THE SCIENCE OPPORTUNITIES
Elementary particle physics—the study of the fundamental constituents and nature of the universe—is poised to take the next significant step in answering questions that humans have asked for millennia: What is the nature of space and time? What are the origins of mass? How did the universe begin? How will it evolve in the future? The next few decades could be one of the most exciting periods in the history of physics.
One of the great scientific achievements of the 20th century was the development of the Standard Model of elementary particle physics, which describes the relationships among the known elementary particles and the characteristics of three of the four forces that act on those particles—electromagnetism, the strong force, and the weak force (but not gravity). However, in the energy regions that physicists are just now becoming able to access experimentally, the incompleteness of the Standard Model becomes apparent. It is unable to reconcile the twin pillars of 20th century physics, Einstein’s general theory of relativity and quantum mechanics. In addition, recent astronomical observations indicate that everyday matter accounts for just 4 percent of the total substance in the universe. The rest of the universe consists of hypothesized entities called dark matter and dark energy that are not described by the Standard Model. Other challenges to the Standard Model are posed by the predominance of matter over antimatter in the universe, the early evolution of the universe, and the discovery that the elusive particles known as
neutrinos have a tiny but nonzero mass. Thus, despite the extraordinary success of the Standard Model, it seems likely that a much deeper understanding of nature will be achieved as physicists continue to study the fundamental constituents of the universe.
Elementary particle physicists use a wide variety of natural phenomena to investigate the properties and interactions of particles. They gather data from cosmic rays and solar neutrinos, astronomical observations, precision measurements of single particles, and monitoring of large masses of everyday matter. In addition, crucial advances historically have come from particle accelerators and the complex detectors used to study particle collisions in controlled environments. Today the most powerful accelerator in the world is the Tevatron at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, which is scheduled to be shut down by the end of the decade. A more powerful accelerator, the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, is scheduled to begin colliding protons in 2007. Both theoretical and experimental evidence suggests that revolutionary new physics will emerge at the energies accessible with the LHC.
Beyond the LHC, physicists around the world are designing a new accelerator known as the International Linear Collider (ILC), which would use two linear accelerators to collide beams of electrons and positrons. Together, the LHC and an ILC will enable physicists to explore the unification of the fundamental forces, probe the origins of mass, uncover the dynamic nature of the “vacuum” of space, deepen the understanding of stellar and nuclear processes, and investigate the nature of dark matter. These tasks cannot be accomplished with the LHC alone.
THE U.S. ROLE IN PARTICLE PHYSICS
For more than half a century, the United States has been a leader in particle physics. But over the next few years, as the flagship U.S. particle physics facilities are surpassed on the energy frontier by new facilities overseas or are converted to other uses, the intellectual center of gravity of the field will move abroad. At the same time, the conclusion of these important experiments creates an opportunity for the United States to consider major new initiatives.
Today, the U.S. program in elementary particle physics is at a crossroads. For the U.S. program to remain relevant in the global context, it must take advantage of exciting new opportunities. Doing so will require decisive actions and strong commitments; it also will require a willingness to assume some risks. Thus, to ensure continued U.S. leadership in this important scientific area, a new strategic framework is needed that can guide the difficult decisions that have to be made.
Seven strategic principles underlie the actions recommended by the committee:
Strategic Principle 1. The committee affirms the intrinsic value of elementary particle physics as part of the broader scientific and technological enterprise and identifies it as a key priority within the physical sciences.
A strong role in particle physics is necessary if the United States is to sustain its leadership in science and technology over the long term. The nation’s investments in basic research in the physical sciences have contributed greatly to U.S. scientific and technological prowess. Elementary particle physics has been a centerpiece of the physical sciences throughout the 20th century. It has inspired generations of young people to become members of the strongest scientific workforce in the world. It also has attracted outstanding scientists from abroad to come to the United States and contribute to the nation’s intellectual and economic vitality.
In addition, particle physics has generated waves of technological innovations that have found applications throughout the sciences and society. The protocols that underlie the World Wide Web were developed at CERN, and the two-way interactions between particle physics and high-performance computing and communications have continued to blossom. Particle physics has generated critical technologies in such areas as materials analysis, medical treatment, and imaging.
Strategic Principle 2. The U.S. program in elementary particle physics should be characterized by a commitment to leadership within the global particle physics enterprise.
In today’s world, leadership in the sciences does not mean singular dominance. Rather, leadership is characterized by taking initiatives on the scientific frontier, accepting risks, and catalyzing partnerships with colleagues at home and abroad. A leadership position enables a country to exploit scientific and technological developments no matter where they emerge. The U.S. program should not only pursue the most compelling scientific opportunities, but it also should establish a clear path for the United States to reach a position of leadership in particle physics.
Strategic Principle 3. As the global particle physics research program becomes increasingly integrated, the U.S. program in particle physics should be planned and executed with greater emphasis on strategic international partnerships. The United States should lead in mobilizing the interests of international partners to jointly plan, site, and sponsor the most effective and the most important experimental facilities.
As experimental facilities become more complex and expensive, the already extensive levels of international collaboration in particle physics will need to intensify further to most effectively address the challenges on the scientific frontier. The committee believes that particle physics should evolve into a truly global collaboration that would enable the particle physics community to leverage its resources, prevent duplication of effort, and maximize opportunities for particle physicists throughout the world. Credible and reliable participation, as well as leadership, in strategic international partnerships require the United States to maintain a healthy and vital particle physics program.
Strategic Principle 4. The committee believes that the U.S. program in elementary particle physics must be characterized by the following to achieve and sustain a leadership position. Together, these characteristics provide for a program in particle physics that will be lasting and continuously beneficial:
A long-term vision,
A clear set of priorities,
A willingness to take scientific risks where justified by the potential for major advances,
A determination to seek mutually advantageous joint ventures with colleagues abroad,
A considerable degree of flexibility and resiliency,
A budget consistent with an aspiration for leadership, and
As robust and diversified a portfolio of research efforts as investment levels permit.
The last of these characteristics—breadth—deserves special consideration. A broad array of scientific opportunities exists in elementary particle physics, and it is not possible to foretell which will yield important new results soonest. Two of the greatest discoveries of the last decade—those of nonzero neutrino masses and dark energy—were quite unexpected and arose from experiments that did not use accelerators, the tools characteristic of many other advances in particle physics. Thus, there is a strong need for supporting a variety of approaches to current scientific opportunities.
It is important to maintain a diverse and comprehensive portfolio of research activities that encompasses university-based students and faculty, national laboratories, and activities conducted in other countries. Even during periods of budgetary stringency, sufficient funding and diversity must be retained in the pipeline of projects so that the United States is positioned to participate in the most exciting science wherever it occurs.
Strategic Principle 5. The Secretary of Energy and the Director of the National Science Foundation, working with the White House Office of Science and Technology Policy and the Office of Management and Budget and in consultation with the relevant authorization and appropriations committees of Congress, should, as a matter of strategic policy, establish a 10- to 15-year budget plan for the elementary particle physics program.
Many important experiments in particle physics require multiyear plans and budgets. Experience with past science projects has shown that uncertainties and shortfalls in annual appropriations can lead to unnecessary cost escalations and to inefficient and unwise, even if expeditious, decisions. The ability to make sustained multiyear commitments is also essential if the United States is to appear credible and serious in the international arena, especially in terms of fostering collaboration and cooperation.
Strategic Principle 6. A strong and vital Fermilab is an essential element of U.S. leadership in elementary particle physics. Fermilab must play a major role in advancing the priorities identified in this report.
Many universities and national laboratories have made vital contributions to particle physics over the years. But in recent years the number of laboratories devoted primarily to particle physics has been declining and will continue to do so, especially as the facilities at the Stanford Linear Accelerator Center and at Cornell University direct their primary focus away from particle physics. Continuing efforts from university groups and other laboratories will be essential to realize the full potential of the U.S. particle physics program. At the same time, Fermilab will play a special role as the only laboratory dedicated chiefly to particle physics.
Strategic Principle 7. A standing national program committee should be established to evaluate the merits of specific projects and to make recommendations to DOE and NSF regarding the national particle physics program in the context of international efforts.
The changing environment in particle physics requires a reexamination of the advisory structure for the field. The combination of unparalleled opportunities in particle physics and inevitable fiscal constraints force the federal government and the particle physics community to make very hard choices and coordinate programs at the various national laboratories and universities. A standing national committee is needed that has sufficient authority to establish a compelling set of priorities and to advise the federal agencies that support particle physics. Such a committee should evaluate the merits of specific proposals and make recommendations regarding the national particle physics program within the context of the
international particle physics program. Existing advisory committees such as the Department of Energy (DOE)/National Science Foundation (NSF) High Energy Physics Advisory Panel (HEPAP) or the Particle Physics Project Prioritization Panel (P5) could be strengthened and broadened to take on this role.
RECOMMENDED ACTION ITEMS
The committee examined several possible scenarios for the funding of particle physics in the United States. Much of the analysis for the next few years was conducted assuming a budget that would rise with the rate of inflation, representing a constant level of effort (though particle physics would represent an ever smaller proportion of the gross domestic product). If, instead, the budget remains flat and without any adjustments for inflation, policy makers will have decided to disinvest in this area of science. This course is incompatible with the goal of leadership for the U.S. program in particle physics.
Recently, both the executive and the legislative branches of the federal government expressed a desire to increase funding for basic research in the physical sciences. Real increases ranging from 2 to 3 percent per year to a doubling over 7 years would enable many exciting experiments to be conducted that cannot be realized in the constant-effort budget.
The committee presents its recommended strategy for the U.S. role in particle physics over the next 15 years in the form of six action items ranked in priority order. The most compelling current scientific opportunity in elementary particle physics is exploration of the Terascale, and this is the committee’s highest priority for the U.S. program. Direct investigations of phenomena at the energy frontier hold the greatest promise for transformational advances. Within this context, the experimental programs at the LHC and at the proposed ILC offer the best means for seizing this opportunity.
The committee’s recommended strategy for exploitation of the LHC and initiation of the ILC addresses projects at radically different stages of realization. On the one hand, the construction phase of the LHC project, including the installation of its massive detectors, is essentially complete, and the global particle physics community is ready to use it. On the other hand, the ILC remains a concept in development, although a substantial amount of R&D demonstrating the feasibility of the technologies selected for the facility has been successfully undertaken during the past decade. Taken together, these two facilities represent a 20-year campaign to seize the opportunities afforded by the opening of the Terascale.
Action Item 1. 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 LHC will be the center of gravity for elementary particle physics over at least the next 15 years as it explores the new phenomena expected to exist at the Terascale. More and more U.S. scientists and students, as well as many others from around the world, are focusing their efforts at this facility, and the United States already has made substantial contributions of resources, people, and equipment to the LHC. U.S. research groups that will carry out experiments at the LHC need to be adequately supported, and the United States should participate in upgrades of experimental facilities as those upgrades are motivated and defined through scientific results obtained from operating the facility.
Action Item 2. The United States should launch a major program of R&D, design, industrialization, and management and financing studies of the ILC accelerator and detectors.
Strong theoretical arguments and accumulating experimental results provide convincing evidence that the Terascale will provide a rich array of physics that will demand exploration by both hadron colliders (such as the LHC) and electron colliders. The consensus of the elementary particle physics community worldwide is that the ILC should be the next major experimental facility to be built. No matter what the LHC finds, an ILC will enable an even greater exploration of the mysteries of the Terascale.
The Global Design Effort (GDE) for the linear collider, which is currently under way, expects to produce an initial cost estimate based on the reference design by the end of 2006, with a full technical design proposal in 2009. An informed decision on the construction of an ILC could be made as soon as a technically credible cost estimate exists; ideally, this decision should be made no later than 2010, by which time the LHC should have revealed the nature of some of the new physics that lies at the Terascale. (The committee provides additional analysis of the path forward in Appendix A.)
Significant R&D is necessary to resolve the remaining technological challenges and to minimize the cost of this multi-billion-dollar facility. Based on evidence presented to the committee and subsequent analysis, U.S. expenditures on R&D for the ILC should be greatly increased. For the accelerator, this commitment should be as high as $100 million in the peak year, with a cumulative investment of $300 million to $500 million over the next 5 years. For the detectors, the appropriate level of resources for R&D would be perhaps $80 million over this period.
Action Item 3. 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.
The United States should move forward in preparing a bid to host the ILC project. Such an aspiration is worthy of a great nation wishing to occupy a leadership position on the scientific and technological frontiers. Building the ILC in the United States will inspire future generations, amply repay the required investments, and lead to a much greater understanding of the universe in which we live. In addition, building and operating the ILC in the United States will provide a focal point to attract talented students and scientists from around the world to U.S. academic research institutions.
One issue that the committee did not address in its analysis was the detailed cost estimate for constructing an ILC. The committee was aware of several preliminary estimates that were developed previously in the United States and other countries, but it concluded that these estimates were based on different design concepts and did not necessarily represent the current plan for the project. The committee also has monitored closely the ongoing GDE, which is currently scheduled to produce by the end of 2006 a Reference Design Report (RDR) that will include a preliminary cost estimate based on the reference design. The committee recognizes the prudence of this approach: A credible estimate of project cost must await a specific set of design parameters and, later, 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.
If the United States is successful in its bid to host the ILC, an increase in resources devoted to particle physics in the United States will be required. A constant-effort budget will not be sufficient to fund the U.S. share of site and mitigation costs, of housing the assembled scientific and engineering staff during construction, and of the construction and operation of the ILC accelerator and detectors.
Although site selection for the ILC will be determined through an international process, the existing physical infrastructure and human capital at Fermilab make it an advantageous site within the United States. As the only national laboratory devoted primarily to particle physics, Fermilab has an opportunity and a responsibility to the national particle physics program to secure the ILC as its top priority.
Action Item 4. 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 participa-
tion in projects where the intellectual and technological capabilities of particle physicists can make unique contributions. The committee recommends that an increased share of the current U.S. elementary particle physics research budget should be allocated to the three research challenges articulated below.
Three major research challenges in astrophysics and cosmology research could lead to discoveries with potentially momentous implications for particle physics:
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.
The United States has already established itself as a leader at the interface of particle physics, astrophysics, and cosmology. Since current commitments to this area from the particle physics budgets are relatively modest compared to the full program, it is the sense of the committee that they should be built up to approximately two to three times the current level.
Action Item 5. 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 charge-parity (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 demonstration that neutrinos have nonzero masses may be one of the first signals of the new physics expected in the years ahead, since the observed masses are in the range predicted by theoretical ideas that unify the forces of nature. In the future, 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 unified theories of the fundamental forces. Furthermore, proton decay experiments might show that the proton is unstable, which would confirm one of the most basic predictions of unified theories.
Full exploitation of large, accelerator-based opportunities in neutrino physics will require planning in an international framework.
Action Item 6. 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 of little or no real growth. Participation in inexpensive, small-scale, high-precision measurements should be encouraged in any budget scenario.
The information from such studies is complementary to that obtainable via direct searches for new particles at the LHC and ILC and has historically played an important role in constraining models of new physics. Types of investigation include a future B factory, lepton-flavor violation and rare-decay studies, precision measurements of the muon g-2 parameter, and searches for electric dipole moments. Some of the latter can be relatively small-scale efforts and should be supported as part of the overall program when they offer significant reach into unexplored physics.
LOOKING TO THE FUTURE
With experimental access to the Terascale at the LHC and the proposed ILC, the particle physics community is poised for discoveries that could revolutionize how we view our world and the universe. Without question, the United States should be a leader in this great scientific adventure.
If these recommendations are carried out in accordance with the committee’s strategic principles, the United States will maintain and enhance, for decades, its position as a leader in this field. Achieving these goals will require increased investment, but this investment will be richly repaid by progress across the science and
technology frontier, the invigoration of particle physics, a boost in the morale of young scientists across a variety of disciplines, and the generation of new high-technology jobs.
If the United States does not win the bid for the ILC or chooses not to pursue this option, the national program still should participate vigorously in the LHC and ILC programs and expand efforts at the interface of particle physics, astrophysics, and cosmology. Without a modest budget increase, the U.S. program would have to rely on international partners to play a leading role in exploring much of the physics of the neutrino sector.
If the United States does not actively participate in exploration of the Terascale and if support for the field continues to decline, it will be clear that the United States has decided to abandon leadership in particle physics. U.S. researchers would then only be able to participate modestly in the LHC and ILC programs, and a U.S. leadership position more than half a century old would be sacrificed.
If a decision is made to host the ILC project in this country, the United States would be expected to shoulder a significant fraction of its costs. Such a course would require growth in the particle physics budget to purchase the right-of-way and to design, build, staff, and operate this forefront scientific facility.
The proposed American Competitiveness Initiative offers one way to realize many of the opportunities described in this report. By committing to a strategic vision in particle physics, the United States can remain a leader in this vital area of science and technology.