Summary

Elementary particle physics is the modern expression of the human quest to understand the basic building blocks of nature and the rules that govern the physical world. Over that long journey, the focus has evolved from atoms to nuclei to quarks and leptons; today, particle physicists seek to understand how space, time, and the universe came to be, how the particles and their interactions are unified, and how quantum mechanics and gravity are connected.

The quest to understand nature at the most basic level has led to the development of tools, from accelerators and medical imaging devices to the World Wide Web; as well as a highly trained workforce, all of which benefit the nation and society more broadly. If history is any guide, the even deeper understanding of matter, energy, space, and time that lies ahead will lead to even more benefits.

Twenty years ago, the big questions driving particle physics included the search for the Higgs boson, the origin of the matter–antimatter asymmetry in the universe, dark matter, and an understanding of dark energy, as well as the unification of the particles and their interactions. Some of the questions have been answered, and others have evolved into richer ones. The field of elementary particle physics is now poised for major advances in our understanding of the fundamental nature of matter, energy, space, and time, and the deep and mysterious connections between them.

Addressing the big questions in particle physics today involves particle colliders, along with a panoply of other approaches, including precision laboratory measurements, ultra-sensitive detectors in underground laboratories, astrophysical observations, gravitational wave measurements, the most powerful computers and computational techniques, and the theoretical exploration of the deep physical and mathematical principles underlying fundamental physics.

Elementary particle physics also depends on discoveries and developments made in many other subfields of physics and engineering, as well as in artificial intelligence, quantum science and technology, and microelectronics. The field is attracting some of the most talented scientists from across disciplines to carry out near-impossible experiments that probe nature at its most fundamental level. The connections that cross boundaries are not only essential to progress in particle physics but enrich the other disciplines involved.

The scope of the program of activities needed to address the agenda of particle physics is broader and more diverse than ever, and it is beyond the resources, both human and fiscal, of any single nation. The science enterprise of particle physics necessarily involves many nations, global and national facilities, and international coordination and cooperation.

The United States is a leader in particle physics today and is well positioned to continue to lead in the future. It has the workforce and material resources needed; it has a powerful system of universities, national laboratories, and industry; and the breadth of its activities in particle physics is unsurpassed. Particle physics is also fortunate to have two major sponsors—the Department of Energy and the National Science Foundation. Finally, the United States has a rich history of accomplishment, leadership, and collaboration in particle physics.

Recognizing the scientific opportunities ahead and the continuing broad benefits over both the short and long term, the Committee on Elementary Particle Physics firmly believes that the United States must continue to be a leader in particle physics. The committee notes that this echoes the overarching recommendation of the 2006 report Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics.¹ And for good reason, the scientific opportunities and potential benefits have only become more compelling.

To maintain U.S. leadership and capitalize on future opportunities and benefits, significant actions and investments are required in the coming years. While the vision outlined in this report extends over a 40-year horizon, timely implementation of its recommendations is crucial for amplifying this leadership and realizing the field’s potential. The committee’s eight recommendations are summarized here and described in more detail in the chapters that follow.

RECOMMENDATIONS

Recommendation 1: The United States should host the world’s highest-energy elementary particle collider around the middle of the century. This requires the immediate creation of a national muon collider research and development program to enable the construction of a demonstrator of the key new technologies and their integration.

A collider with approximately 10 times the energy of the Large Hadron Collider (LHC) is crucial for addressing the big questions of particle physics and making discoveries. A 10-TeV muon collider on the Fermi National Accelerator Laboratory (Fermilab) site would have similar discovery reach as a 100-TeV proton collider. A muon collider combines the physics advantages of an electron–positron and a proton–proton collider, with a much smaller size. A machine that collides unstable elementary particles has never been attempted before and requires significant research and development (R&D), international coordination, and a demonstrator project to establish its feasibility. Developing a U.S.-hosted muon collider—an unprecedented machine requiring considerable research, development, and a feasibility demonstrator—would solidify U.S. leadership in particle physics and drive accelerator innovation. The “middle of the century” timeline reflects the project’s inherent complexities, which are elaborated in Chapter 2, and underscores the need for global coordination.

Recommendation 2: The United States should participate in the international Future Circular Collider Higgs factory currently under study at CERN to unravel the physics of the Higgs boson.

Participation in the design and building of an international “Higgs factory” on a shorter timescale than a muon collider is also critical. Such a machine will explore the mysteries of the Higgs, including its possible links to cosmology, as well as probing other electroweak phenomena. U.S. participation and contributions from its universities, national laboratories, and industry will help ensure the success of such a machine. The partnership between CERN and the United States is important for both as well for the health of the field.

Recommendation 3: The United States should continue to pursue and develop new approaches to questions ranging from neutrino physics and tests of fundamental symmetries to the mysteries of dark matter, dark energy, cosmic inflation, and the excess of matter over antimatter in the universe.

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¹ National Research Council, 2006, Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics, The National Academies Press, https://doi.org/10.17226/11641.

The evolution of particle physics and the discoveries made over the past two decades have expanded its scope and have led to new approaches for exploring the fundamental nature of matter, energy, space, and time. Understanding the mysteries of neutrinos will involve accelerators, reactors, and experiments that search for rare processes. Cosmological measurements can also probe neutrinos as well as dark matter, dark energy, and the earliest moments of the universe, complementing what can be learned from colliders.

Recommendation 4: The United States should explore new synergistic partnerships across traditional science disciplines and funding boundaries.

There are growing connections to other scientific disciplines, including astronomy, particle astrophysics, nuclear physics, gravitation, atomic physics, and quantum science. For example, quantum sensing and instrumentation can provide new, more powerful techniques for detecting dark matter. In some cases, measurements of trapped ions and molecules can explore energy scales beyond those achieved by the highest-energy accelerators. Crossing the boundaries of traditional disciplines is necessary to advance the mission of elementary particle physics and benefits all the fields involved.

Recommendation 5: The United States should invest for the long journey ahead with sustained research and development funding in accelerator science and technology, advanced instrumentation, all aspects of computing, emerging technologies from other disciplines, and a healthy core research program.

As particle physics has matured, many of the undertakings now involve turning the presently impossible into the merely very difficult. Project timescales and costs have grown. Particle physics thrives on innovation, and thus, sustained investment in R&D is essential to its long-term success. Equally important is adequate support for the researchers whose commitment to the long journey makes the discoveries possible.

Recommendation 6: The federal government should provide the means and the particle physics community should take responsibility for recruiting, training, mentoring, and retaining the highly motivated student and postdoctoral workforce required for the success of the field’s ambitious science goals.

As they have in the past, the discoveries and advances in elementary particle physics will depend on an ambitious, determined, and highly trained workforce, especially students, postdocs, and other early-career researchers. The challenges in training, supporting, and retaining the workforce needed to fully realize the field’s dreams are significant and will require concerted action by the funding agencies and the particle physics community.

Recommendation 7: The United States should engage internationally through existing and new partnerships and explore new cooperative planning mechanisms.

Elementary particle physics is now a global scientific enterprise involving almost 100 nations. Its continued and future success depends on international planning, cooperation, and partnerships. The world leaders of particle physics—including the United States—must strive to improve the planning, cooperation, and partnership mechanisms and to involve the Global South² to a fuller extent.

Recommendation 8: Funding agencies, national laboratories, and universities should work to minimize the environmental impact of particle physics research and facilities.

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² The Global South broadly refers to the “developing economies” in Africa; Latin America and the Caribbean; Asia, with the exception of Japan, South Korea, and Israel; and Oceania, with the exception of Australia and New Zealand (United Nations Conference on Trade and Development, 2022, Handbook of Statistics 2022, https://unctad.org/system/files/official-document/tdstat47_en.pdf).

Particle physics, like the other sciences, must strive to make its science energy efficient while minimizing the impact on the environment. With its large accelerator facilities, which require significant amounts of energy, this will be challenging. An added benefit of meeting the challenge is that many of the solutions that the field creates will have broader applicability across science and society.

If the committee’s recommendations are implemented, this is the future for particle physics that can be anticipated 40 or so years from now:

Circa 2065, we will have gained a much deeper understanding of the physical world, answered some of the mysteries that bedevil us today, and will be puzzling about new ones. What we know today will fit into a grander, more unified understanding of matter, energy, space, and time. It is impossible to predict the discoveries that will come or when they will occur; but it is easy to predict that our deeper understanding about the physical world will have impact across the sciences as well as new benefits for humankind. The global effort that made all of this possible will include an international Higgs Factory with strong U.S. involvement, surprising us with new insights about the Higgs; a muon collider that was once thought impossible to build, hosted by the Fermi National Accelerator Laboratory and making unexpected discoveries; and a program of dark matter studies and cosmic surveys revealing new insights into particle physics and cosmology. And the connections made across the subfields of physics and the sciences more broadly will be illuminating the power of working across discipline boundaries to the larger benefit of science. Particle physics will not only be a shining example of how the world can make progress on seemingly impossible problems, but it will also be inspiring the next generations, here and around the globe, with its amazing discoveries and mysteries yet to be revealed. In short, the future for particle physics will be even brighter than it is today.

ORGANIZATION OF THE REPORT

The chapters ahead provide a more detailed discussion of U.S. elementary particle physics in the global context and the committee’s eight recommendations and the motivations behind them. Chapter 1 begins with an overview of particle physics today—the science, how it is carried out, and progress since the 2006 National Research Council study.

Chapter 2, “The Next 40 Years,” is the heart of the report. It describes what is necessary to make the committee’s vision for particle physics a reality and lays out in detail Recommendations 1 through 5. Realizing the opportunities that lie ahead requires a talented and well-trained workforce with cutting-edge skills that cut across scientific disciplines.

Chapter 3, “Workforce,” describes both the challenges to doing so and the actions that must be taken to ensure particle physics continues to have the talented workforce needed to achieve its goals, which motivates Recommendation 6.

Chapter 4, “International,” describes the current state of international collaboration and cooperation and how it has evolved. Because particle physics is a global enterprise involving close to 100 nations, the future of U.S. particle physics is dependent on integrating its aspirations and plans with those of other countries, and this forms the basis for Recommendation 7.

Chapter 5, “Benefits,” describes some past and continuing benefits of elementary particle physics. They include benefits to other scientific disciplines and both short- and long-range benefits to the United States and humankind. Historically, the quest to understand the deepest inner workings of nature has made today’s world possible—from materials, medicines, and energy to the devices that underpin the current information age.

Chapter 6, “Energy and Environmental Management,” calls attention to the importance of minimizing the environmental impact of energy-intensive particle-physics facilities and the broader benefits that can result from doing so. This is the basis for the committee’s final recommendation, Recommendation 8.

The final chapter, Chapter 7, lists all of the report’s findings, conclusions, and recommendations.