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Particle Physics Today

THE SCIENCE

Elementary particle physicists seek to understand how the universe works by discovering the most elementary constituents of matter and energy and the interactions between them, exploring the basic nature of space and time itself, and probing the deepest connections between matter, energy, space, and time.

Matter and Energy

Matter and energy were unified in Einstein’s theory of relativity, famously in the equation E = mc². Relativistic quantum mechanics implies that for every particle there exists an anti-particle of the same mass with opposite charges. Hence, with enough energy, any and all particles can be produced in the laboratory, no matter how massive. Particles are smashed together to make energetic collisions that can produce new indivisible, or elementary, particles that are the building blocks of matter.

Quantum mechanics teaches that particles behave as waves: the higher the energy of the particle, the smaller the wavelength. Thus, high-energy particle colliders are in essence high-resolution microscopes that can be used to probe the structure of matter and study the forces that shape the collisions. Ernest Rutherford used such collisions to deduce the structure of the atom; today, such collisions are used to discover and to measure the properties of the subatomic elementary particles hidden within the nucleus of atoms and thereby to unravel the weak and strong nuclear forces.

By the end of the 20th century, particle physicists had completed a comprehensive theory of matter and forces—the Standard Model—that describes all the observed elementary particles and forces acting on them (see Figure 1-1). There are three atomic and subatomic forces: the force of electricity and magnetism that govern the properties of atoms and molecules; the weak nuclear force, responsible for radioactivity and the transmutation of matter; and the strong nuclear force that governs the properties of nuclei. These forces are described by an extension of electromagnetism wherein elementary particles have more than one type of charge.

The elementary particles of the Standard Model consist of quarks (that make up nuclei) and leptons (electrons and neutrinos). The quarks carry three (“color”) charges that source the strong nuclear force, which confines the quarks within the proton. The quarks and leptons carry two (“flavor”) charges that source the weak nuclear force, and both quarks and electrons carry one (electric) charge that shapes the structure of the atom. Both quarks and leptons come in three “families” with identical charges, but wildly disparate masses. Ordinary

matter is constituted of the lightest family, while the more massive quarks and leptons decay rapidly to their lighter cousins.

Electromagnetism implies the existence of electromagnetic waves—light. In quantum mechanics, these waves are described as particles—photons, the quanta of light. Photons are said to “carry” the electromagnetic force. The other forces are also associated with particles as force carriers. The weak interactions are “carried” by massive particles known as W and Z bosons, while the strong interactions are “carried” by gluons, that together with quarks are confined in protons and neutrons.

The final crucial ingredient in the Standard Model is the Higgs particle, discovered in 2012, which is responsible for producing the mass of the elementary particles and the carriers of the weak nuclear force. To date, after about a half century of exploration, every measurement has confirmed the predictions of the Standard Model. No additional particles or forces and no hints of substructure of quarks, leptons, the electroweak and strong force mediators, or the Higgs boson have been observed.

Although the Standard Model has proven immensely successful, many of its aspects remain mysterious—the masses of the elementary particles that span more than 12 orders-of-magnitude with no clear pattern or explanation,

the excess of matter over antimatter (made of anti-particles) in the universe, and most importantly the properties of the Higgs boson. In addition, the matter content and structure of the forces suggest that all the forces and all the matter within it are unified at extremely small distances. Straightforward attempts at unification have not been confirmed by experiment; the telltale signs of unification, such as proton decay, have yet to be observed.

Recent decades have seen the emergence of decisive evidence for new forms of matter and energy that dominate the universe on the largest scales: dark matter, which comprises 85 percent of the matter content of the universe, and dark energy, which comprises 70 percent of the total content of the Universe. These new forms of matter (cold dark matter [CDM], and possibly or other types of dark matter) and energy (dark energy [Λ]) are essential components of the enormously successful standard model of cosmology: ΛCDM. This model describes much of the universe’s history, which plausibly goes back to a period of explosive, accelerated expansion known as inflation and ends by producing a hot bath of elementary particles. Tiny quantum fluctuations during inflation give rise to small under- and over-densities of matter in the late universe. Around the time when the universe cools enough to allow atoms to form, regions with slightly more dark matter begin to gravitationally collapse, forming basins in which ordinary matter clumps into galaxies and stars. Finally, about 5 billion years ago, dark energy took over, leading to the current epoch of accelerated expansion that currently dominates the evolution of the universe.

The standard models of particle physics and of cosmology, despite their successes, have raised new urgent questions. The Standard Model of particle physics does not account for neutrino masses, the dark matter particle (or particles), or dark energy; nor does it explain the origin of the excess of matter over antimatter. The ΛCDM model of cosmology relies on a tiny quantum vacuum energy to account for the dark energy, slowly moving dark matter particles to explain the origin of large-scale structure, and a new spinless field—the inflaton—to explain the origin of the small, primeval, over- and under-densities needed to seed galaxies.

Research in elementary particle physics, astronomy, and cosmology have become intrinsically linked as researchers endeavor to produce and study dark matter in the laboratory and understand the properties of the quantum vacuum that could explain the existence and magnitude of dark energy. To this end, many new experimental tools, from sensitive deep-underground detectors for the detection of dark matter, to powerful telescopes for observing the expanding universe, play an important role in the research program of elementary particle physics.

Space and Time

Space and time were unified in Einstein’s theory of special relativity and shown to be dynamical in the theory of general relativity, in which matter and energy are the source of the curvature of space-time, giving rise to gravity. The laws of space-time and of quantum mechanics are fundamental to particle physics; indeed, the very definition of a particle is in terms of an excitation of the quantum vacuum whose most essential properties are dictated by space-time symmetries.

The study of elementary particles and the laws of nature at the highest energies and shortest distances is also deeply linked to cosmology and the evolution of the universe on the largest scales. While the universe today is expanding and cooling, back in time it was a hot and dense plasma of elementary particles continually colliding at high energies. The understanding of elementary particles is necessary to describe this hot and dense environment. In this way, particle physics is essential to make the science of early universe cosmology possible, in much the same way as the understanding of atoms at the turn of the past century made modern astronomy and the understanding of stars possible. In turn, cosmology opens up a number of fascinating new questions and opportunities for particle physics, from the nature and origin of dark matter and dark energy, to developing a more fundamental understanding for the early inflationary phase of the universe. And at the deepest level, the nature of quantum gravity and space-time are fundamentally linked to cosmology and the origin and fate of the universe.

At low energies and large distances, gravity is a weak force, and quantum effects are negligible. But simple thought experiments involving quantum mechanics and gravity lead to the startling conclusion that the current theory of space-time breaks down at “Planckian” distances a billion billionth the size of the nucleus, due to violent quantum fluctuations in the vacuum. Space-time itself may even be an approximate concept that emerges from more primitive building blocks.

Remarkably, the scale of distance and energy where a theory of quantum gravity is required is close to the scale where the Standard Model forces seem to unify. The very successes of the current framework also suggest the need for revolutionary new foundations of fundamental physics that could unify all the forms of matter and energy and reveal the origin of space and time itself.

PROGRESS SINCE THE 2006 NATIONAL RESEARCH COUNCIL REPORT

The field of elementary particle physics has seen significant structural change, scientific advances, and technological progress since 2006. The shutdown of the Fermi National Accelerator Laboratory’s (Fermilab’s) Tevatron collider in 2011, following the 2008 closure of the Stanford Linear Accelerator Center (SLAC) B-factory, marked the end of U.S.-hosted collider experiments and a more than 20-year run of the United States hosting the highest-energy particle accelerator. The Tevatron fell just short of discovering the Higgs boson, with strong evidence but not enough to claim discovery.

The successful commissioning of the Large Hadron Collider (LHC) at CERN in 2010 and shutdown of the Tevatron led to a major shift in particle physics. Europe is now hosting the most powerful accelerator and attracting scientists from almost 100 countries—including the United States—to carry out their research at the LHC. The large U.S. scientific contingent at CERN underscores the ongoing importance of the United States in particle physics.

Having decided not to build the electron–positron linear collider recommended in the 2006 report Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics,¹ the United States has focused on its strong involvement at the LHC, its neutrino program at Fermilab, and a broader program of particle physics research, especially in the search for dark matter and increasingly precise cosmological surveys.

Science

Without a doubt, the biggest scientific highlight in particle physics since 2006 was the discovery of the Higgs boson at CERN’s LHC, which was announced on July 4, 2012. U.S. scientists, who accounted for about 20 percent of the researchers on the two LHC experiments involved, played central roles in this milestone event. The Higgs discovery completed the simplest version of the Standard Model of particle physics but also raised many new questions in both particle physics and cosmology. Since then, the properties of the Higgs have been under intense study at the LHC, and there is planning around the world for new accelerator facilities dedicated to the study of the Higgs.

Other highlights include the discovery of a new kind of neutrino oscillation, from the neutrino associated with the electron to the heaviest neutrino. Scientists from the United States played a leading role in this experiment that used neutrinos from the Daya Bay nuclear reactors in China. This result, which was confirmed by neutrino experiments in Korea, France, Japan, and the United States, was welcomed news for Fermilab’s neutrino program, making it very likely that the Deep Underground Neutrino Experiment (DUNE) will be able to detect charge parity (CP) violation among neutrinos.²

Precision measurements of the cosmic microwave background made by NASA’s Wilkinson Microwave Anisotropy Probe satellite, the European Space Agency’s Planck Satellite, and ground-based experiments in Chile and at the South Pole, together with measurements of the large-scale structure of the universe by the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES), firmly established ΛCDM as the standard model of cosmology. Both the SDSS and DES involved particle physicists, and in 2024, the Department of Energy (DOE)led Dark Energy Spectroscopic Instrument (DESI) reported its first results. While DESI’s results are consistent with ΛCDM, there is a hint that dark energy may not be as simple as the cosmological constant Λ. A long-standing

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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.

² CP violation refers to a lack of symmetry between the properties and interactions of particles and anti-particles and is crucial to understanding the small excess of quarks over antiquarks that must have existed in the early universe to explain the existence of matter—and not antimatter—in the universe today.

disagreement between measurements of the Hubble constant using different techniques could also indicate that new physics might be needed.

In February 2016, the first direct observation of gravitational waves by the Laser Interferometer Gravitational Wave Observatory detectors in Hanford, Washington, and Livingston, Louisiana, was announced. The tiny ripples detected in the fabric of space-time were produced by the coalescence of two 30 solar mass black holes more than a billion light years away. This event verified the last major prediction of general relativity, showing that merging black holes behave as predicted.

Since then, hundreds of mergers have been detected, including a few that involved neutron stars. While not central to particle physics, gravitational waves now provide a new window on the universe and on fundamental physics—for example, on the equation of state of nuclear matter and the equality of the speed of light and gravitational waves.

In all of science, experimentation is the ultimate arbiter of scientific truth; but in particle physics, theory plays an especially important role. Established theory is essential to performing the extremely delicate, complex, and intricate measurements at the frontiers of particle physics. Theorists also play an essential role in interpreting new experimental results, often leading to new theories. Speculative theory plays an indispensable role both in framing the scientific questions driving the current experimental program and in suggesting new directions for experiment. But even more importantly, progress on the deepest questions demands bold and fundamentally new theoretical ideas. Theory has time and again opened up previously unimagined new intellectual vistas, exposing hidden connections between particle physics and other sciences—from nuclear physics to solid-state physics, cosmology, quantum information, and mathematics.

Theory has seen remarkable new developments on many fronts since the 2006 report. Powerful new analytic tools have enabled the calculation of high-energy processes to unparalleled accuracy. Lattice gauge theory has advanced into a mature numerical tool for precise calculations of strong dynamics. These calculations are crucial for looking beneath the billions of events at colliders, most of which arise from well-understood processes, for signs of new particles. Theoretical exploration has developed new techniques, based on machine learning and artificial intelligence (AI), for analyzing data and understanding the implications for new physics. Conceptual breakthroughs involving Stephen Hawking’s black hole information paradox, as well as the discovery of new connections between gravity and quantum theory, continues to illuminate the deep connections between matter, energy, space, and time.

Revealing the Hidden Nature of Space and Time anticipated the early detection of supersymmetric partners of the Standard Model particles at the LHC, as well as signs of a dark matter particle. Although these discoveries have not yet been made, the ongoing searches and explorations have inspired important new theoretical ideas about the nature and origin of the Higgs and dark matter, how to search for them, and how the forces and particles might be unified.

Technology

Particle physicists are often early adopters of innovative technologies, sometimes even being the first to use them on a large scale. Frequently, they drive technological advances pushing the boundaries of what is possible to ensure the success of their experiments. This interplay between science and technology fuels progress in both. The examples are numerous, and a few are mentioned here.

Advances in superconducting technologies, originally developed by condensed matter and nuclear physics and further developed by particle physicists, have led to the creation of high-gradient, high-Q superconducting radio frequency (SRF) cavities. These SRF cavities are now deployed in the Linac Coherent Light Source-II at the SLAC National Accelerator Laboratory. Particle physicists are also leaders in developing niobium-tin superconductor technology, which is crucial for the High Luminosity LHC and proposed future colliders. Additionally, these superconducting technologies have become a focal point for quantum processor developments at the Superconducting Quantum Materials and Systems Center at Fermilab and other national laboratories, advancing quantum computing and sensing. These same superconducting technologies also open up possibilities for using high-power beams in industry, with applications ranging from medicine and semiconductor manufacturing to the treatment of some of the most persistent “forever” plastics.

There have been dramatic improvements in particle detectors. The LHC can now boast of 200 m² of silicon tracking in a single detector. “Fast AI” has been developed and is already integrated into triggers for the Compact Muon Solenoid (CMS) experiment. Large liquid time projection chambers with automated event reconstruction have been developed and deployed successfully in Fermilab’s MicroBooNE. Innovations in microelectronics include the development of the Endcap Timing Read-Out Chip for the CMS detector at the LHC, with its pioneering of embedded “AI on a chip.” AI and machine learning have been integrated into particle physics research ubiquitously over the past decade.

The full deployment of distributed grid computing at a global scale for distributing and analyzing the data from the LHC experiments was a major advance, which also served as a learning platform for commercial cloud providers.

PARTICLE PHYSICS TODAY

Particle physics today is a global enterprise, characterized by increasing connections and exchanges of ideas and techniques with many scientific fields beyond astrophysics and cosmology.

Accelerators continue to play a central role in particle physics as the most powerful tools to directly explore new physics. The challenge of building more powerful accelerators, both in terms of energy and beam intensity, has increased the importance of accelerator science and technology to the future of the field, which has brought broader applications to other sciences and to society (discussed in Chapter 5).

The field today extends far beyond its accelerator-based core. Physicists of many disciplines are building ultra-sensitive detectors to detect the signatures of dark matter particles in the halo of the Milky Way, with experiments housed in deep underground laboratories. Telescopes are being used across the electromagnetic spectrum to probe the origin and evolution of the universe, to search for clues about the nature of the dark matter, and to understand the mysterious dark energy.

Neutrinos are currently a major focus of particle-physics research, with the goal of better understanding their masses and mixing angles, as well as the differences between neutrinos and antineutrinos. The experimental program involves particle accelerators, nuclear reactors, ultra-sensitive underground detectors, and kilometer-sized arrays under ice in Antarctica.

The search for tiny deviations from Standard Model predictions, which could provide essential clues to new phenomena, include flavor violation and CP symmetry tests and also involves nuclear and atomic physicists. They use trapped particles of all kinds, and a range of table-top experiments, to look for new forces of nature at all scales.

Theoretical particle physics has become more expansive as well. What used to be separate areas of inquiry are now much more closely connected, ranging from the discovery of deep connections between quantum field theory, string theory, and gravitational physics to fresh interactions with condensed matter, quantum information science, and pure mathematics. Moreover, some theorists are more tightly connected to the experiments than ever, designing and even leading experimental efforts.

The World

Over the past few decades, particle physics has evolved from an international science dominated by the United States to a fully global science enterprise, with almost 100 nations participating in elementary particle physics research today and no dominant nation or region. Even projects on the smallest scale involve international collaboration. This shift to a global science enterprise has occurred for many reasons: the realization of the importance of the science both for its own sake and for its larger benefits; the fact that the scale of the effort required exceeds the resources, both fiscal and human, of any one nation or region; and the United States’ successful role in attracting many other countries to participate in particle physics.

The largest undertakings in particle physics are hosted by world laboratories that are open to scientists from around the world and receive support from many nations. The major accelerator laboratories are CERN in Geneva, Switzerland, which hosts the LHC; Fermilab in Batavia, Illinois, which hosts DUNE; and the High Energy Accelerator Research Organization in Japan, which operates the neutrino beam for the Hyper-Kamiokande

(Hyper-K) experiment and hosts the Belle II experiment. All of these laboratories are planning for future accelerators; in addition, the Chinese Academy of Sciences has identified the Circular Electron Positron Collider as its top priority for a future particle accelerator.

The United States

The U.S. program is well integrated into the global program of particle physics. In addition, it also maintains its unique, national aspects. On the largest scale, the United States and U.S. scientists play an outsized role at the LHC. U.S. contributions to the building and upgrading of the LHC have been essential to its success, and particle physicists from the United States—especially early-career researchers—have had the opportunity to be involved in the exciting discoveries being made at the LHC. Fermilab, the U.S. national laboratory for particle physics, is hosting DUNE, an international effort with significant contributions from CERN and more than 30 countries.

Despite its continuing growth in scale and complexity, U.S. particle physics remains firmly rooted in universities. This is especially beneficial given the emerging intersections with other areas of science and technology—for example quantum and AI—since cross-disciplinary interactions are most easily facilitated at universities. U.S. graduate schools continue to attract many of the brightest students from around the world, who are often drawn to particle physics and who go on to illustrious careers here and abroad.

A unique aspect of U.S. particle physics is the close interaction between university researchers, the system of National Laboratories,³ and industry. The National Laboratories carry out research and provide the infrastructure and expert engineering needed to bring projects to scale and help to facilitate multi-university collaborations. Particle physicists collaborate with industry experts to develop novel accelerator and other technologies. These partnerships lead to advancements in accelerator design, beam diagnostics, and control systems. This university–national laboratory–industry triad provides the strong foundation of U.S. particle physics, which is one of the reasons that the United States continues to be an international “partner of choice.”

CONFRONTING THE BIG QUESTIONS

The current program in particle physics is global and rich in the size and diversity of its efforts. It targets the big questions of the field (some of which are described in Box 1-1). For example, the LHC is intensively studying the properties of the Higgs boson, searching for evidence of dark matter and for new physics. The properties of neutrinos will be addressed soon by accelerator experiments in the United States (DUNE), in Japan (Hyper-K), and in China (Jiangmen Underground Neutrino Observatory). Multi-ton, underground detectors are seeking to detect the dark matter particles that comprise the halo of the Milky Way (the LUX-ZEPLIN experiment in the United States, XENONnT in Italy, and the Particle and Astrophysical Xenon Detector in China). Cosmic surveys, across multiple wavelengths, are under way to unravel the mystery of dark energy and to search for the gravitational-wave signature of inflation.

In Appendix E, the committee briefly summarizes the major experiments under way or planned around the world. The discussion there draws from the Snowmass 2021⁴ and Particle Physics Project Prioritization Panel⁵ reports, where the interested reader will find more details about the full extent of the global program in place.

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³ The National Laboratories are a system of 17 laboratories run by DOE. Four of the laboratories—Argonne National Laboratory, Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and SLAC National Accelerator Laboratory—have high-energy physics programs, and one—Fermi National Accelerator Laboratory—is a single purpose laboratory devoted to high-energy physics. The laboratory complex possesses unique, globally unmatched instruments and facilities that enable them to address large-scale, complex research and development challenges with a multidisciplinary approach, while also emphasizing the translation of basic science into innovation through collaboration with industry, academia, and other government agencies to drive technological advancements (Department of Energy, 2020, “The State of the DOE National Laboratories: 2020 Edition,” https://www.energy.gov/articles/state-doe-national-laboratories-2020-edition).

⁴ J.N. Butler, R.S. Chivukhala, A. de Gouvêa, et al., 2023, “Report of the 2021 U.S. Community Study on the Future of Particle Physics (Snowmass 2021) Summary Chapter,” https://doi.org/10.48550/arXiv.2301.06581.

⁵ S. Asai, A. Ballarino, T. Bose, et al., 2024, “Pathways to Innovation and Discovery in Particle Physics: Report of the 2023 Particle Physics Project Prioritization Panel,” https://doi.org/10.48550/arXiv.2407.19176.

The remit of this committee is to define a long-term scientific vision for the field and a strategy for realizing these goals. In Chapter 2, the committee outlines a future program for the United States that builds on the current program and that is necessary to realize the vision. This program involves accelerators, international collaborations, and a broad spectrum of activities that go beyond accelerators, involving connections with other disciplines. This plan can only be achieved with a highly talented and well-supported workforce, and therefore the urgent workforce issues are addressed in a separate chapter, Chapter 3.