Findings and Recommended Actions
THE SCIENTIFIC AGENDA FOR ELEMENTARY PARTICLE PHYSICS
New technological capacities now make it possible to address long-stand ing questions in particle physics that are encompassed within the following three questions:
Can all the forces between particles be understood in a unified framework?
What do the properties of particles reveal about the nature and origin of matter and the properties of space and time?
What are dark matter and dark energy, and how has quantum mechanics influenced the structure of the universe?
Two special considerations give these questions an urgency. First, a rare opportunity currently exists for the U.S. program to collaborate with international partners to transform today’s understanding of all three of these questions. This window of opportunity will not remain open for long. Second, the U.S. effort in particle physics has been without a compelling, clearly articulated, and widely held strategic vision since the cancellation of the Superconducting Super Collider, and the lack of such a vision has now become critical. The committee’s recommended agenda for the U.S. role in particle physics addresses both considerations.
The committee’s recommended agenda not only seizes the tremendous opportunity for intellectual transformation as the discoveries of the Terascale be-
come available, but it also calls for a transformation in how particle physicists interact with one another at the national and international levels; this turning point in particle physics is extremely compelling.
In considering the actions recommended in this chapter, it is important to understand what the committee means by “priority.” The elements of the scientific agenda that it recommends have been prioritized based on its analysis of the importance of the underlying scientific opportunities combined with its assessment of the technical readiness and feasibility of experimental facilities located in the United States and abroad.
It also is important to keep in mind the strategic principles outlined in Chapter 4. In particular, it is important to recall the strategic necessity of mounting, regardless of budgetary constraints, a comprehensive program that reflects a diversity of scientific opportunities and approaches to the scientific challenges facing particle physics. Under no circumstances, therefore, should the committee’s top two or three priorities be permitted to exhaust the entire available budget. Indeed, in the most pessimistic budget scenario, where maintaining a position of leadership is unrealistic (Scenario B), the resources invested in the priorities outlined below would need to be adjusted, but the need for pursuing a diversified research portfolio would be unchanged.
The capacity of the Large Hadron Collider (LHC) and the International Linear Collider (ILC) to explore the Terascale directly offers the promise of deep insights into such matters as the Higgs boson, supersymmetry, dark matter candidates, and hidden spatial and quantum dimensions. At the same time, explorations of unification and particle astrophysics, including both space-based and underground observations, promise to shed light on dark energy, dark matter, and inflationary models of the universe. Moreover, new and planned precision studies of lepton and quark properties and their interactions may reveal the role of neutrinos in the universe, explain how matter came to dominate antimatter, or uncover entirely new phenomena. The committee discusses each of these scientific opportunities and the associated action items in priority order, assuming, for the moment, the constant-effort budget (Scenario A).
DIRECT EXPLORATION OF THE TERASCALE
The most compelling current scientific opportunity in elementary particle physics is aggressive exploration of the Terascale, and this is the committee’s highest priority for the U.S. program. A two-part strategy using two world-leading
accelerators is required, as described below. Direct investigation of phenomena at the energy frontier holds the greatest promise for transformational advances. Realizing the scientific potential of the Terascale requires experiments using particle accelerators. Within this context, the LHC and the proposed ILC experimental programs offer the best routes. The committee’s recommended strategy is also predicated on the observation that, at the current time, a higher risk, higher reward strategy is necessary if the United States is to sustain a leadership position in the decades ahead. To accomplish this, our nation must take the initiative in aggressively exploring the most compelling new science opportunities.
Finding 1: The LHC Experimental Program. The study of LHC physics will be at the center of the U.S. particle physics program during the coming decade.
The LHC is scheduled for start-up in 2007 at CERN and will be at the center of exciting developments in elementary particle physics over at least the next 15 years. By colliding particles at the TeV energy scale (the Terascale), the LHC will provide the first look at a landscape expected to be rich in answers to questions about the origins of mass, hidden dimensions, and the limits to current understandings of the quantum universe. As U.S.-based facilities conclude their particle physics programs over the next few years, more and more U.S. scientists and students, as well as many others from around the world, will be focusing their efforts at the LHC, which soon will become the center of gravity for experimental particle physics.
The United States has already made a very substantial contribution of human and financial capital to the development of the ATLAS and CMS detectors at CERN, as well as to construction of the accelerator, but it is critical to adequately support the U.S. research groups that will carry out experiments at the LHC and to participate in the upgrades of the LHC’s experimental facilities. In addition, U.S. centers for data analysis have been set up at Fermilab and Brookhaven National Laboratory, as have smaller data centers at other national laboratories and universities, and these critical analysis and computing facilities must be supported during the LHC’s lifetime.
The committee notes the importance of devoting resources to the continued development of cyberinfrastructure if the United States is to play a leadership role in the LHC program. In 2007, for the first time, the United States will enter an era when its primary experimental research in particle physics will be based in a foreign country. Careful attention to networking, data access, and collaborative tools will be required to ensure the full participation of U.S. scientists and students in the global LHC effort and realization of the scientific opportunities offered by the LHC.
As the LHC physics program unfolds over the next decade, proposals for upgrades will likely become better formulated. For instance, the detectors might be upgraded to cope with increases in accelerator event rates, or the accelerator might be significantly upgraded to double the energy. Early discoveries at the LHC are likely to guide the pace of the surrounding discussions.
Action Item 1: The LHC Experimental Program. The highest priority for the U.S. national effort in elementary particle physics should be to continue as an active partner in realizing the physics potential of the LHC experimental program.
The number of U.S. researchers working at the LHC will continue to increase as operations begin and the earliest results of Terascale science appear. As the most immediate scientific opportunity for U.S. researchers, the LHC should receive U.S. support in line with the level of involvement of U.S. researchers and the need to maintain the detectors. The committee expects that full support of the growing U.S. participation will consume a larger share of resources than at present. As potential upgrades to the detectors and the accelerator are motivated and defined by scientific results, the U.S. particle physics program should consider in-kind contributions where appropriate. With regard to LHC accelerator operations, direct funding should only be considered within the context of international discussions that address the broad suite of international scientific collaborations both inside and outside particle physics.
Finding 2: Achieving Readiness for the ILC. An aggressive approach to realization of the ILC needs to be the central element in a new strategic plan for the U.S. program in particle physics.
Exploration of the Terascale is the highest priority for particle physics, and a linear collider is the next critical element required to meet this objective and carry the science program well beyond the next decade. The science program for the proposed ILC addresses the major contemporary challenges in particle physics and extends the discovery reach for Terascale physics. The ILC is therefore the most important new experimental facility in elementary particle physics. It is envisioned to have a total energy of 500 GeV during its initial phase, with a planned capability for a subsequent increase to 1 TeV. It is to discover the nature and meaning of results from the LHC that an electron-positron collider such as the ILC is required. As a result, the elementary particle physics community worldwide is in consensus that the ILC should be the next major experimental facility to be built for particle physics. Furthermore, the scientific, technological, and industrial expertise needed to build and operate the facility is becoming available, and complete capabilities will fall within reach alongside a comprehensive R&D effort.
The study of physics at the ILC will be at the center of the U.S. particle physics program beyond 2015.
Additional R&D is necessary to resolve the remaining technological issues and to formulate a set of design and manufacturing requirements that will minimize the cost of this multi-billion-dollar facility. The Global Design Effort (GDE) currently under way expects to prepare the Reference Design Report (RDR) with a baseline cost estimate by the end of 2006, with a full technical design ready in 2009. The GDE is currently setting the strategies and priorities for the work of hundreds of scientists and engineers at universities and laboratories around the world. More than $50 million was expended for these efforts last year in Europe, a similar amount was spent in Japan, and $25 million in the United States. This research is essential to reduce the technical and cost risks before such a project can be approved.
Clearly, a project as large and complex as the proposed ILC can be pursued only if an international consortium can be formed to pursue the design and remaining R&D and to devise a fair mechanism for sharing the costs, scientific leadership, and participation in the ILC program. While there is a great deal to be learned from existing models of international collaboration—examples include ITER, the Atacama Large Millimeter Array (ALMA), and CERN—the ILC may require a unique form of collaboration. To date, international collaboration on the scientific and technical issues surrounding the ILC has been excellent, as shown by the consensus on the ILC technology selection, ongoing R&D collaboration, and initiation of the GDE. (Appendix A describes some of this progress.)
Strong theoretical arguments and past experimental results provide convincing evidence that the Terascale will offer a rich spectrum of physics that demands exploration by both proton and electron colliders.1 An informed decision on the construction of the ILC could be made as soon as a credible cost estimate exists and an appropriate governance structure takes shape; ideally it should be made no later than 2010, by which time the LHC should have revealed some of the nature of the new physics that lies at the Terascale. Timely and responsive decisions on the ILC will optimize the forward momentum and continuity of the field’s research pursuits. This time frame is compatible with the expected start-up date of the LHC and with the conclusion of current accelerator-based experimental programs in the United States.
See, for example, LHC/ILC Study Group, Physics Interplay of the LHC and ILC, 2004, available online at <http://arxiv.org/abs/hep-ph/0410364> (last accessed February 1, 2006).
Action Item 2: Achieving Readiness for the ILC. The United States should launch a major program of R&D, design, industrialization, and management and financing studies of the ILC accelerator and detectors.
U.S. expenditures on R&D for the ILC should be very significantly expanded. The key objective of this R&D program is to reduce both the technical and cost risks of the ILC and to initiate a program that will allow for industrialization of significant portions of ILC components. This effort should continue in the tradition of the broad international collaboration that has been the hallmark of the ILC project to date. The United States should prepare for long-term involvement in the physics program of the ILC as well.
For the accelerator, this commitment should be at a level as high as $100 million in the peak year and could represent a cumulative amount on the order of $300 million to $500 million over the time period prior to the decision to proceed with construction.2 For detector R&D, the commitment would be near $80 million over the same period,3 financed in part by the redirection of some university and national laboratory efforts.
The committee believes strongly that it is in the best long-term interests of the U.S. program in particle physics to make a significant investment in this R&D program and to become a leading center for ILC R&D well before a construction decision is made. This is a critical element of the committee’s recommended strategy: The United States must take the initiative now to be in a position to make a
These levels of effort are based on tables provided to the committee by the GDE and the U.S. Linear Collider Steering Group and the High Energy Physics Advisory Panel on May 15, 2005, in response to a written request from the committee for information. With certain adjustments and small differences in emphasis, this investment profile is in agreement with expectations from the GDE for the necessary worldwide R&D to prepare for a construction decision. The committee reviewed the proposed investment profile carefully and used its best judgment to characterize the entire planned effort with robust figures. Finally, the proposed schedule does separate out the “globally shareable investment” in ILC research and development and the additional investment required for the United States to provide and certify a site for a bid to host the ILC (for instance, the analysis includes figures for meeting environment, safety, and health labor regulations; budget management; and contingency). Accepting the current cooperation brokered by the GDE, the committee posited that the United States would partake in an equal third of the globally shareable investment in addition to the expected costs for developing a U.S. site. Based on its collective experience in project management and cost projections, the committee came to agreement on the proposed range and schedule of investment as proposed in the text.
These numbers are based on an updated report by the ILC Detector R&D Panel: J.-C. Brient et al., ILC Detector Research and Development: Status Report and Urgent Requirements for Funding, January 2006, available online at <http://physics.uoregon.edu/~lc/wwstudy/R&D%20Reportfinal.pdf> (last accessed July 1, 2006).
credible bid to host the ILC when the R&D effort is complete. Clearly, however, the decision to proceed with actual construction would require the establishment of an international governance structure, an international decision on a site for the ILC, and reliable and robust cost estimates.
The ILC project has been conceived and planned in a manner very different from the Superconducting Supercollider (SSC) project of the late 1980s and early 1990s. The committee concurs that successful implementation of the ILC will require different approaches than were employed with the SSC. The ILC planning and R&D activities to date have been managed by the scientific community as a truly international effort. These activities have laid a solid foundation for the eventual development of an international governance structure and cost-sharing agreement, both of which need to be in place prior to the start of construction. The committee also concurs with the need for a rigorous R&D and industrialization program before a decision to construct in order to minimize technical risks and uncertainties in cost and schedule.
In addition, the committee believes that the U.S. effort to become the host site for the project should take full advantage of the existing infrastructure and expertise of Fermilab. Using Fermilab as the host laboratory would avoid start-up issues associated with a new site like those that affected the SSC. The committee also notes that DOE has adopted a new project management system since the SSC (under DOE Order 413) that strengthens oversight and cost management of major projects. In fact, since the mid-1990s, the Office of Science at DOE has a remarkably good record of meeting performance, schedule, and budget targets.4 The committee expects that the United States and its international partners will employ state-of-the-art information systems to aid in the management and reporting of project implementation activities. However, the committee anticipates that some procedural aspects of the DOE project management system may need to be modified to accommodate international participation in the project. This issue should be addressed in more detail as part of the planning for the United States to bid to host the ILC.
Finding 3: The Benefits of Hosting the ILC. Hosting the ILC would inspire students, attract talented scientists from throughout the world, create a suite of high-technology jobs, and strengthen national leadership in science and technology.
For historical reference, see L. Edward Temple, Jr., “Office of Energy Research Project Performance,” Department of Energy, Office of Energy Research, Construction, Environment, and Safety Division, February 1986. Project performance since 2000 has been guided by DOE Order 413.3, and some evaluations are available at the DOE Office of Science’s Office of Project Assessment, available at <http://www.science.doe.gov/opa/>.
The ILC will be a flagship scientific facility. It will focus on some of the most profound and mind-stretching questions in science. This challenge will fire the creativity and imagination of many of the nation’s brightest young minds. Hosting this exciting project can be expected to increase student interest in science and engineering and thus enhance the nation’s scientific and technological workforces, just as many of today’s engineers and scientists were attracted to these fields by the nation’s commitment to the space program in the 1960s and 1970s.
In addition, the ILC will attract thousands of talented scientists and students from around the world. If history is any guide, many of these highly talented and motivated scientists will remain in the United States and continue to contribute to the nation’s technological leadership, which in turn will stimulate domestic economic growth through scientific and technological innovation. Moreover, some of the world’s best scientists undoubtedly will join the nation’s universities to be close to the project, thereby enriching these institutions. A key component of the success of U.S. academic research institutions has been direct and easy access to the world’s premier research facilities and infrastructure; hosting the ILC would extend this pattern of success into the 21st century. In short, constructing and operating a world-class facility will create an unparalleled intellectual environment and stimulate innovation and creativity.
The nation to host the ILC will require a substantial number of highly trained accelerator physicists, engineers, and technicians to operate the facility. The ILC will be one of the world’s premier training facilities for bright young people entering accelerator and particle physics. Many of these young accelerator physicists will take their skills to other areas of science and technology such as biomedical applications and materials studies and fabrication. This could contribute to the creation of jobs across the nation, not only in high-technology sectors but in all sectors that benefit from a strong economy and the creation of knowledge.
Finally, investments by other nations in the facility will be very significant, principally through in-kind contributions. These contributions can add value to the host nation by leveraging the skills and abilities of the U.S. technical workforce and industry.
The intellectual benefits of hosting a flagship scientific facility are illustrated by the LHC at CERN. Although the United States is an important partner in this enterprise, the scientific soul is in Geneva. To participate in and significantly contribute to the scientific program of the LHC, U.S. researchers regularly travel (and will continue to travel) to Switzerland to conduct their research on-site at the LHC. Furthermore, scientists directing graduate students find it increasingly important for their young particle physicists to spend a year or more at an operating experiment to fully appreciate and understand the principles that connect day-to-day operations with the underlying quest for physics.
If the United States is successful in attracting the ILC, the actual construction and operation of the facility will require not only international partners but an increase in the resources devoted to the U.S. program. The constant-effort budget underlying the control scenario (Scenario A) will not be sufficient to fully fund the U.S. share of the construction and operation of the ILC’s accelerator and detectors.
Although the U.S. community of particle physicists, accelerator scientists, and engineers does not now have the capacity to undertake an effort as large as the construction and operation of the ILC, there is every expectation that the excitement of such an initiative would attract more than sufficient talent to the field to make it possible by the time a construction decision is announced. That is, the U.S. physical science and engineering community has a lot of latent expertise that could be tapped for the ILC and, the committee believes, would allow the United States to host the ILC.
It is not certain that the aggressive pursuit of both ILC R&D and a bid to host the facility would ensure that the project moves toward construction or that the site chosen would be in the United States. It is the committee’s view, however, that deferring or avoiding an investment in R&D and mounting a compelling bid to host the ILC will not achieve a leadership position for the U.S. program, even if it might be less risky. The committee explicitly acknowledges that focusing U.S. efforts on such a major and singular enterprise would expose the U.S. particle physics program to risk, and while various creative strategies might mitigate the risk, it cannot be entirely avoided.
The decision to take an aggressive approach to the realization of the ILC does not in and of itself constitute a compelling strategy for the U.S. particle physics program. However, it is an essential element in any strategy based on an initial constant-effort (or better) budget and would provide an opportunity for the U.S. program to achieve a well-defined and distinct position of leadership in the next 15 years. The committee strongly believes that the risk-adjusted return on an investment that enables the United States to become a major center for ILC R&D and to prepare a bid to host it is the best chance the U.S. particle physics program has to occupy a distinct position of leadership over the next decade and beyond.
Action Item 3: The Path Forward for the ILC. The United States should announce its strong intent to become the host country for the ILC and should undertake the necessary work to provide a viable site and mount a compelling bid.
As the host country for the ILC, the United States would likely need to commit to a higher cost share than its international partners. Based on experience, it is expected that 30 to 35 percent of the total cost would be for conventional con-
struction activities at the site. As part of the planning for hosting the project, the DOE and NSF should undertake a study of alternative methods for financing the cost of conventional construction work at the host site. These alternatives could include greater assurance for public appropriations (e.g., lump-sum or advance appropriations), nonfederal contributions, and federally backed third-party financing. Such methods would smooth the construction budget profile and could involve financing program expenditures over multiple years. (For instance, in accord with Strategic Principle 5, a construction project might arrange third-party financing by future planned appropriations.) Experience with science projects shows that uncertainties and shortfalls in annual appropriations can be a leading cause of unnecessary cost escalations and inefficient and unwise, though expeditious, decisions. Alternative methods of financing conventional construction could provide greater funding stability and, in turn, greater certainty that cost and schedule goals will be achieved. Reliance on annual federal appropriations to finance these costs could require a significant increase in the annual budget for the U.S. particle physics program when construction begins.
Locating the ILC near Fermilab would give U.S. elementary particle physics a vibrant center in the coming decades. The existing infrastructure at Fermilab, as well as initial assessments of its geological stability and useable space, make it a logical choice for the ILC in the United States. As the only national laboratory devoted solely to particle physics, Fermilab’s top priority should be to secure the ILC.
One issue that the committee did not address in its analysis was a detailed cost estimate for the ILC.5 The committee was aware that several preliminary estimates had been developed in the United States and elsewhere, but it concluded that these estimates were based on different design concepts and did not necessarily reflect the current plan for the project. The committee also has monitored closely the ongoing GDE, which is currently scheduled to produce an RDR by the end of 2006 that will include an RDR-based preliminary cost estimate. Successful completion of the RDR exercise will be an important demonstration of the feasibility of an ILC. The committee recognizes the prudence of this approach: A credible estimate of project cost must await a specific set of design parameters and, later, the international selection of a viable site. In general, the committee notes that the scale,
complexity, and engineering challenges of the ILC are expected to be very roughly comparable to those associated with the LHC.6
The committee’s recommendations on the path forward toward the ILC are based on the premise that the GDE will produce an RDR that is acceptable to decision makers in the United States and other countries as a basis on which to proceed. This statement may appear to indicate only conditional support for the ILC, but the opposite is true: The committee believes that the ILC is such a tremendous opportunity that it must be pursued vigorously and wholeheartedly. The committee’s recommendations are intended to help guide the next phases of the R&D and the detailed design and international collaboration processes. They are also intended to form a set of expectations in advance of a decision to proceed with the project.
The elements of the committee’s highest priority recommendations and the relationship among them may require additional explanation. Exploration of the Terascale is the committee’s highest priority because it offers the most compelling science opportunities. This exploration will begin at the LHC within 2 years, while the detailed explorations of the Terascale at the ILC are perhaps a decade or more away. Exploiting the LHC and taking a leadership role in ILC R&D must proceed along different paths of action because the construction phase of the LHC is essentially complete and the global particle physics community is ready to conduct experiments with it, whereas the ILC is in an embryonic stage. The United States has invested heavily in the LHC, and U.S. scientists are preparing for operations. The effort to design and build the ILC has, relatively speaking, just begun, with the GDE representing the first phase of an internationally (but informally) coordinated program; the creation of an institution to oversee the building of the ILC is still in the future. The committee strongly believes that a firm commitment to the R&D phase of the ILC and the development of a bid to host the project are necessary to give the United States a scientifically advantageous position when the construction decision becomes tenable.
In summary, within the strategic principles outlined in Chapter 4, the committee believes that the two highest priorities for the U.S. particle physics program are aggressive support for the LHC and ILC programs. In the peak years of the planning and R&D for the ILC accelerator and detectors, support for the LHC and ILC experimental programs will demand a large fraction of the U.S. particle physics budget.
EXPLORATIONS OF PARTICLE ASTROPHYSICS AND UNIFICATION
Finding 4: Opportunities at the Interface of Particle Physics, Astrophysics, and Cosmology. Elementary particle physicists have an extraordinary opportunity to make breakthrough discoveries by engaging in astrophysics and cosmology research that probes energies and physical conditions that are not available in an accelerator laboratory. The investigations simultaneously search for new laws of nature and advance understanding of the origin, evolution, and future of the universe.
The United States has already established a leadership position at the interface of particle physics, astrophysics, and cosmology, but with international activity growing rapidly, further investment will be needed to maintain that leadership.
The committee has identified three major research challenges that are ripe for pursuit:
The direct detection of dark matter in terrestrial laboratories, the results of which could then be combined with measurements of candidate dark matter particles produced in accelerators.
The precision measurement of the cosmic microwave background (CMB) polarization, which would probe the physics during the inflation that appears to have occurred within a tiny fraction of a second following the big bang.
The measurement of key properties of dark energy.
Action Item 4: Coordination of Efforts at the Interface of Particle Physics, Astrophysics, and Cosmology. Scientific priorities at the interface of particle physics, astrophysics, and cosmology should be determined through a mechanism jointly involving NSF, DOE, and NASA, with emphasis on DOE and NSF participation in projects where the intellectual and technological capabilities of particle physicists can make unique contributions. The committee recommends that a larger share of the current U.S. elementary particle physics research budget should be allocated to the three research challenges articulated above.
NASA has historically played a critical role in this area, and it should continue to do so. Projects that cut across agencies and research communities require an additional level of planning and coordination to ensure success, especially when multiple research communities are involved that have overlapping scientific priorities. The key element is coordination of efforts to respond to scientific opportunities. For instance, dark energy can be explored from space and from the ground,
and researchers from particle physics, astrophysics, and cosmology have all expressed interest in doing so. Ways to explore dark energy should be pursued jointly by all three constituencies and the three agencies.
There are existing mechanisms that could provide such coordination. A good example is the broader astronomy and astrophysics decadal survey process, which has provided strategic advice to NSF and NASA in the form of a list of scientific priorities for each decade.7 Not all opportunities at this interface require the simultaneous involvement of all three agencies, of course. Typically, ground-based projects, such as the Large Synoptic Survey Telescope, require coordination between DOE and NSF, while space-based projects, such as the Joint Dark Energy Mission, require coordination between DOE and NASA. Between DOE and NSF, both HEPAP and P5 have started taking an active role in providing coordination of the joint portfolio at this scientific interface. Finally, the Astronomy and Astrophysics Advisory Committee (AAAC), chartered by Congress in the NSF Authorization Act of 2002, is a relatively new mechanism that has started to provide tactical guidance to all three agencies about the implementation of joint projects.
Since current commitments from the particle physics budgets to opportunities at the interface are relatively modest compared to the full particle physics program, it is the sense of the committee that they should be built up to two to three times their current level.
Finding 5: Probes of Neutrinos and Proton Decay. A program of neutrino physics including, eventually, a detector sensitive enough for proton decay offers a probe of unification physics.
In the past 10 years, it became clear that neutrinos have tiny but nonzero masses. This is a departure from the Standard Model and may be a signal of the unification of particle forces.
There are now opportunities to extend this hint of unification. Proton decay experiments might show that the proton is unstable, a monumental discovery that would confirm one of the most basic predictions of unified theories. Neutrinoless double-beta decay experiments could demonstrate that the neutrino is its own antiparticle, which would greatly strengthen the case for interpreting neutrino masses in terms of unification. Experiments that measure the neutrino mixing angle θ13 and the CP-violating parameter that affects neutrino oscillations could provide additional information about particle unification. Finally, important clues
about unification could come from other experiments, including observation of the polarization of the CMB (one of the particle astrophysics priorities recommended above) and axion searches.
A recent study of neutrino physics by the American Physical Society8 identified a set of important questions that need to be addressed and laid out a progressive program of research. The two highest priority recommendations were (1) to establish whether or not the neutrino is its own antiparticle through a phased program of neutrinoless double-beta decay experiments and (2) to carry out a program of experiments to establish the remaining parameters in the neutrino mixing matrix with the goal of understanding CP violation for neutrinos. The second program depends on the value of θ13. It could include reactor experiments sensitive to θ13, long-baseline accelerator experiments sensitive to θ13 and capable of determining the ordering of neutrino masses by observing matter effects, and, eventually, a large-scale, long-baseline experiment with a large multipurpose underground detector capable of detecting CP violation. This large underground detector also could search for proton decay. DOE recently announced a set of mission needs in neutrino physics along the lines recommended by the APS study.
Full exploitation of neutrino physics requires diverse modalities of experimentation, including accelerator beams, reactors, underground experiments, and, especially, underground neutrinoless double-beta decay experiments. Significant efforts are under way in Asia and Europe, as well as in the United States and Canada. In the United States, some neutrino experiments are supported by the nuclear physics community, so effective coordination is essential within DOE’s Office of Science. The NSF is overseeing a process to develop proposals for a U.S. deep underground science and engineering laboratory (DUSEL) that would provide scientists from physics, geology, and biology with a technical infrastructure to conduct investigations deep underground. Neutrinoless double-beta decay and proton decay experiments are examples of projects that could take advantage of such a facility.
Action Item 5: A Staged Neutrino and Proton Decay Research Program. The committee recommends that the properties of neutrinos be determined through a well-coordinated, staged program of experiments developed with international planning and cooperation.
A phased program of searches for the nature of neutrino mass (using neutrinoless double-beta decay) should be pursued with high priority.
DOE and NSF should invite international partners in order to initiate a multiparty study to explore the feasibility of joint rather than parallel efforts in accelerator-based neutrino experiments. Major investments in this area should be evaluated in light of the outcome of this study.
Longer-term goals should include experiments to unravel possible CP violation in the physics of neutrinos and renewed searches for proton decay. There may be a valuable synergy between these important objectives, as the neutrino CP violation measurements might require a very large detector that, if placed deep underground, would also be the right instrument for detecting proton decay.
The committee believes that in order to give U.S. researchers access to the best scientific opportunities in a timely manner, international cooperation and coordination are essential from start to finish. Existing individual projects in these areas, especially neutrinoless double-beta decay, exemplify strong and diverse international participation. The committee recommends, however, that experiments in this area should be globally rationalized from start to finish. Facilitating such a rationalization process is a necessary part of the U.S. commitment to leadership. The effort would also work to avoid unnecessary duplication and would most efficiently deploy the worldwide investments in the field. For instance, efforts are under way in Japan to finish the construction of a high-intensity proton source that has important applications for certain neutrino experiments; the United States is exploring opportunities for experiments that could use different baselines to achieve different sensitivities; and there are proposals for neutrino beams at CERN. The objective of investigating the feasibility of a joint program is not simply to avoid unnecessary overlap or duplication of experiments. Rather, in the constrained budget environment facing the international particle physics community, it is to explore whether pooling resources can lead to a more robust scientific program and achieve key experimental results more quickly.
Finding 6: Precision Probes of Physics Beyond the Standard Model. Studies of the patterns of weak interactions (particularly rare decays and CP violation in the quark sector), dipole moments, tabletop tests of gravity, and lepton flavor and lepton number violation could expand our understanding of and more precisely define the physics that might lie beyond the Standard Model.
The information from such studies is complementary to that obtainable from direct searches for new particles at the LHC and ILC and has historically played an important role in constraining models of new physics. The current B factories and CLEO-c will conclude their programs in this arena by the end of 2008. Future B
physics efforts include the LHCb and a possible future super-B factory (under consideration in Japan and Italy). Lepton flavor violation studies offer an important window on new physics, as do searches for rare kaon decays. Some of these studies require meson beams from a proton facility, such as J-PARC in Japan; others can proceed using a high-intensity electron-positron collider, such as the Beijing Electron-Positron Collider facility in China. Precision measurements of the muon g-2 parameter and searches for electric dipole moments also offer new constraints on physics beyond the Standard Model. Some of the latter can be relatively small-scale efforts; experiments with significantly improved reach are possible within the next few years.
Action Item 6: Precision Probes of Physics Beyond the Standard Model. U.S. participation in large-scale, high-precision experiments that probe particle physics beyond the Standard Model should continue, but the level of support that can be sustained will have to be very sensitive to the overall budget picture. Only very limited participation will be feasible in budget scenarios with little or no real growth. Participation in inexpensive, small-scale, high-precision measurements should be encouraged in any budget scenario.
This is an area where investment and collaboration in joint international projects can offer significant opportunities and leverage to all parties. Small-scale experiments should be supported as part of the overall program when they offer significant reach into unexplored physics.
IMPLICATIONS OF THE STRATEGIC AGENDA UNDER DIFFERENT BUDGET SCENARIOS
The committee recognizes that the United States could pursue more than one strategy in particle physics in the next decade. It outlines here the strategy that will have the highest risk-adjusted return and, in its best judgment, will be most likely to sustain U.S. leadership in particle physics. Failure to participate in active exploration of the Terascale would surrender U.S. leadership because the nation would not be able to pursue the most compelling science opportunities. Finally, the committee considered strategies that abandoned accelerators altogether; these approaches were rejected because they did not lead to a national program of sufficient health and vitality to sustain itself.
As the committee considered the constant-effort budget (Scenario A), it estimated that such a budget could support the LHC and ILC efforts it recommends, as well as an expansion of the current efforts in particle astrophysics through 2010. Moreover, as long as the program adheres to the strategic principles outlined in Chapter 4, there would be adequate funds to support some smaller programs that
are important to the field. However, the Scenario A budget would not provide much funding for neutrino-mass measurements, nor would it allow proceeding with any major accelerator-based neutrino program based in the United States without significant foreign contributions. The more ambitious initiatives in neutrino physics would need to rely on forging alliances with colleagues abroad.
The straightforward set of priorities articulated in action items 1 through 6 could take the U.S. program through the next 5 years, but if a decision is made to go ahead with the proposed ILC, important new considerations will enter the picture. Most important, the ILC is a multi-billion-dollar facility, and if the United States were to be the host, as the committee has recommended, it would be expected to shoulder a significant fraction of the costs. While the ongoing U.S. program could and should provide a significant share of the necessary funds, it cannot fully cover the expected contribution of the host country. Funding beyond that assumed in Scenario A would be required to build and operate the ILC.
The committee also considered the appropriate set of priorities under Scenario B. In this more pessimistic case, the committee still recommends that the highest priority be participation in the LHC and the ILC programs over the next 5 years, so that the U.S. particle physics program could provide U.S.-based scientists and students with opportunities to participate in the most exciting aspects of elementary particle physics. However, the committee believes that in Scenario B the United States would not be able to host the ILC; rather, it would be a strong participant in such a facility hosted abroad. Indeed, in Scenario B, full U.S. participation in the exploration of the Terascale might well be jeopardized. In this scenario, the United States could expand its efforts in particle astrophysics and participate in globally coordinated neutrino experiments abroad. However, distinctive and distinguished U.S. leadership in particle physics would very likely be sacrificed. Certainly the scientific influence of the U.S. program in particle physics would be much diminished. Even with an ILC located overseas, U.S. participation would require budget increases well above the Scenario B level after the initial 5 years.
It was apparent to the committee that Scenario C could provide a significant portion of both the capital and operating resources necessary for the United States to host the ILC. This goal and participation in the LHC would remain the top priorities. However, this scenario would allow an expanded program in particle astrophysics (the committee’s next highest priority) and a fuller program with international collaborators in neutrino physics and proton decay and in flavor physics. This scenario would go a long way toward ensuring retention of the infrastructure and expertise required for the ILC and toward securing a U.S. presence among the leaders in this field. The more optimistic scenario, D, would offer opportunities to engage strongly in all aspects of the science described in this
report and to recover more securely the U.S. role as a leader in this field. Under this scenario, operation of a facility on the scale of ILC would probably not require additional funding, though additional support during construction might still be needed.
REALIZING THE STRATEGIC VISION FOR ELEMENTARY PARTICLE PHYSICS
As some important elementary particle physics experiments at U.S. national laboratories complete their current objectives, Fermilab will have a special role as the only national laboratory devoted completely to particle physics. In framing the future role for the United States in elementary particle physics, the committee would like to emphasize not only the importance of a flourishing and dynamic Fermilab but also the necessity of ensuring that the overall resources of the U.S. particle physics community be deployed to best effect.
Implementing the committee’s recommended priorities will require strong leadership and a strong commitment to a common vision from all stakeholders (including the particle physics community, federal research agencies, the Office of Science and Technology Policy, the Office of Management and Budget, and Congress). Previous planning efforts were sometimes not fully realized because they lacked coherence, a clear consensus on relative priorities, and a commitment to implementation. Moreover, in the past, planning at the national laboratories was not tied to an overall national plan. When adequate resources were available, this democracy of ideas was a strong point of the program; now, somewhat more centralized planning and implementation are necessary. To move forward, the community must adopt a new way of making decisions, and those decisions need to follow from a strategic plan.
Elementary particle physics is poised to make potentially transformative discoveries. If the United States commits to a strategic vision such as the one the committee has laid out, the nation can continue to occupy a position of leadership in this vibrant and exciting science. Such an aspiration is worthy of a great nation wishing to remain on the scientific and technological frontiers. It will inspire future generations, repay the investments many times over, and provide a fuller understanding of mankind’s place in the cosmos.
Every 10 years or so, the National Research Council’s Board on Physics and Astronomy (BPA) engages in a decadal survey of physics. The current survey, Physics 2010, is under way and is expected to be completed over a 5-year period following its inception in 2005.
The Physics 2010 decadal survey is focused on an assessment of and outlook for each branch of physics. Each assessment will be conducted by an independent study committee appointed by the National Research Council based on the advice and recommendations of the BPA. This decadal survey of physics serves two broad purposes: (1) it provides a periodic snapshot of the field that is useful for tracking and understanding the evolution of the science, and (2) it provides a process whereby compelling emerging opportunities can be identified and developed.
The Physics 2010 project will include reports on atomic, molecular, and optical science; plasma physics; condensed matter and materials physics; elementary particle physics; and nuclear physics. The Committee on Elementary Particle Physics in the 21st Century undertook the preparation of this volume, the first of this series.