E

Current Global Program

The current program in elementary particle physics is global and far-reaching, targeting the most urgent questions of the field discussed in Chapter 1. A schematic timeline of ongoing or planned major efforts is shown in Figure E-1. The discussion here draws from the Snowmass 2021¹ and Particle Physics Project Prioritization Panel² reports, where the full breadth and details of experimental efforts may be found.

ENERGY FRONTIER

Because higher energies correspond to shorter distances and the ability to produce particles of higher mass, the “energy frontier” of particle physics allows the deepest exploration into the world of the elementary particles. While higher energies existed in the Big Bang and arise in rare cosmic-ray interactions today, particle accelerators, with their well-controlled conditions and sophisticated detectors, provide the most powerful way to explore the energy frontier.

For the next 20 years or so, the Large Hadron Collider (LHC) at CERN will be the energy frontier accelerator. The LHC collides both protons and heavier ions at multi-teraelectronvolt center-of-mass energies. The LHC commenced operations in 2009, discovered the Higgs boson in 2012, and has ramped up its collision energy to 13.6 TeV. Nearly 10,000 physicists from more than 70 countries from around the globe are involved in experiments at the LHC, with very significant participation from the United States. Under the current plan, the LHC will operate until at least 2040.

Two of the main experiments at the LHC, ATLAS and CMS, continue to study the Higgs boson, and measurements to date are consistent with expectations predicted by the Standard Model at the 10 percent level. In 2029, when the operation of the High Luminosity LHC (HL-LHC) begins, this is expected to improve to the level of a few percent.

More generally, the LHC provides a controlled environment in which the Standard Model itself is well understood, which can be used to explore the unknown and make discoveries that can illuminate the big mysteries of

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¹ 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.

² 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.

particle physics, which include discovering the dark matter particle, searching for hints about the unification of the particles and forces, explaining neutrino mass, and understanding the relative lightness of the Higgs.

In particular, supersymmetry, which predicts that every particle has a heavier “superpartner” whose spin differs by a half-integer unit, is an attractive means of doing so. To naturally explain the small value of the Higgs mass, the masses of the superpartners must themselves be close to the teraelectronvolt scale, and several were expected to be copiously produced at the LHC, which has not happened yet. The absence of any clear sign of supersymmetry—“the dog that didn’t bark”—might indicate more dramatic revolutions on the horizon in terms of the organizing principles that govern the highest energy scales.

RARE PROCESSES AND PRECISION MEASUREMENTS

Precision measurements of rare processes allow for a complementary approach to searching for physics beyond the Standard Model. The LHCb experiment at the LHC and the Belle II experiment at the High Energy Accelerator Research Organization in Japan study the production, oscillation, and decays of mesons containing bottom quarks, which provides sensitivity to physics beyond the Standard Model. LHCb has the capability to study kaon decays to the same end, along with the NA62 experiment at CERN and the KOTO Experiment at the Japan Proton Accelerator Research Complex.

The anomalous magnetic moment of the muon (g-2) is a precisely measured quantity—to one part in 10 million—whose experimental value displays a long-standing discrepancy with the Standard Model prediction at a few standard deviations. The Muon g-2 experiment at the Fermi National Accelerator Laboratory (Fermilab) has not yet released its final measurement. Thus far, the discrepancy persists, although a recent theoretical study suggests that the Standard Model prediction may only be one standard deviation from the measured value.

The decay of a muon into an electron and a photon is expected to be extremely small in the Standard Model and thus could easily be influenced by the presence of new physics related to lepton mass. The MEG-II experiment in Switzerland currently provides the best bound on the decay process and is expected to finish operations in 2025. The related process in which a muon converts into an electron in the electromagnetic field of an atom

probes similar physics and will be pushed even further by the Muon-to-electron-Conversion Experiment, which is currently under construction at Fermilab, and the Coherent Muon to Electron Transition experiment, under construction at the Japan Proton Accelerator Research Complex.

NEUTRINOS

The discovery that neutrinos oscillate from one type to another is evidence for small neutrino masses, and three kinds of such oscillations are now known. Neutrino masses do not arise in the Standard Model; further, their values—a factor of a million or more smaller than the next lightest particle, the electron—point to the need for a deeper understanding of neutrinos.

Neutrino oscillations raise the possibility of charge-parity (CP) violation among neutrinos as is seen in the hadrons; for example, the K-mesons and b-mesons. 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.

The Jiangmen Underground Neutrino Observatory in China, the Hyper-Kamiokande in Japan, and the Deep Underground Neutrino Experiment (DUNE; Figure E-2) in the United States are all expected to advance understanding of neutrinos, including most importantly CP-violation over the next decade or so.

A related question is whether neutrinos are their own anti-particles. The process of neutrino-less double beta-decay, in which a heavy nucleus simultaneously experiences two neutrons converting into protons by emitting two electrons without the usual attendant neutrinos, is predicted to occur only if neutrinos are their own anti-particles. To date, this process remains unobserved, but more sensitive detectors—nEXO (based in the United States), CUORE Upgrade with Particle Identification and LEGEND (in Italy), and the Neutrino Experiment with a Xenon TPC-100 (in Spain)—all aim to extend the sensitivity to this process.³

DARK MATTER

Dark matter is one of the great links between and mysteries of particle physics and cosmology. Most of the matter in the universe is dark and is not made of atoms or any other particle in the Standard Model. Candidates for the dark matter particle abound, but there is no conclusive evidence for an actual dark matter particle.

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³ In the United States, neutrinoless double beta decay is funded by the Office of Nuclear Physics at the Department of Energy and at the National Science Foundation.

The ATLAS and CMS experiments continue to search for the dark matter particle. Beyond this, there are broad programs searching for the dark matter. “Direct detection” experiments seek to observe the ambient dark matter near Earth interacting with detectors, whereas “indirect detection” seeks to detect Standard Model particles produced when the dark matter annihilates with itself.

Three large, underground detectors are searching for weakly interacting massive particles (WIMPs) in the halo of the Milky Way, which interact with electroweak strength with ordinary matter—the LUX-ZEPLIN experiment in the Homestake Mine in South Dakota, the XENONnT experiment in the Gran Sasso Laboratory in Italy, and the Particle and Astrophysical Xenon Detector-4T experiment in China.

Beyond these large efforts, there is a plethora of smaller-scale efforts looking for other dark matter candidates, including the axion. The absence of a signal from WIMPs to date has motivated much of the recent theoretical activity aimed at proposing new candidates and devising new search strategies sensitive to a wider range of dark matter mass.

COSMIC SURVEYS

Observations of the universe as well as cosmic relics from earlier times contain valuable information about fundamental physics. In addition to the evidence for dark matter, measurements of the expansion of the universe revealed the presence of dark energy, and precision measurements of the cosmic microwave background (CMB) have provided the strongest evidence for an earlier period of cosmic acceleration—inflation—best explained by the physics of a spinless inflaton. These observations have opened our eyes to phenomena that cannot be accessed at accelerators, highlighting the unique role that cosmology plays in modern particle physics.

The largest projects involve cosmic surveys, of galaxies and of the CMB. The Dark Energy Spectroscopic Instrument (DESI) survey is in progress, and it will amass 30 million galaxy redshifts when complete—more than the total measured by astronomers to date. DESI will study the expansion of the universe with unprecedented precision in order to better understand the nature of the mysterious dark energy that accounts for about 70 percent of energy content of the universe. Its early results have revealed hints that dark energy may be changing with time and, if so, is something more complicated than the quantum energy of the vacuum (Λ).

The 10-year Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory in Chile is scheduled to begin in 2025. The 3.2 gigapixel camera, the largest ever built, will image the entire visible sky every three to four nights, discovering 17 billion stars and 20 billion galaxies that have never been seen before. High on the list of its science objectives are dark matter and dark energy.

DESI and LSST are jointly funded by the National Science Foundation (NSF) and the Department of Energy (DOE) and involve hundreds of particle physicists. Two other galaxy surveys will be carried out from space—the European Space Agency’s Euclid mission and NASA’s Roman mission. Although neither mission has funding from DOE or NSF, the major science objectives of each include dark matter and dark energy. Furthermore, NASA’s Roman mission grew out of an unsuccessful collaboration between DOE and NASA, illustrating the connections between particle physics and other fields—in this case astronomy.

Precision measurements of the anisotropy and polarization of the CMB have been crucial to establishing the Λ-cold dark matter model, and there is additional information to be gained from high-precision study of the CMB. Two of the biggest prizes include the detection of the polarization signature of the gravitational waves produced during inflation and a determination of the sum of neutrino masses (oscillation experiments only determine mass differences). Currently, two CMB experiments are operating at the South Pole—the Keck/Background Imaging of Cosmic Extragalactic Polarization array and the South Pole Telescope. In the Atacama Desert of Chile, one experiment is operating (the Cosmology Large Angular Scale Surveyor), and another is beginning its commissioning (the Simons Observatory).

The Cosmic Microwave Background Stage Four—a major, joint effort of DOE and NSF—is still under consideration. It builds on the experience gained by the South Pole and Chilean experiments and greatly expands the scientific reach by adding new telescopes. The necessary access to the South Pole is in competition with the South Pole infrastructure upgrades, and so the schedule is uncertain at this time.