1
Overview

According to the big bang theory, our Universe began in a state of unimaginably high energy and density, contained in a space of subatomic dimensions. At that time, unlike today, the fundamental forces of nature were presumably unified and the particles present were interacting at energies not attainable by present-day accelerators. The features of the Universe we observe today, like the large-scale distribution of luminous and dark matter and the preponderance of matter over antimatter, resulted from the behaviour of unknown elementary particles in that primordial epoch. The physics of this earliest of states can be assessed through the search for certain spontaneous but very rare phenomena in matter and through the detection of the weak effects of highly elusive particles. Underground laboratories provide the conditions needed to investigate these processes and have succeeded in discovering the first clear evidence for physics beyond the Standard Model: namely, that those extremely elusive particles—neutrinos—are massive and that flavor lepton numbers are not conserved. These laboratories now appear to be the gateway to understanding the physics of the grand unification of the forces of nature.

STUDY BACKGROUND

The DUSEL Program

Within the confines of the existing underground mines and laboratories in the United States and abroad, the U.S. research community has always played a leading



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1 Overview According to the big bang theory, our Universe began in a state of unimagi- nably high energy and density, contained in a space of subatomic dimensions. At that time, unlike today, the fundamental forces of nature were presumably unified and the particles present were interacting at energies not attainable by present-day accelerators. The features of the Universe we observe today, like the large-scale distribution of luminous and dark matter and the preponderance of matter over antimatter, resulted from the behaviour of unknown elementary particles in that primordial epoch. The physics of this earliest of states can be assessed through the search for certain spontaneous but very rare phenomena in matter and through the detection of the weak effects of highly elusive particles. Underground laboratories provide the conditions needed to investigate these processes and have succeeded in discovering the first clear evidence for physics beyond the Standard Model: namely, that those extremely elusive particles—neutrinos—are massive and that flavor lep- ton numbers are not conserved. These laboratories now appear to be the gateway to understanding the physics of the grand unification of the forces of nature. STUDY BACKGROUND The DUSEL Program Within the confines of the existing underground mines and laboratories in the United States and abroad, the U.S. research community has always played a leading 5

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an assessment science ProPosed dUsel 6 of the for the role in underground science. Examples of landmark experiments in this country include the first observation of solar neutrino oscillations by the Brookhaven solar neutrino experiment begun in the 1960s in the Homestake mine in Lead, South Dakota;1 the first limits imposed by the Grand Unification Theory (GUT) on pro- ton decay at the Irvine-Michigan-Brookhaven experiment in a Morton salt mine;2 and the pioneering solid-state direct-detection dark matter experiment, Cryogenic Dark Matter Search, in the Soudan mine.3 As the required sensitivity and scale of underground experiments grow, the need for new underground laboratory space has drawn the attention and proposals of research communities around the world (see Chapter 2). The U.S. particle and nuclear physics communities have identified certain underground experiments as a top priority for their fields in their long- range plans. Efforts to develop a major facility in the United States have resulted in a proposal for a facility, the Deep Underground Science and Engineering Laboratory (DUSEL), to be located in the abandoned Homestake gold mine.4 The research to take place at DUSEL is described by the proponents as being built upon “four pillars,” or four physics quests of critical scientific importance— the search for dark matter, the study of neutrino oscillations, and investigations into whether protons decay and whether atoms can undergo neutrinoless double-beta decay. In the proposed initial suite of experiments, these four quests are addressed by the apparatus of three experiments (see Chapter 3). The proponents of DUSEL also describe three research tenets—that the facility provide opportunities for a diverse set of research efforts in subsurface engineering, the geosciences, and biosciences; that it allow other well-motivated experiments to take advantage of the unique capabilities of a world-class underground research facility; and that it provide a significant education and outreach program for visitors and the com- munities near the laboratory. The principal underground laboratory space is to be located at 4,850 ft, where plans call for installing five or six physics experiments and at least one earth science experiment. The proponents’ plans also call for a deeper site, at 7,400 ft, where two smaller physics experiments and an earth science experiment would be located. Other research facilities could be installed at other levels, depending upon require- ments of the experiments. Recently plans were developed that would allow for the installation of a liquid argon detector for the neutrino oscillation experiment at 1 R.Davis, Jr. 1964. Solar Neutrinos: II. Experimental. Physical Review Letters 12: 303. Becker-Szendy, C.B. Bratton, D.R. Cady, et al. 1990. Search for proton decay into e + + π0 in the 2 R. IMB-3 detector. Physical Review D 42: 2974-2976. 3 D.S. Akerib, J. Alvaro-Dean, M.S. Armel-Funkhouser, et al. 2004. First results from the cryogenic dark matter search in the Soudan Underground Laboratory. Physical Review Letters 93: 211301. 4 “How much better to get wisdom than gold, to choose understanding rather than silver.” Proverbs 16:16 (New International Version).

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overview 7 800 ft, where ramp, as opposed to vertical, access can be provided. Finally, the facil- ity would include a large research space on the surface to support the underground experiments and allow for the development of future experiments. A significant personnel and facilities infrastructure is called for to manage the ongoing facility as well as to plan for future expansion. Management must ensure that the facilities needs of each experiment—sufficient excavated space, ventilation, power, data transfer capabilities, and possible shielding, among others—are met. Safety concerns are paramount for space so deep underground, so measures would be taken to develop, maintain, and ensure compliance with all safety standards. Because one of the principal experiments at DUSEL would be the neutrino oscilla- tion experiment associated with a neutrino source at the Fermi National Accelera- tor Laboratory (Fermilab), management would have to coordinate the scientific and facility development taking place at DUSEL with similar efforts at Fermilab. The program would also include a process for evaluating the merit of proposed future experiments. Once selected, those experiments would be integrated with the current suite of experiments, either by incorporating them into the existing space or by excavating new space and expanding support services. This Study As part of the process for developing the DUSEL program, the National Science Foundation (NSF) and the Department of Energy (DOE) jointly commissioned this study. The principal charge to the committee is as follows: The committee will undertake an assessment of the proposed DUSEL program, including: • An assessment of the major physics questions that could be addressed with the pro- posed DUSEL and associated physics experiments, • An assessment of the impact of the DUSEL infrastructure on research in fields other than physics, • An assessment of the impact of the proposed program on the stewardship of the research communities involved, • An assessment of the need to develop such a program in the U.S., in the context of similar science programs in other regions of the world, • An assessment of broader impacts of such an activity, including but not limited to education and outreach to the public. This report is the response to that charge. However, several events transpiring over the course of this study caused the committee to interpret the statement of task in a manner that merits explanation. Just before the committee’s first meeting, in December 2010, the National Science Board (NSB), the governing organization for the NSF, elected not to provide bridge funding for the further development of the DUSEL facility. NSF’s FY2012 budget request, submitted several months thereafter,

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an assessment science ProPosed dUsel 8 of the for the indicated that the decision not to provide this funding was part of a larger deter- mination by the NSB that the scope and likely cost of the project lay outside NSF’s core mission. As a consequence, NSF would not be proceeding as principal steward of the DUSEL facility. The committee was also informed that there would be very limited follow-up to the preliminary design report being prepared by proponents of the DUSEL program and that it would mainly serve as general input for evaluating future opportunities. As it releases this report, the committee understands that DOE and NSF have been discussing whether to proceed with some or all of what has been described as the DUSEL program but that no firm decision has been made. Given these uncertainties and developments, the committee has interpreted the portions of its charge by which it is to assess the science that might take place in the proposed DUSEL program (the first and second bullets in the statement of task) as directing the committee to evaluate the intellectual merit of the science to be addressed by the slate of experiments that were to be included in the initial DUSEL program, as described at the committee’s first meeting. In particular, the committee did not assess any future experimental opportunities that would be enabled by the existence of an underground research facility but that had not been included in the initial suite of experiments. In assessing the impact of the DUSEL infrastructure on fields other than physics, the committee considered the suite of experiments in the biosciences, geosciences, and subsurface engineering that were presented to it as being indicative of the type of nonphysics questions that could be addressed rather than as specifying the DUSEL nonphysics program. The committee assessed the science questions in the general context of frontier research worldwide; it did not compare them with any particular alternative project or investment. In responding to the remaining bulleted items in the statement of task—the impact such a program would have on the stewardship of research communities; the need to develop such a program in the United States given similar science programs elsewhere; and the broader impacts of such a program—the committee elected not to restrict its assessment to the specifics of “the proposed DUSEL pro- gram” at the Homestake site. Rather, it set forth more general considerations that it believes should be taken into account by policy makers in evaluating the impact that an underground laboratory facility in the United States—either a DUSEL-like national laboratory or a more limited facility—would have on advancing the goals of the U.S. research communities. In conformity with the charge, the committee assessed only the options associated with developing some form of underground research facility in the United States and did not assess the project costs or budget- ary impacts of the facilities and experiments discussed. This chapter provides an overview of the science questions that an under- ground research facility could address and of the broader impacts such a facility would have on the relevant research communities and summarizes the report’s principal findings and conclusions. Chapter 2 discusses the general parameters of

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overview 9 underground research space and the status of the principal underground research facilities around the world. Chapter 3 contains a detailed assessment of the princi- pal science questions that would be addressed with the DUSEL program. Chapter 4 concludes the report by describing the broader impacts of the program, including the education and outreach opportunities such a program might provide. SCIENCE OVERVIEW Physics Experiments The committee finds that three of the proposed physics experiments—a direct detection dark matter experiment on the scale of one to tens of tons; a long-baseline neutrino oscillation experiment; and a ton-scale, neutrinoless double-beta decay experiment—to be of paramount and comparable importance. These experiments are judged to be of paramount scientific importance because each would be a central component of an ongoing science program that would seek to address at least one crucial unanswered question whose eventual answer will greatly enhance our scientific understanding. As a consequence, each experiment would have the potential to make a breakthrough discovery upon which the future of particle, nuclear, and astrophysics will build. In this sense, each of these three experiments is essential and could radically transform scientific understanding and progress. Dark Matter The direct detection dark matter experiment will provide unprecedented sen- sitivity for the direct detection of the dark matter omnipresent in the Universe and an opportunity to discover the unknown particle nature of dark matter. Because dark matter makes up approximately 80 percent of the material Universe, the dis- covery of its nature would be of enormous significance to the fields of astrophysics and particle physics. The direct detection of dark matter would complement its indirect detection by astrophysics and accelerator-based searches for its produc- tion. Direct detection would strongly support the idea that dark matter candidates identified at an accelerator are indeed the mysterious dark matter pervading the Universe. Experiments to directly detect dark matter must be massive in order to capture weakly interacting dark matter particles. Experiments at the ton scale (~1 ton to tens of tons, depending upon the technology) will achieve the sensitivity levels needed to discover and study dark matter. The background from cosmic rays that mimic the signals of dark matter demand that the experiments be performed underground. Moreover, the challenges to instrumentation posed by residual back- grounds demand that the world should have at least two experiments of this scale implementing different techniques. Because resolving the nature of dark matter is

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an assessment science ProPosed dUsel 10 of the for the so vital and U.S. scientists have had a leading role in addressing this problem, the United States should take a leading role in mounting one of the two direct detection dark matter experiments on this scale, and support for U.S. scientists participating in the second direct detection experiment, wherever it is, would be appropriate. Neutrino Oscillation The long-baseline neutrino oscillation experiment would provide a great advance in the study of neutrino properties, particularly when coupled with a neutrino beam produced at Fermilab using a new high-intensity proton source that is under development. By significantly improving sensitivity to the so-called mixing angle between the lightest and heaviest of the neutrinos, to the hierarchy of neutrino masses, and to matter-antimatter asymmetry in neutrino oscillations, this experiment will probe for more signs of the new physics that revealed itself when neutrino mass was discovered in earlier oscillation experiments, and it will elucidate the processes of the early Universe. As discussed in more detail later in this chapter, this experiment will also provide increased sensitivity for the possible detection of proton decay and neutrinos from supernovas. Although these rare phe- nomena are not very likely to be observed, the detection of either proton decay or neutrinos from a nearby supernova would be of momentous scientific significance. The detection and identification of neutrinos require massive, sensitive detectors to capture the weakly interacting neutrinos. Detectors that weigh between tens and hundreds of kilotons, depending on the technology, are required for sensitivity to the neutrino-antineutrino asymmetries, and underground sites are needed to control experimental backgrounds, with depth depending also on detector technology. Two detector technologies, the traditional water Cherenkov detector and a more novel liquid argon tracking calorimeter, are under study. Both would require a scaling up of present detectors to achieve the sensitivities needed to advance the fields. Accu- mulating enough information to untangle the subtle interplay of neutrino param- eters demands intense neutrino beams as well as massive detectors. With its plans to exploit the full potential of the existing Fermilab accelerator complex to provide a high-intensity neutrino beam, and with the possibility of an even higher intensity neutrino beam generated by Project X at Fermilab, the United States will be in a good position to conduct the next-generation long-baseline neutrino experiment. Neutrinoless Double-Beta Decay The neutrinoless double-beta decay experiment could determine whether neutrinos are their own antiparticles and could measure, or at least constrain, the neutrino masses. Resolution of the particle-antiparticle question will contribute in a critical way to our understanding of how particles came into existence in

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overview 11 the early Universe and of why matter dominates over antimatter in the Universe today and will therefore contribute to our understanding of how the Universe has evolved. This experiment is the only practical way to address the particle-antipar- ticle question. Moreover, the masses of the neutrinos are fundamental parameters of the Standard Model and they cannot be accessed directly by studying neutrino oscillations. Neutrinoless double-beta decay experiments must be massive in order to con- tain enough nuclei to allow observation of such rare decays. Experiments at the ton scale may be needed to achieve the sensitivity required to observe neutrinoless dou- ble-beta decay and/or to determine the neutrino masses. As with the direct detec- tion of dark matter, backgrounds from cosmic rays that mimic the signals sought demand that the experiments be performed underground, and the challenges posed by residual backgrounds to the design of detector instrumentation demand that the world have at least two ton-scale experiments using different experimental techniques. Because it is so important to resolve the particle-antiparticle nature of the neutrinos and to determine their mass scale, and because U.S. scientists have been playing a leading role in addressing this challenge, the United States should take a leading role in mounting one of the ton-scale experiments, and the support of U.S. scientists participating in the other such experiments, perhaps elsewhere in the world, would be appropriate. Together, these two neutrino experiments—the long-baseline neutrino experi- ment and the neutrinoless double-beta decay experiment—form a complementary program to address the outstanding questions of neutrino physics and to complete our understanding of this portion of the particle world and its key cosmological role. The marked improvements in sensitivity that will be afforded by next-gener- ation underground experiments for detecting dark matter and studying neutrinos will enable significant advances in these matters of fundamental and critical sci- entific importance. Proceeding with plans to build in the United States a world- leading long-baseline neutrino oscillation experiment, and taking a leadership role in developing within the United States both a direct detection dark matter experiment on a scale of one to tens of tons and a neutrinoless double-beta decay experiment on the ton scale will bring an extraordinary opportunity for the U.S. scientific community. The program would put U.S. scientists in a good position to have leadership in these crucial experimental undertakings. The benefits to the U.S. particle and nuclear physics communities would be greatest if, in addition to the neutrino oscillation experiment, the dark matter and neutrinoless double-beta decay experiments are both installed at a U.S. facility. However, if a U.S. site is not available for one or both of them, a U.S.-led experiment at an appropriate facility abroad would still be of significant benefit to the U.S. research communities. (See Chapter 4 for a discussion of appropriate facilities.)

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an assessment science ProPosed dUsel 12 of the for the Conclusion: Three underground experiments to address fundamental ques- tions regarding the nature of dark matter and neutrinos would be of para- mount and comparable scientific importance: • The direct detection dark matter experiment, • The long-baseline neutrino oscillation experiment, and • The neutrinoless double-beta decay experiment. Each of the three experiments addresses at least one crucial question upon whose answer the tenets of our understanding of the Universe depend. Conclusion: The three major physics experiments would not only provide an exceptional opportunity to address scientific questions of paramount importance, they would also have a significant positive impact upon the stewardship of the particle physics and nuclear physics research communi- ties, and would have the United States assume a visible leadership role in the expanding field of underground science. The U.S. particle physics program is especially well positioned to build a world-leading long-baseline neutrino experiment due to the combined availability of an intense neutrino beam from Fermilab and a suitably long baseline from the neutrino source to an appropriate underground site such as the proposed DUSEL. In light of the leading roles played by U.S. scientists in the study of dark matter and double- beta decay, together with the need to build two or more large experiments for each of these two areas, U.S. particle and nuclear physicists are also well positioned to assume leadership roles in the development of one direct detection dark matter experiment on the ton- to multiton scale and one neu- trinoless double-beta decay experiment on the scale of a ton. While installa- tion of such U.S.-developed experiments in an appropriate foreign facility or facilities would significantly benefit scientific progress and the research communities, there would be substantial advantages to the communities if these two experiments could be installed within the United States, possibly at the same site as the long-baseline neutrino experiment. An underground research facility would also offer the opportunity to under- take several other important physics studies. In addition to investigating neutrino characteristics, the detector in the long-baseline neutrino oscillation experiment would provide the increased sensitivity needed for the study of proton decay and would allow collecting valuable data if a supernova event occurred in the nearby universe during the course of operation. The facility could also be the site for an accelerator-based study on nuclear cross sections that are critical for understanding a wide array of astrophysical events.

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overview 13 Proton Decay The massive detector of the long-baseline neutrino oscillation experiment will have a sensitivity to proton decay greater than that of current detectors. The stability of the proton is a matter of considerable scientific interest. There are com- pelling theoretical reasons to expect that the proton is unstable, and proton decay would provide a unique and direct window onto the physics of the GUT and the origin of matter. Because the lifetime of the proton is very long, the probability of observing any individual proton decay is very small, necessitating very massive detectors and an extremely large number of protons. As noted above, the detector of the long-baseline neutrino oscillation experiment will be more massive than previous detectors, allowing more sensitivity that could produce a major discovery. Nevertheless, the detectors being proposed are only large enough to improve sen- sitivity for many important decay modes by less than an order of magnitude over a 10-year operational period relative to what current instruments could achieve over the same time. As a result, the added reach is not sufficient for proton decay to be the primary factor in decisions on neutrino detector technology or siting. Supernova Studies The large detector of an underground long-baseline neutrino oscillation exper- iment could make a unique and valuable contribution to the study of supernovas. These remarkable phenomena play a crucial role in the history of the Universe as well as in the life of galaxies, yet they are not well understood. Detection of neutrinos from a supernova would provide a wealth of information about the dynamics of supernovas not available from astronomical observations, making this capability a significant feature of a long-baseline neutrino experiment. (Either a water Cherenkov detector or a liquid argon detector would detect a large number of neutrinos from a supernova in our galaxy.) Nonetheless, although a supernova (SN1987A) has previously been observed by particle detectors, supernovas close enough to Earth to be observed occur only rarely, approximately once or twice per century, and none may occur during the long lifetime of the experiment. However, should a nearby supernova occur, having more than one large neutrino detector in the world would ensure its observation and maximize the scientific output. Conclusion: Two additional capabilities of the long-baseline neutrino exper- iment would be of great scientific interest and would add significant value to that experiment: • Its sensitivity to the study of proton decay and • Its sensitivity to the detection of neutrinos from supernovas.

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an assessment science ProPosed dUsel 14 of the for the The stability of the proton is a crucial, fundamental scientific question. Moreover, the detection of neutrinos from supernovas would make a unique and valuable contribution to our understanding of one of the most impor- tant astrophysical phenomena. However, these sensitivities are not so impor- tant as to make them primary considerations in choosing neutrino detector technology or a site for the experiment. Nuclear Astrophysics The committee found that an accelerator-based study to measure the low- energy nuclear cross sections needed to elucidate astrophysical processes would be scientifically important. These cross sections are critical for advancing our under- standing of the nuclear processes that generate stellar energy and explain certain aspects of solar neutrinos and the abundances of the elements and their isotopes in the Universe. For example, with the recent greatly improved measurements of the solar neutrino flux, nuclear cross sections are now the dominant uncertainty in using the neutrino flux to extract information on neutrino properties as well as on solar structure and composition. Measuring nuclear cross sections at stellar ener- gies requires high luminosities and low backgrounds, which can be provided by an underground accelerator facility. Such a facility would be an effective complement to the new Facility for Rare Isotope Beams (FRIB) in advancing nuclear astrophys- ics science. Because a large number of cross section measurements are needed and the cross sections to be measured have low counting rates, more than one such low-energy facility is called for worldwide. The proposed U.S. facility would thus enable scientifically important measurements beyond the number that can be made at the LUNA facility at the Gran Sasso Laboratory in Italy. Conclusion: A small underground accelerator to enable measurements of low-energy nuclear cross sections would be scientifically important. These measurements are needed to elucidate fundamental astrophysical processes such as thermonuclear reactions and the production of heavy elements in the Sun and the stars. Nonphysics Experiments Subsurface Engineering, the Geosciences, and the Biosciences Access to extensive underground space and the ability to conduct regulated long- term and large-scale tests would afford unparalleled research opportunities for fields

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overview 15 such as subsurface engineering, the geosciences, and the biosciences. Such opportu- nities would inform the study of the complex hydraulic, chemical, mechanical, and thermal forces at play underground and how they interact in existing fracture systems subject to tectonic and gravitational forces, affecting deformation and slippage, pro- ducing earthquakes at faults, and influencing life in its extreme forms. Conclusion: The ability to perform long-term experiments in the regulated environment of an underground research facility could enable a paradigm shift in research in subsurface engineering and would allow other valuable experiments in the geosciences and biosciences. BROADER IMPACTS Co-location of Experiments Underground research such as the major physics experiments described above, requires experienced personnel and extensive infrastructure to provide access, power, and ventilation, as well as surface facilities for the assembly and maintenance of apparatus. Safety is, as always, important, particularly because of the inherent danger in working underground. Much infrastructure and personnel could be effi- ciently and effectively shared among contemporary experiments located at a single site, and among future experiments as well. A common site could also provide other benefits, such as opportunities for increased interactions and synergy among sci- entists engaged in different experiments. It could also heighten the visibility of the research, to the public here and to the international research community abroad. Conclusion: The co-location of the three main underground physics experi- ments at a single site would be a means of efficiently sharing infrastructure and personnel and of fostering synergy among the scientific communi- ties. The infrastructure at the site would also facilitate future underground research, either as extensions of the initial research program or as new research initiatives. These additional benefits, along with the increase in visibility for U.S. leadership in the growing field of underground science, would be important considerations when choosing a site for the three main physics experiments. Conclusion: If co-located with one or more of the main underground physics experiments in the United States, a small underground accelerator facility to enable measurements of low-energy nuclear cross sections important to

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an assessment science ProPosed dUsel 16 of the for the nuclear astrophysics would benefit from shared infrastructure, personnel, and expertise. Conclusion: In light of the potential for valuable experiments in subsurface engineering, the geosciences, and the biosciences that could be offered by an underground research facility, if such facility is constructed in the United States for physics experiments, scientists in other fields would greatly benefit by having a mechanism in place that would allow them to perform research there. National Facility Access to underground research laboratories is vital to research programs in particle and nuclear physics and to the biological, geological, and subsurface engi- neering sciences of the subsurface. Indeed, underground facilities are essential for addressing some of the most important questions in science, such as the nature of the neutrino, the stability of the proton, and the nature of dark matter—and they enable valuable long-term experiments in a regulated environment. There are at present underground laboratories in several places in the world. However, the grow- ing number of underground experiments and the need for multiple experiments for the direct detection of dark matter, for neutrinoless double-beta decay, and for nuclear astrophysics, and the growing size of those experiments mean increased demand for underground laboratory space around the world. For this reason, many of the world’s existing laboratories have plans—as yet unrealized—for expansion, and there are proposals, most notably in China, to create an international under- ground facility. The Soudan Underground Laboratory, in Soudan, Minnesota, is the only general underground research laboratory in the United States, although some underground experiments are also performed at DOE’s Waste Isolation Pilot Plant (WIPP) and the Sanford Underground Laboratory at Homestake, a facility developed in conjunction with the DUSEL program. Although each proposed or future experiment could be located in some existing, expanded, or new facil- ity, stewardship of the research communities requires providing them access to adequate, appropriate underground research facilities. A national underground research facility in the United States would supplement and complement facilities elsewhere in the world by providing increased underground research space and future expansion capability, as well as appropriate infrastructure and safety sys- tems. Although the final decision to build a national underground facility will be made taking into account many other factors, including the programmatic goals of the funding agencies and the costs of different options, significant advantages

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overview 17 would accrue to the pertinent U.S. research communities if such a facility were to be built here.5 Conclusion: A facility for underground research would have a significant positive impact on the stewardship of the research communities involved. Such a facility would offer the particle and nuclear physics communities access to the underground research space they need to undertake a range of scientifically critical experiments, and it would allow the bioscience, geosci- ence, and subsurface engineering communities to perform valuable long- term experiments in a regulated environment. An underground research facility in the United States could offer advantages over underground facilities in other places in the world. Foremost, a large neu- trino oscillation experiment in this country could be coupled with the present and future capabilities of the Fermilab accelerator complex, which would provide an intense neutrino beam at a suitably long baseline, making the United States a world leader in neutrino physics. At present, no other location in the world offers a fully competitive combination of future neutrino intensity and an appropriate under- ground site for a very large neutrino detector. Furthermore, some underground science programs, such as the programs that are imperative in direct detection of dark matter and neutrinoless double-beta decay, require multiple large experi- ments using complementary techniques. As these international programs evolve, it becomes reasonable to expect that the hosting and supporting of large experiments will be shared by underground laboratories in different countries. Meanwhile, biological and, particularly, geological and subsurface engineering experiments need to be performed in many different environments. The site proposed for the DUSEL program offers some special features for particular engineering science experiments. Finally, an underground research facility in the United States would offer advantages to the U.S. research communities, reinforcing their stewardship of the research. It would provide a research site that does not involve travel to dis- tant places and would facilitate graduate student training as well as the research 5 Following the NSB’s decision not to proceed with stewardship of the DUSEL program, the DOE initiated a study to evaluate the financial aspects of several options, including funding a facility with many of the components of the original DUSEL program and funding a program in which several experiments are constructed at the Sudbury Neutrino Observatory near Sudbury, Ontario, Canada. February 28, 2011, Letter from the Office of the Director, Department of Energy, to Dr. Jay Marx and Mr. Mark Reichanadter. The results of that study were made available around the time of the release of this report. Available at http://www.dusel.org/dusel/recent/Marx_Review_of_Underground_Science_ Report_Final.pdf. Last accessed on October 19, 2011.

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an assessment science ProPosed dUsel 18 of the for the enterprise. It could also help ensure access to underground space for experiments led by U.S. scientists, who have historically been leaders in underground science. At the same time, it would guarantee the United States a leadership role in the expand- ing global underground science community while being a principal component of the growing U.S. world-class particle physics program at the Intensity Frontier. Conclusion: Development of an underground research facility in the United States would supplement and complement underground laboratories around the world. A U.S. facility could build upon the unique position of the United States that would allow it to develop a long-baseline neutrino experiment using intense beams from Fermilab. It could accommodate one of the large direct detection dark matter experiments and one of the large neutrinoless double-beta decay experiments that are needed by the international effort to delve into these critical scientific issues, while sharing infrastructure among the three experiments, which are of comparable import. It could also host and share infrastructure with other underground physics experiments, such as an accelerator to study nuclear astrophysics, and with underground experiments in other fields. An underground research facility would benefit the U.S. research communities and would guarantee the United States a lead- ership role in the expanding global field of underground science.