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5 Conclusions One of the more intriguing developments of recent years has been the growing connection between understanding the physics of the fundamental constituents of nature and understanding the universe. These constituents and their interactions shaped the very early history of the universe, as well as its evolution to the present state. The complex sets of questions involved in untangling this picture are being probed through a multiprong approach with experiments deep underground, on land, and in space. This close coupling between the science at the largest scales known and the science at the smallest scales imaginable is manifested in the science initiatives considered in this report. The proposal to develop a cubic-kilometer-scale neutrino observatory will exploit the properties of elementary particles to open a window into an unexplored region of our universe. The proposals to develop an underground laboratory describe a national facility hosting a variety of experiments that will probe some of today’s most compelling questions in elementary particle physics, astrophysics, and cosmology. The committee’s scientific evaluation of the IceCube and deep underground initiatives presented in this report should be viewed in the context of the broader planning for future projects in physics and astronomy. The NRC report Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century addresses a set of important questions at the interface of astronomy and physics, citing the goals to “determine the neutrino masses, the constituents of the dark matter, and
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the lifetime of the proton.”1 The report recommends “that DOE and NSF work together to plan for and to fund a new generation of experiments to achieve these goals. We further recommend that an underground laboratory with sufficient infrastructure and depth be built to house and operate the needed experiments.”2 By their nature, IceCube and a deep underground laboratory are interdisciplinary and have strong overlaps with existing fields. The recent DOE/NSF long-range plan for nuclear physics states, “We strongly recommend immediate construction of the world’s deepest underground science laboratory.”3 It gives a new deep underground laboratory second highest priority for future projects. The neutrino science of IceCube has less overlap with the scientific goals of nuclear physics and is therefore not included in that report. The DOE/NSF long-range plan for particle physics has also endorsed both initiatives, although it ranks them below the highest scientific priority—participation in the worldwide efforts to build a linear collider. Regarding the scientific goals of IceCube, the long-range plan says that it is an “example of a mutually beneficial cross-disciplinary effort between astrophysics and particle physics,”4 and that experiments in a deep underground laboratory “will make important contributions to particle physics for at least the next twenty years, and should be supported by the high energy physics community.”5 The committee’s assessments of the scientific opportunities presented by IceCube and a new deep underground laboratory are consistent with these reports. The committee finds that the scientific opportunities for both in astrophysics, nuclear physics, particle physics, and their intersections make for impressive and exciting research programs. The committee believes that both are well worth pursuing. ICECUBE Experiments that detect very high energy particles from space can explore the physics of extreme conditions in the universe. For example, gamma-ray bursts, among the most powerful explosions since the big bang, may be sources of ultra- 1 National Research Council, Connecting Quarks with the Cosmos: Eleven Science Questions for the New Century, Washington, D.C., National Academies Press, 2003, p. 7. 2 Ibid. 3 DOE/NSF Nuclear Science Advisory Committee, Opportunities in Nuclear Science, 2002, p. 2. 4 DOE/NSF High-Energy Physics Advisory Panel, Subpanel Report on Long Range Planning for U.S. High-Energy Physics, 2002, p. 80. 5 Ibid., p. 77.
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high-energy neutrinos and cosmic rays. Astrophysical sources are capable of accelerating particles to energies well beyond what we can produce here on Earth. So it is no surprise to find that experiments studying such ultrahigh-energy phenomena have important consequences for our understanding of both the universe and the physics of the basic constituents of nature. IceCube is an exploratory experiment in a new area of science, the detection of high-energy neutrinos from astrophysical sources. That is, the primary objective is to determine whether astrophysical neutrinos exist (and are detectable) and, if so, what they can tell us about the far and extreme universe. Possible sources include gamma-ray bursts, active galaxies and quasars, and neutron stars. Since IceCube is breaking new ground, there is significant discovery potential not only for these sources but also for new and unexpected sources. Neutrinos traverse the universe almost unimpeded, making them a powerful new probe, but because of their small interaction cross sections, a very large detector is needed to make detections possible. To achieve sufficient sensitivity, experiments on the scale of a cubic kilometer (a billion tons of detector mass) are required. The idea in IceCube and underwater detectors is to use the material of Earth itself (ice or water) as a converter and to detect the products of these neutrino interactions. IceCube is to be constructed at the South Pole, making use of Antarctic ice as the detecting medium. Although IceCube is a major undertaking in a rather remote location, its design and prospects for success are bolstered by the strength of the existing infrastructure at the South Pole and, in particular, by the successful deployment and operation of AMANDA, a smaller precursor to IceCube. IceCube will substantially improve upon AMANDA’s capabilities, by virtue of a larger detection volume and improved technology. IceCube is ready for construction now, while the underwater detectors in the Mediterranean are in a preliminary development stage. The IceCube project is international and involves collaboration between scientists from institutions in the United States, Belgium, Germany, Japan, Sweden, the United Kingdom, and Venezuela. The plan is to incrementally build IceCube over about a 6-year period. The committee notes that if construction starts promptly IceCube will become the first detector to embark on these high-energy neutrino observations, an important requirement for such an exploratory experiment. By operating the partially completed detector even as it is being constructed, the project team will have early performance feedback to guide its work; furthermore, initial results could be available even before the complete detector is finished. Technically, the IceCube concept is well founded, based on an existing U.S. effort at the South Pole. The AMANDA project has demonstrated the feasibility of deep-ice neutrino detectors, and engineering efforts have advanced to the stage
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that a full-scale detector for IceCube can be constructed that meets the performance requirements of the experiment. Before construction can begin, it will be necessary to install appropriate management for the project, make final technical and design decisions, and strengthen the collaboration to leverage the experiment to its full potential. To summarize, the committee’s scientific assessment is that 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. A NEW DEEP UNDERGROUND LABORATORY Laboratories deep underground are required for several reasons: They make it possible to study rare forms of penetrating radiation (e.g., neutrinos and dark matter) and rare processes (e.g., double beta decay, proton decay, etc.) in a low-background environment. A variety of physics areas have been examined in some detail to determine what will be needed to address the critical and exciting scientific questions that have recently emerged on topics such as solar neutrinos, double beta decay, dark matter, long-baseline neutrinos, proton decay, and stellar processes. In all cases, numerous experiments of varying size and complexity are being devised, proposed, and discussed that would greatly increase our knowledge of these complex physics phenomena. The committee finds that a common feature of the future experimentation in this field is the importance of depth. Most of the experiments envisaged for exploring solar neutrinos, double beta decay, and dark matter require an overburden of about 4,500 meters water equivalent (mwe) or more. There are a few other experiments that, because of special detector features, may be done at 2,000 mwe, but even they would benefit from greater depth. The depth requirements for long-baseline neutrino experiments and searches for proton decay are less stringent and depths of 2,000 mwe are deemed adequate, but both of these types of experiment share a need for large, massive detectors and hence a sufficiently large site for the underground lab. Greater depth could still be an asset in accomplishing the physics goals. These requirements, of course, apply for studies undertaken with our present knowledge. Historically, it has always been prudent to anticipate unexpected backgrounds, more stringent requirements, or new physics whose study requires greater sensitivity—contingencies that argue for greater depths to leverage the science further, although there are necessarily other considerations when making a siting decision.
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To optimize long-baseline studies of neutrino oscillations, a new underground facility should be located farther than 1,000 km from existing, high-intensity proton accelerators. The United States has advantages in this siting requirement: the large size of the North American continent and the proven and expandable capability for producing intense neutrino beams at Brookhaven National Laboratory and at Fermilab. A new laboratory should have the potential to host a broad spectrum of experiments. Both significant depth and sufficient underground space will be needed to realize the full range of opportunities. This will result in economies coming from shared resources, as well as the development of a stimulating scientific center. A compelling collection of science experiments that could utilize a deep underground laboratory to address some of the most fundamental questions in particle physics and cosmology are, or soon will be, feasible. The committee finds that the science motivation for mounting large-scale experiments underground has increased markedly in the recent past and that the prospects for the next generation of experiments are particularly bright. Physicists in the United States pioneered the use of underground locations to conduct the sensitive experiments required to detect rare phenomena. Today, U.S. physicists continue to play a leading role in initiating and implementing many of the important subterranean experiments. In recent years, however, some of the most important new experiments have been sited outside the United States, not because there is insufficient U.S. participation, but because the major facilities for underground experiments are located in other countries. The breadth and quality of the future experimental program requiring an underground location suggest a major opportunity for the United States if it can soon develop a large new underground facility able to meet the requirements of the broad range of proposed experiments. To do this will require detailed planning over a complex and extensive set of scientific goals to determine the best site and a detailed strategy for an experimental program. In summary, the committee’s assessment is that a deep underground laboratory in the United States 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, together with new ideas and technologies, make possible a broad and rich experimental program. Considering the commitment of the U.S. community and the existing scientific leadership in this field, the time is ripe to build such a unique facility.
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REDUNDANCY AND COMPLEMENTARITY The two scientific initiatives assessed in this report (IceCube and a deep underground laboratory) have largely distinct science goals. Although neutrinos play a prominent role in both projects, the origins of the neutrinos, their energies, and the science they address are very different. IceCube takes advantage of the very clear ice available at the South Pole to develop an observatory for ultrahigh-energy neutrinos that might be produced by energetic sources in the universe. IceCube has secondary goals too: the detection of neutrinos from supernovae and the search for some forms of dark matter. A deep underground laboratory would host a very broad range of science experiments in fundamental physics and astronomy, including studies of the underlying nature of neutrinos, direct searches for dark matter, studies of proton decay, solar neutrino measurements, and experiments on neutrino oscillations. Direct dark-matter experiments at an underground laboratory are different from and complementary to searches that might use the IceCube detector, as they are suited to different mass ranges and different types of interactions of dark-matter particles with nuclei. Likewise, the large detectors for proton decay and long-baseline neutrino oscillation studies deep underground could also serve as a detector for supernovae. The committee finds essentially no redundancy in the primary science goals and capabilities of IceCube and those of a deep underground laboratory. Although some of the science may overlap between the two projects, both are critical investments that address key science questions in different ways. Finally, the committee finds that on the international scene each project has exciting potential and much-needed scientific value. IceCube will employ what looks to be a unique technology for gigaton-sized detectors and will take advantage of the opportunity for high-energy neutrino detection. A national underground laboratory offers the United States some vital scientific opportunities that will affect a number of important international efforts and provide a center in the United States for some of the most exciting physics at the beginning of the 21st century.
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