Annual Review of Nuclear and Particle Science - Current Issue

Over the past 60 years, particle physics has seen the maturation of its Standard Model and an enormous change in the character of the experiments that have defined it. I have had the good fortune to participate in and help shape this evolution.

For many decades, the main source of information on the top-left corner element of the Cabibbo–Kobayashi–Maskawa quark mixing matrix, Vud, was superallowed nuclear β decays with an impressive 0.01% precision. This precision, apart from experimental data, relies on theoretical calculations in which nuclear structure–dependent effects and uncertainties play a prime role. This review is dedicated to a thorough reassessment of all ingredients that enter the extraction of the value of Vud from experimental data. We try to keep balance between historical retrospect and new developments, many of which occurred in just the past 5 years. They have not yet been reviewed in a complete manner, not least because new results are forthcoming. This review aims to fill this gap and offers an in-depth yet accessible summary of all recent developments.

We present a review of the conceptual basis, current knowledge, and recent progress regarding global analysis of nuclear parton distribution functions (PDFs). After introducing the theoretical foundations and methodological approaches for the extraction of nuclear PDFs from experimental data, we discuss how different measurements in fixed-target and collider experiments provide increasingly precise constraints on various aspects of nuclear PDFs, including shadowing, antishadowing, the EMC effect, Fermi motion, flavor separation, deuteron binding, and target-mass and other higher-twist effects. Particular emphasis is given to measurements carried out in proton–lead collisions at the Large Hadron Collider, which have revolutionized the global analysis during the past decade. These measurements include electroweak boson, jet, light hadron, and heavy flavor observables. Finally, we outline the expected impact of the future Electron Ion Collider and discuss the role and interplay of nuclear PDFs with other branches of nuclear, particle, and astroparticle physics.

Fast radio bursts (FRBs) are brief, highly dispersed bursts detected in the radio band that originate from cosmological distances. The only such event detected in the Milky Way Galaxy, FRB 20200428D—which was associated with an X-ray burst emitted by a magnetar named SGR J1935+2154—revealed the first case of a multiwavelength counterpart of an FRB. Counterparts in other wavelengths accompanying or following FRBs, as well as the bright emission associated with the progenitor of the FRB engine, have been proposed in various FRB models, but no robust detection has been made so far. In general, FRBs as we know them are not favored multimessenger emitters. Nonetheless, possible neutrino and gravitational wave emission signals associated with FRBs or FRB-like events have been discussed in the literature. Here I review these suggested multiwavelength and multimessenger counterparts of FRBs or FRB-like events and the observational progress in searching for these signals. Topics include multiwavelength (X-rays, γ-rays, optical) emission and neutrino emission from FRBs within the framework of the magnetar source models and possible FRB-like events associated with gravitational waves.

Amid all candidates of physics beyond the Standard Model, string theory provides a unique proposal for incorporating gauge and gravitational interactions. In string theory, a four-dimensional theory that unifies quantum mechanics and gravity is obtained automatically if one posits that the additional dimensions predicted by the theory are small and curled up—a concept known as compactification. The gauge sector of the theory is specified by the topology and geometry of the extra dimensions, and the challenge is to reproduce all of the features of the Standard Model of particle physics from them. We review the state of the art in reproducing the Standard Model from string compactifications and summarize the lessons drawn from this fascinating quest. We describe novel scenarios and mechanisms that string theory provides to address some of the Standard Model puzzles as well as the most frequent signatures of new physics that could be detected in future experiments. We then comment on recent developments that connect, in a rather unexpected way, the Standard Model with quantum gravity and that may change our field theory notion of naturalness.

The field of nuclear science has considerably advanced since its beginning just over a century ago. Today, the science of rare isotopes is on the cusp of a new era with theoretical and computing advances complementing experimental capabilities at new facilities internationally. In this article we present a vision for the science of rare isotope beams (RIBs). We do not attempt to cover the full breadth of the field; rather, we provide a perspective and address a selection of topics that reflect our own interests and expertise. We focus in particular on systems near the drip lines, where one often finds nuclei that are referred to as exotic and where the role of the nuclear continuum is only just starting to be explored. An important aspect of this article is its attempt to highlight the crucial connections between nuclear structure and the nuclear reactions required to fully interpret and leverage the rich data to be collected in the next years at RIB facilities. Further, we connect the efforts in structure and reactions to key questions of nuclear astrophysics.

This is a golden age for galaxy formation: Existing and especially new telescopes are providing observations that challenge and illuminate rapidly improving theory and simulations. This review describes the formation of the cosmic web and the structure of the dark matter halos that provide the scaffolding of the Universe. It then summarizes how empirical models, semianalytic models, and hydrodynamic simulations attempt to account for key properties of the galaxy population, including the main sequence of star-forming galaxies, the inefficiency of star formation, the shape evolution and color bimodality of galaxies, and the phenomena that cause galaxies to quench their star formation. It concludes with a summary of observations that have challenged the cosmological constant cold dark matter (ΛCDM) paradigm of galaxy formation—including the Hubble and S8 tensions, bright galaxies in the early Universe, an extragalactic background light mystery, missing satellite galaxies, the diversity of dwarf galaxies, the cusp–core problem, the too-big-to-fail problem, stellar clumps, planes of satellite galaxies, and galaxies without dark matter—and solutions that have been proposed.

This review provides a conceptual and technical survey of methods for parameter estimation of gravitational-wave signals in ground-based interferometers such as Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo. We introduce the framework of Bayesian inference and provide an overview of models for the generation and detection of gravitational waves from compact binary mergers, focusing on the essential features that are observable in the signals. Within the traditional likelihood-based paradigm, we describe various approaches for enhancing the efficiency and robustness of parameter inference. This includes techniques for accelerating likelihood evaluations, such as heterodyne/relative binning, reduced-order quadrature, multibanding, and interpolation. We also cover methods to simplify the analysis to improve convergence, via reparameterization, importance sampling, and marginalization. We end with a discussion of recent developments in the application of likelihood-free (simulation-based) inference methods to gravitational-wave data analysis.

Experimental activities involving multi-TeV muon collisions are a relatively recent endeavor. The community has limited experience in designing detectors for lepton interactions at center-of-mass energies of 10 TeV and beyond. This review provides a short overview of the machine characteristics and outlines potential sources of beam-induced background that could affect the detector performance. The strategy for mitigating the effects of the beam-induced background on the detector at  TeV is discussed with a focus on the machine–detector interface, detector design, and implementation of reconstruction algorithms. The physics potential at this center-of-mass energy is evaluated using a detailed detector simulation that incorporates the effects of the beam-induced background. This evaluation concerns the Higgs boson couplings and the Higgs field potential sensitivity, which then are used to obtain confidence on the expectations at 10 TeV. The physics and detector requirements for an experiment at  TeV, outlined here, form the foundation for the initial detector concept at that center-of-mass energy.

A major focus in particle physics has been on understanding the interactions of the Higgs boson. Tremendous progress has been made in determining the strength of the couplings of the Higgs boson to fermions and vector bosons, but its self-interaction has yet to be established. Understanding the Higgs self-coupling and the form of the potential function of the Higgs field will illuminate the process by which the Higgs boson acquires a vacuum expectation value and could provide insight into the early Universe and, perhaps, its eventual fate. The most natural way to probe the Higgs self-interaction is via searches for Higgs boson pair (HH) production. Since the Standard Model makes a definite prediction for the Higgs self-coupling, enhanced rates and modified kinematic properties of HH production are a smoking-gun signature for new physics. This article reviews the current experimental status of HH searches, discusses the experimental challenges and limitations, and provides an outlook for the future of the field.

We review dark matter (DM) candidates of a very low mass appearing in the window below the traditional weakly interacting massive particle (mχ ≲ 10 GeV) and extending down to mχ ≳ 1 meV, somewhat below the mass limit at which DM becomes wavelike. Such candidates are motivated by hidden sectors such as hidden valleys, which feature hidden forces and rich dynamics, but have evaded traditional accelerator searches for New Physics because of their relatively weak coupling to the Standard Model (SM). Such sectors can still be detected through dedicated low-energy colliders, which, through their intense beams, can have sensitivity to smaller couplings, or through astrophysical observations of the evolution of DM halos and stellar structures, which, through the Universe's epochs, can be sensitive to small DM interactions. We also consider mechanisms whereby the DM abundance is fixed through the interaction with the SM, which directly motivates the search for light DM in terrestrial experiments. The bulk of this review is dedicated to the new ideas that have been proposed for direct detection of such DM candidates of a low mass through nuclear recoils, electronic excitations, or collective modes such as phonons and magnons. The rich tapestry of materials and modes in the condensed matter landscape is reviewed along with specific prospects for detection.

The recently completed PREX-2 campaign measured the density distribution of neutrons in the lead nucleus as a function of momentum transfer (the form factor), confirmed a relatively large extent of the neutrons beyond the protons in the nucleus (the neutron skin), and provided a precise determination of the density of protons and neutrons at the center of a heavy nucleus. In turn, the measured form factor can be related to various nuclear and neutron star properties. The NICER X-ray telescope has inferred the masses and radii of some X-ray pulsars (neutron stars), although complications arise when determining these quantities independently. Further improvements in NICER have enabled simultaneous mass–radius determinations that had not previously been possible. During the next decade, measurements in astrophysics, gravitational-wave astronomy, and nuclear physics are expected to provide a wealth of more precise data. In this review, we present an overview of the current state of neutron skin measurements and offer insights into prospects for the future.

Nuclei will play a prominent role in searches for physics beyond the Standard Model as the active material in experiments. In order to reliably interpret new physics signals, one needs an accurate model of the underlying nuclear dynamics. In this review, we discuss recent progress made with quantum Monte Carlo approaches for calculating the electroweak structure of light nuclei. We place particular emphasis on recent β decay, muon capture, neutrinoless double β decay, and electron scattering results.

The Borexino experiment has developed, in its 32 years of activity, techniques and methods that allow for unprecedented radiopurity levels, which continue to be the current state of the art. These pioneering techniques and methods represent a new standard for ultra-low-background physics, a legacy that Borexino leaves to future experiments studying low-energy neutrinos and searching for rare events with detectors operating deep underground. The Borexino experiment leaves an equally influential scientific legacy with its discoveries and precise measurements of the nuclear processes that cause the Sun and stars to shine. Thanks to its unparalleled low background in the energy window of ∼150 keV to ∼15 MeV, the Borexino experiment also has contributed significantly to the understanding of neutrino oscillations with the observation of the energy-dependent matter to vacuum-dominated flavor conversion probability of solar neutrinos. Along with this textbook-quality body of solar neutrino results, the Borexino experiment has contributed to the study of the Earth's mantle radioactivity with background-free measurements of geoneutrinos. This article presents an overview of the long-lasting Borexino results and of the experimental efforts required to achieve them.

Recent strategy updates by the international particle physics community have confirmed strong interest in a next-generation energy frontier collider after completion of the High-Luminosity LHC program and construction of a e⁺e⁻ Higgs factory. Both hadron and muon colliders provide a path toward the highest energies, and both require significant and sustained development to achieve technical readiness and optimize the design. For hadron colliders, the energy reach is determined by machine circumference and the strength of the guiding magnetic field. To achieve a collision energy of 100 TeV while limiting the circumference to 100 km, a dipole field of 16 T is required and is within the reach of niobium–tin magnets operating at 1.9 K. Magnets based on high-temperature superconductors may enable a range of alternatives, including a more compact footprint, a reduction of the cooling power, or a further increase of the collision energy to 150 TeV. The feasibility and cost of the magnet system will determine the possible options and optimal configurations. In this article, I review the historical milestones and recent progress in superconducting materials, design concepts, magnet fabrication, and test results and emphasize current developments that have the potential to address the most significant challenges and shape future directions.

Accelerator searches for new resonances have a long-standing history of discoveries that have driven advances in our understanding of nature. Since 2010, the Large Hadron Collider (LHC) has probed previously inaccessible energy scales, enabling searches for new heavy resonances predicted by a wide range of theories beyond the Standard Model (BSM). In particular, resonance decays into fermionic final states are often seen as golden channels since they provide a clear signal—typically a peak in the invariant mass of the decay products over a smoothly falling background distribution. This review summarizes the key concepts of the experimental searches for new resonances decaying to fermions, in the context of the BSM theories that motivate them, and presents the latest results of the ATLAS and CMS experiments, focusing on the complete LHC Run 2 dataset. Future prospects at the High-Luminosity LHC and potential future colliders are also surveyed.

Major advances in experimental nuclear and particle physics are often motivated by the need to answer challenging questions. In 2009, Monreal and Formaggio were motivated by the problem of measuring the absolute mass of the neutrino to propose the technique that would come to be called cyclotron radiation emission spectroscopy (CRES). They needed to measure the energies of the electrons from tritium beta decay with extremely high precision, which could be achieved by measuring the frequency of the cyclotron radiation from many individual magnetically trapped electrons. The technique was put into practice first by the Project 8 Collaboration and then by the He6-CRES Collaboration for the study of nonstandard weak interactions. In this review, we present the CRES experiments that have been performed to date, describe the phenomenology of CRES that has so far been explored, and cover potential applications of CRES that have been proposed.

Will neutrinos find uses outside basic science? It may be too early to say, but neutrino physicists have already imagined a variety of possibilities from the relatively modest to the more blue-sky. In this review, we survey the range of proposed applications, most involving nuclear reactors and other fission sources. We give special attention to the most recent proposals, including verifying submarine reactor integrity, safeguarding advanced nuclear power plants, and monitoring spent nuclear fuel. All of these concepts take advantage of the fact that neutrinos pass through barriers other signals cannot penetrate. That same fact creates the central challenge for neutrino applications: the size and complexity of detectors needed to collect a signal. Although the weakly interacting nature of neutrinos makes them fundamentally difficult to use, developments in detector technology are making some ideas more feasible.

For well over half a century, precision studies of neutron and nuclear β decays have been at the forefront of searches for exotic electroweak physics. Recent advances in nuclear ab initio theory and the widespread use of effective field theories mean that the modern understanding of β decay is going through a transitional phase. This has been propelled by current tensions in the global dataset leading to renewed scrutiny of the theoretical ingredients. In parallel, novel techniques and methods are being investigated that can sidestep many traditional systematic uncertainties and require a diverse palette of skills and collaboration with material science and condensed matter physics. This review highlights the current opportunities and open questions to facilitate the transition to a more modern understanding of β decay.

Liquid argon time-projection chambers (LArTPCs) have become a prominent tool for experiments in particle physics. Recent years have yielded significant advances in the techniques used to capture the signals generated by these cryogenic detectors. This article summarizes these novel developments for detection of ionization electrons and scintillation photons in LArTPCs. New methods to capture ionization signals address the challenges of scaling traditional techniques to the large scales necessary for future experiments. Pixelated readouts improve signal fidelity and expand the applicability of LArTPCs to higher-rate environments. Methods that leverage amplification in argon enable measurements in the keV regime and below. Techniques to enhance collection of argon scintillation photons improve calorimetry and expand the physics program for very large detectors. Future efforts aim to demonstrate systems for the combined detection of both electrons and photons.