Annual Review of Nuclear and Particle Science - Volume 69, 2019

Sidney David Drell, professor emeritus at Stanford University and senior fellow at the Hoover Institution, died shortly after his 90th birthday in Palo Alto, California. In a career spanning nearly 70 years, Sid—as he was universally known—achieved prominence as a theoretical physicist, public servant, and humanitarian.

Sid contributed incisively to our understanding of the electromagnetic properties of matter. He created the theory group at the Stanford Linear Accelerator Center (SLAC) and led it through the most creative period in elementary particle physics. The Drell-Yan mechanism is the process through which many particles of the Standard Model, including the famous Higgs boson, were discovered.

Sid advised Presidents and Cabinet Members on matters ranging from nuclear weapons to intelligence, speaking truth to power but with keen insight for offering politically effective advice. His special friendships with Wolfgang (Pief) Panofsky, Andrei Sakharov, and George Shultz highlighted his work at the interface between science and human affairs. He advocated widely for the intellectual freedom of scientists and in his later years campaigned tirelessly to rid the world of nuclear weapons.

Precise predictions for collider observables require the computation of higher orders in perturbation theory. This task usually involves the evaluation of complicated multiloop integrals, which typically give rise to complicated special functions. This article discusses recent progress in understanding the mathematics underlying multiloop Feynman integrals and discusses a class of functions that generalizes the logarithm and that often appears in multiloop computations. The same class of functions is an active area of research in modern mathematics, which has led to the development of new powerful tools to compute Feynman integrals. These tools are at the heart of some of the most complicated computations ever performed for a hadron collider.

Mergers of binary neutron stars and black hole–neutron star binaries are among the most promising sources for ground-based gravitational-wave (GW) detectors and are also high-energy astrophysical phenomena, as illustrated by the observations of GWs and electromagnetic (EM) waves in the event of GW170817. Mergers of these neutron star binaries are also the most promising sites for r-process nucleosynthesis. Numerical simulation in full general relativity (numerical relativity) is a unique approach to the theoretical prediction of the merger process, GWs emitted, mass ejection process, and resulting EM emission. We summarize the current understanding of the processes of neutron star mergers and subsequent mass ejection based on the results of the latest numerical-relativity simulations. We emphasize that the predictions of the numerical-relativity simulations agree broadly with the optical and IR observations of GW170817.

Most strong-interaction resonances have decay channels involving three or more particles, including many of the recently discovered X, Y, and Z resonances. In order to study such resonances from first principles using lattice QCD, one must understand finite-volume effects for three particles in the cubic box used in calculations. We review efforts to develop a three-particle quantization condition that relates finite-volume energies to infinite-volume scattering amplitudes. We describe in detail the three approaches that have been followed, and present new results on the relationship between the corresponding results. We show examples of the numerical implementation of all three approaches and point out the important issues that remain to be resolved.

A well-established knowledge of nuclear phenomena including fission, reaction cross sections, and structure/decay properties is critical for applications ranging from the design of new reactors to nonproliferation to the production of radioisotopes for the diagnosis and treatment of illness. However, the lack of a well-quantified, predictive theoretical capability means that most nuclear observables must be measured directly and used to calibrate empirical models, which in turn provide the data needed for these applications. In many cases, either there is a lack of data needed to guide the models or the results of the different measurements are discrepant, leading to the development of evaluation methodologies to provide recommended values and uncertainties. In this review, we describe the nuclear data evaluation process and the international community that carries it out. We then discuss new measurements and improved theory and/or modeling needed to address future challenges in applied nuclear science.

Direct dark matter detection experiments will soon be sensitive to neutrinos from astrophysical sources, including the Sun, the atmosphere, and supernovae, which will set an important benchmark and open a new window into neutrino physics and astrophysics. The detection of these neutrinos will be complementary to accelerator- and reactor-based experiments that study neutrinos over the same energy range. We review the physics and astrophysics that can be extracted from the detection of these neutrinos, highlighting the potential of identifying New Physics in the form of light mediators that arise from kinetic mixing and hidden sectors, as well as ∼eV-scale sterile neutrinos. We discuss how the physics reach of these experiments will complement searches for New Physics at the LHC and dedicated neutrino experiments.

We address the phenomenology of light sterile neutrinos, with an emphasis on short-baseline neutrino oscillations. After reviewing the observed short-baseline neutrino oscillation anomalies, we discuss the global fit of the data and the current appearance–disappearance tension. We also review briefly the effects of light sterile neutrinos in β decay, neutrinoless double-β decay, and cosmology. Finally, we discuss future perspectives of the search for the effects of eV-scale sterile neutrinos.

This article discusses some of the history of parity-violation experiments that culminated in the Qweak experiment, which provided the first determination of the proton's weak charge . The guiding principles necessary to the success of that experiment are outlined, followed by a brief description of the Qweak experiment. Several consistent methods used to determine from the asymmetry measured in the Qweak experiment are explained in detail. The weak mixing angle sin²θw determined from is compared with results from other experiments. A description of the procedure for using the result on the proton to set TeV-scale limits for new parity-violating semileptonic physics beyond the Standard Model (BSM) is presented. By also considering atomic parity-violation results on cesium, the article shows how this result can be generalized to set limits on BSM physics, which couples to any combination of valence quark flavors. Finally, the discovery space available to future weak-charge measurements is explored.

Neutrinoless double-beta decay is a forbidden, lepton-number-violating nuclear transition whose observation would have fundamental implications for neutrino physics, theories beyond the Standard Model, and cosmology. In this review, we summarize the theoretical progress to understand this process, the expectations and implications under various particle physics models, and the nuclear physics challenges that affect the precise predictions of the decay half-life. We also provide a synopsis of the current and future large-scale experiments that aim to discover this process in physically well-motivated half-life ranges.

With myriads of detection events from a prospective Galactic core-collapse supernova, current and future neutrino detectors will be able to sample detailed, time-dependent neutrino fluxes and spectra. This will offer significant possibilities of inferring supernova physics from the various phases of the neutrino signal, ranging from the neutronization burst through the accretion and early explosion phases to the cooling phase. The signal will constrain the time evolution of bulk parameters of the young proto–neutron star, such as its mass and radius, as well as the structure of the progenitor; probe multidimensional phenomena in the supernova core; and constrain the dynamics of the early explosion phase. Aside from further astrophysical implications, supernova neutrinos may also shed light on the properties of matter at supranuclear densities and on open problems in particle physics.

In recent years, the combination of precise quantum Monte Carlo (QMC) methods with realistic nuclear interactions and consistent electroweak currents, in particular those constructed within effective field theories (EFTs), has led to new insights in light and medium-mass nuclei, neutron matter, and electroweak reactions. For example, with the same chiral interactions, QMC calculations can reproduce binding energies and radii for light nuclei, n–α scattering phase shifts, and the neutron matter equation of state. This compelling new body of work has been made possible both by advances in QMC methods for nuclear physics, which push the bounds of applicability to heavier nuclei and to asymmetric nuclear matter, and by the development of local chiral EFT interactions up to next-to-next-to-leading order and minimally nonlocal interactions including Δ degrees of freedom. In this review, we discuss these recent developments and give an overview of the exciting results for nuclei, neutron matter and neutron stars, and electroweak reactions.

The nuclear shell model has perhaps been the most important conceptual and computational paradigm for the understanding of the structure of atomic nuclei. While the shell model has been used predominantly in a phenomenological context, there have been efforts stretching back more than half a century to derive shell model parameters based on a realistic interaction between nucleons. More recently, several ab initio many-body methods—in particular, many-body perturbation theory, the no-core shell model, the in-medium similarity renormalization group, and coupled-cluster theory—have developed the capability to provide effective shell model Hamiltonians. We provide an update on the status of these methods and investigate the connections between them and their potential strengths and weaknesses, with a particular focus on the in-medium similarity renormalization group approach. Three-body forces are demonstrated to be important for understanding the modifications needed in phenomenological treatments. We then review some applications of these methods to comparisons with recent experimental measurements, and conclude with some remaining challenges in ab initio shell model theory.

The Short-Baseline Neutrino (SBN) program consists of three liquid argon time-projection chamber detectors located along the Booster Neutrino Beam at Fermi National Accelerator Laboratory. Its main goals include searches for New Physics—particularly eV-scale sterile neutrinos, detailed studies of neutrino–nucleus interactions at the GeV energy scale, and the advancement of the liquid argon detector technology that will also be used in the DUNE/LBNF long-baseline neutrino experiment in the next decade. We review these science goals and the current experimental status of SBN.

After 10 years of physics at the Large Hadron Collider (LHC), the particle physics landscape has greatly evolved. Today, a staged Future Circular Collider (FCC), consisting of a luminosity-frontier highest-energy electron–positron collider (FCC-ee) followed by an energy-frontier hadron collider (FCC-hh), promises the most far-reaching physics program for the post-LHC era. FCC-ee will be a precision instrument used to study the Z, W, Higgs, and top particles, and will offer unprecedented sensitivity to signs of new physics. Most of the FCC-ee infrastructure could be reused for FCC-hh, which will provide proton–proton collisions at a center-of-mass energy of 100 TeV and could directly produce new particles with masses of up to several tens of TeV. This collider will also measure the Higgs self-coupling and explore the dynamics of electroweak symmetry breaking. Thermal dark matter candidates will be either discovered or conclusively ruled out by FCC-hh. Heavy-ion and electron–proton collisions (FCC-eh) will further contribute to the breadth of the overall FCC program. The integrated FCC infrastructure will serve the particle physics community through the end of the twenty-first century. This review combines key contents from the first three volumes of the FCC Conceptual Design Report.

The ultrarelativistic heavy-ion programs at the Relativistic Heavy Ion Collider and the Large Hadron Collider have entered an era of quantitative analysis of quantum chromodynamics (QCD) at high temperatures. The remarkable discovery of the strongly coupled quark–gluon plasma (sQGP), as deduced from its hydrodynamic behavior at long wavelengths, calls for probes that can reveal its inner workings. Charm- and bottom-hadron spectra offer unique insights into the transport properties and the microscopic structure of the QCD medium created in these collisions. At low momentum the Brownian motion of heavy quarks in the sQGP gives access to their diffusion constant, at intermediate momentum these quarks give insight into hadronization mechanisms, and at high momentum they are expected to merge into a radiative-energy loss regime. We review recent experimental and theoretical achievements on measuring a variety of heavy-flavor observables, characterizing the different regimes in momentum and extracting pertinent transport coefficients to unravel the structure of the sQGP and its hadronization.

We present an introductory review of the early-time dynamics of high-energy heavy-ion collisions and the kinetics of high-temperature quantum chromodynamic matter. The equilibration mechanisms in the quark–gluon plasma uniquely reflect the nonabelian and ultrarelativistic character of the many-body system. Starting with a brief exposé of the key theoretical and experimental questions, we provide an overview of the theoretical tools employed in weak coupling studies of the early-time nonequilibrium dynamics. We highlight theoretical progress in understanding different thermalization mechanisms in weakly coupled nonabelian plasmas, and discuss their relevance in describing the approach to local thermal equilibrium during the first fm/c of a heavy-ion collision. We also briefly discuss some important connections to the phenomenology of heavy-ion collisions.

The recent discoveries of high-energy cosmic neutrinos and gravitational waves from astrophysical objects have led to a new era of multimessenger astrophysics. In particular, electromagnetic follow-up observations triggered by these cosmic signals have proved to be highly successful and have brought about new opportunities in time-domain astronomy. We review high-energy particle production in various classes of astrophysical transient phenomena related to black holes and neutron stars, and discuss how high-energy emission can be used to reveal the underlying physics of neutrino and gravitational-wave sources.