source neutrino signal in IceCube, however, could still be significant as it would restrict the broad class of models for cosmic acceleration.) The unique and important opportunity to observe the expected high-energy neutrinos makes the experiment very attractive and worth undertaking. The committee finds that there is evidence that the universe contains a variety of sources of very high energy neutrinos and that their detection would reveal much about how nature accelerates particles, as well as the inner workings of supermassive black holes and the mysterious gamma-ray bursts. The technology exists to build the enormous detectors necessary to detect neutrinos from across the universe, and the infrastructure exists at the South Pole. The time is right to open this new window on the universe.

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Assessment: The planned IceCube experiment can open a new window on the universe by detecting very high energy neutrinos from objects across the universe. The science is well motivated and exciting, the detection technique is proven, and the experiment appears ready for construction. IceCube has completed its research and development (R&D), prototyping, and conceptual design phases. When the funding is approved, it will be ready to transition to the construction phase. This will require putting into place appropriate project management, making final technical and design decisions, and ensuring that the collaboration is strong enough to support a project of this importance and magnitude. A NEW DEEP UNDERGROUND LABORATORY The science of underground physics was pioneered in the United States by Raymond Davis, Jr., more than 35 years ago. He detected electron-type neutrinos coming from the Sun, confirming Hans Bethe’s theory that a chain of thermonuclear reactions takes place in the solar core. He then made the profoundly significant observation that the actual number of detected solar neutrinos was much lower than predicted, giving the first hint of new physics. Underground experiments at Japanese and Canadian mines have recently suggested the explanation, providing dramatic evidence that neutrinos oscillate from one type to another, in turn implying that neutrinos have nonzero mass. With these discoveries energizing the rapidly growing field of underground physics, and recognizing both the large U.S. commitments being made to underground facilities abroad and the future science opportunities for such facilities, it is now very timely to consider the building of a new deep underground facility in the United States. In fact, the development of a new underground laboratory with characteristics that are well matched to the needs of future experiments could regain for the United States its leadership in this important area of science. Laboratories deep underground make it possible to study rare forms of penetrating radiation (e.g., neutrinos and dark-matter particles) and rare processes (e.g., double beta decay and proton decay) in a low background environment. To meet the unique challenges of the many possible experiments considered in this review, any future underground laboratory must have several key attributes. First, it must provide the ability to place experiments as deep as 4,500 mwe (the equivalent of 4,500 meters of water), with the future possibility of siting experiments down to 6,000 mwe. (Although 4,500 mwe would likely satisfy the needs of many upcoming experiments, the potential for greater depth would result in a truly

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unique and longer-lived facility with even less risk of interference from background processes.) Second, a facility located at large distances—over 1,000 km— from accelerator facilities capable of producing intense neutrino beams will be essential for the next generation of neutrino oscillation experiments and would represent another unique capability. The proposals that are currently under consideration for a deep underground laboratory allow for the development of a flexible multipurpose infrastructure to support a full suite of experiments. The actual experiments would be proposed separately, peer reviewed, and then funded for implementation at the laboratory. Every effort should be made to closely integrate the actual development of a new laboratory with the program of experiments that would be performed. A significant advantage of a central facility is the opportunity to share common technical and equipment support among the various experiments. There are many other research uses for sufficiently shielded underground laboratory space, including various geophysics and geobiology projects, but the committee had neither the expertise nor sufficient time to make additional evaluations. The committee finds that to fully exploit the potential science opportunities, a new underground facility must provide depths great enough for those experiments that require it, together with flexibility in siting experiments that need less over-burden but more space. It must afford a long-term future for science at minimal cost. Siting the facility within the continental United States would offer another important advantage: the presence of powerful existing accelerators with proven and expandable capabilities for neutrino beam production, necessary for potential long-baseline experiments. A new, deep underground laboratory with this combination of features could fully exploit the science opportunities described in this report. Assessment: A deep underground laboratory can house a new generation of experiments that will advance our understanding of the fundamental properties of neutrinos and the forces that govern the elementary particles, as well as shed light on the nature of the dark matter that holds the universe together. Recent discoveries about neutrinos, new ideas and technologies, and the scientific leadership that exists in the United States make the time ripe to build such a unique facility. It will require considerable strategic and technical guidance to construct a deep underground laboratory expeditiously and in synergy with an experimental research program. Critical decisions that are beyond the scope of this report remain: choosing between several viable site options, defining the laboratory’s scope and the nature of its staff and its management organization, and determin-

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ing the site infrastructure and the level of resident technical support. Developing sound proposals for experiments will require early access to deep underground facilities to perform the necessary preliminary R&D. Therefore, it is important to complete the process of setting the laboratory’s scope and goals, soliciting and reviewing proposals, and building up the necessary infrastructure to allow timely initiation of the experimental research program. REDUNDANCY AND COMPLEMENTARITY The exploratory physics envisioned for IceCube and the broad science program enabled by a deep underground laboratory are truly distinct. IceCube would concentrate on very high energy neutrinos from astrophysical sources that require a detector of much larger size than is possible in an underground laboratory, while an underground laboratory would focus on experiments, including neutrino experiments, that require the low backgrounds available deep underground. The committee finds essentially no overlap or redundancy in the primary science goals and capabilities of IceCube and those of a deep underground laboratory. On the international scene of present and planned experiments, IceCube is unique in its technology and location (using ice as a detection medium at the South Pole) and is the most advanced project for gigaton-scale high-energy neutrino telescopes. Separately, the wealth of experimental opportunities available in an underground laboratory ensures that an additional underground laboratory would contribute substantially to international science efforts. While it is true that each particular experiment proposed for the underground lab could be individually sited elsewhere, there are likely to be scientific leadership, economic, and administrative advantages to a centralized national underground facility.