Many-body physics aims to understand emergent properties of systems made of many interacting objects. This review examines recent progress on the topic of radiative heat transfer in many-body systems consisting of thermal emitters interacting in the near-field regime. Near-field radiative heat transfer is a rapidly emerging field of research in which the cooperative behavior of emitters gives rise to peculiar effects that can be exploited to control heat flow at the nanoscale. Using an extension of the standard Polder and van Hove stochastic formalism to deal with thermally generated fields in $N$-body systems, along with their mutual interactions through multiple scattering, a generalized Landauer-like theory is derived to describe heat exchange mediated by thermal photons in arbitrary reciprocal and nonreciprocal multiterminal systems. In this review, this formalism is used to address both transport and dynamics in these systems from a unified perspective. The discussion covers (i) the description of nonadditivity of heat flux and its related effects, including fundamental limits as well as the role of nanostructuring and material choice; (ii) the study of equilibrium states and multistable states; (iii) the relaxation dynamics (thermalization) toward local and global equilibria; (iv) the analysis of heat transport regimes in ordered and disordered systems composed of a large number of objects, density, and range of interactions; and (v) the description of thermomagnetic effects in magneto-optical systems and heat transport mechanisms in non-Hermitian many-body systems. The review concludes with a listing of outstanding challenges and promising future research directions.

Self-assembly is a ubiquitous process in synthetic and biological systems, broadly defined as the spontaneous organization of multiple subunits (macromolecules, particles, etc.) into ordered multiunit structures. The vast majority of equilibrium assembly processes give rise to two states: one consisting of dispersed disassociated subunits and the other consisting of a bulk-condensed state of unlimited size. This review focuses on the more specialized class of self-limiting assembly, which describes equilibrium assembly processes resulting in finite-size structures. These systems pose a generic and basic question, how do thermodynamic processes involving noncovalent interactions between identical subunits “measure” and select the size of assembled structures? This review begins with an introduction to the basic statistical mechanical framework for assembly thermodynamics that is used to highlight the key physical ingredients ensuring that equilibrium assembly will terminate at finite dimensions. Then examples of self-limiting assembly systems are introduced, and they are classified within this framework based on two broad categories: self-closing assemblies and open-boundary assemblies. These include well-known cases in biology and synthetic soft matter (micellization of amphiphiles and shell and tubule formation of tapered subunits) as well as less widely known classes of assemblies, such as short-range attractive or long-range repulsive systems and geometrically frustrated assemblies. For each of these self-limiting mechanisms, the physical mechanisms that select equilibrium assembly size, as well as the potential limitations of finite-size selection, are described. Finally, alternative mechanisms for finite-size assemblies are discussed, and contrasts are drawn with the size control that these can achieve relative to self-limitation in equilibrium, single-species assemblies.

Events with vector bosons produced in association with jets have been extensively studied at hadron colliders and provide high-accuracy tests of the standard model. A good understanding of these processes is of paramount importance for precision Higgs physics, as well as for searches for new physics. In particular, associated production of $\gamma$, $W$, or $Z$ bosons with light-flavor and heavy-flavor jets is a powerful tool for testing perturbative QCD calculations, Monte Carlo event generators, and can also constrain the parametrizations used to describe the parton content of the proton. Furthermore, events with a $W$ or $Z$ boson produced with two well-separated jets can be used to distinguish between electroweak and strong production mechanisms, and to set limits on contributions of physics beyond the standard model. This review summarizes the historical theoretical developments and the state-of-the-art in the modeling of vector-boson-plus-jet physics while focusing on experimental results by the LHC collaborations in run 1 and run 2 and including comparisons with recent measurements at the Tevatron.

The physics of quantum materials is dictated by many-body interactions and mathematical concepts such as symmetry and topology that have transformed our understanding of matter. Angle-resolved photoemission spectroscopy (ARPES), which directly probes the electronic structure in momentum space, has played a central role in the discovery, characterization, and understanding of quantum materials ranging from strongly correlated states of matter to those exhibiting nontrivial topology. Over the past two decades, ARPES as a technique has matured dramatically with ever-improving resolution and continued expansion into the space, time, and spin domains. Simultaneously, the capability to synthesize new materials and apply nonthermal tuning parameters in situ has unlocked new dimensions in the study of all quantum materials. These developments are reviewed, and the scientific contributions they have enabled in contemporary quantum materials research are surveyed.

Quantum-mechanical effects at the macroscopic level were first explored in Josephson-junction-based superconducting circuits in the 1980s. In recent decades, the emergence of quantum information science has intensified research toward using these circuits as qubits in quantum information processors. The realization that superconducting qubits can be made to strongly and controllably interact with microwave photons, the quantized electromagnetic fields stored in superconducting circuits, led to the creation of the field of circuit quantum electrodynamics (QED), the topic of this review. While atomic cavity QED inspired many of the early developments of circuit QED, the latter has now become an independent and thriving field of research in its own right. Circuit QED allows the study and control of light-matter interaction at the quantum level in unprecedented detail. It also plays an essential role in all current approaches to gate-based digital quantum information processing with superconducting circuits. In addition, circuit QED provides a framework for the study of hybrid quantum systems, such as quantum dots, magnons, Rydberg atoms, surface acoustic waves, and mechanical systems interacting with microwave photons. Here the coherent coupling of superconducting qubits to microwave photons in high-quality oscillators focusing on the physics of the Jaynes-Cummings model, its dispersive limit, and the different regimes of light-matter interaction in this system are reviewed. Also discussed is coupling of superconducting circuits to their environment, which is necessary for coherent control and measurements in circuit QED, but which also invariably leads to decoherence. Dispersive qubit readout, a central ingredient in almost all circuit QED experiments, is also described. Following an introduction to these fundamental concepts that are at the heart of circuit QED, important use cases of these ideas in quantum information processing and in quantum optics are discussed. Circuit QED realizes a broad set of concepts that open up new possibilities for the study of quantum physics at the macro scale with superconducting circuits and applications to quantum information science in the widest sense.

Despite major advances in climate science over the last 30 years, persistent uncertainties in projections of future climate change remain. Climate projections are produced with increasingly complex models that attempt to represent key processes in the Earth system, including atmospheric and oceanic circulations, convection, clouds, snow, sea ice, vegetation, and interactions with the carbon cycle. Uncertainties in the representation of these processes feed through into a range of projections from the many state-of-the-art climate models now being developed and used worldwide. For example, despite major improvements in climate models, the range of equilibrium global warming due to doubling carbon dioxide still spans a range of more than 3. Here a promising way to make use of the ensemble of climate models to reduce the uncertainties in the sensitivities of the real climate system is reviewed. The emergent constraint approach uses the model ensemble to identify a relationship between an uncertain aspect of the future climate and an observable variation or trend in the contemporary climate. This review summarizes previous published work on emergent constraints and discusses the promise and potential dangers of the approach. Most importantly, it argues that emergent constraints should be based on well-founded physical principles such as the fluctuation-dissipation theorem. This review will stimulate physicists to contribute to the rapidly developing field of emergent constraints on climate projections, bringing to it much needed rigor and physical insights.

Over the last decade impressive progress has been made in the theoretical understanding of transport properties of clean, one-dimensional quantum lattice systems. Many physically relevant models in one dimension are Bethe-ansatz integrable, including the anisotropic spin-$1/2$ Heisenberg (also called the spin-$1/2$ $\mathit{XXZ}$ chain) and the Fermi-Hubbard model. Nevertheless, practical computations of correlation functions and transport coefficients pose hard problems from both the conceptual and technical points of view. Only because of recent progress in the theory of integrable systems, on the one hand, and the development of numerical methods, on the other hand, has it become possible to compute their finite-temperature and nonequilibrium transport properties quantitatively. Owing to the discovery of a novel class of quasilocal conserved quantities, there is now a qualitative understanding of the origin of ballistic finite-temperature transport, and even diffusive or superdiffusive subleading corrections, in integrable lattice models. The current understanding of transport in one-dimensional lattice models, in particular, in the paradigmatic example of the spin-$1/2$ $\mathit{XXZ}$ and Fermi-Hubbard models, is reviewed, as well as state-of-the-art theoretical methods, including both analytical and computational approaches. Among other novel techniques, matrix-product-state-based simulation methods, dynamical typicality, and, in particular, generalized hydrodynamics are covered. The close and fruitful connection between theoretical models and recent experiments is discussed, with examples given from the realms of both quantum magnets and ultracold quantum gases in optical lattices.

Topological semimetals (TSMs) are characterized by bulk band crossings in their electronic structures, which are expected to give rise to gapless electronic excitations and topological features that underlie exotic physical properties. The most famous examples are Dirac and Weyl semimetals, in which the corresponding low-energy fermionic excitations, i.e., the Dirac and Weyl fermions, are direct analogs of elementary particles in quantum field theory. The last decade has witnessed an explosion of research activities in the field of TSMs thanks to precise theoretical predictions, well-controlled material synthesis, and advanced characterization techniques including angle-resolved photoemission spectroscopy, scanning tunneling microscopy, magnetotransport measurements, optical spectroscopy, etc. Here recent progress in three-dimensional TSMs is reviewed with an emphasis on their characteristic bulk electronic structures, including dimensionality (such as zero-dimensional nodal points, one-dimensional nodal lines, and two-dimensional nodal surfaces), degeneracy (twofold, threefold, fourfold, sixfold, or eightfold) of the band crossing, the slope (type I and type II) and order (linear, quadratic, or cubic) of the band dispersion near the crossing, the characteristic topological invariants (such as monopole charges), and the crystallographic symmetries that stabilize the band crossings. The distinct signatures of the various topological semimetal phases, such as the nontrivial surface states (including Fermi arcs of Dirac and Weyl semimetals) and the unique transport and optical responses (such as chiral anomaly-induced negative magnetoresistance in Dirac and Weyl semimetals), are also reviewed.

Laser-cooled and trapped atomic ions form an ideal standard for the simulation of interacting quantum spin models. Effective spins are represented by appropriate internal energy levels within each ion, and the spins can be measured with near-perfect efficiency using state-dependent fluorescence techniques. By applying optical fields that exert optical dipole forces on the ions, their Coulomb interaction can be modulated to produce long-range and tunable spin-spin interactions that can be reconfigured by shaping the spectrum and pattern of the laser fields in a prototypical example of a quantum simulator. Here the theoretical mapping of atomic ions to interacting spin systems, the preparation of complex equilibrium states, and the study of dynamical processes in these many-body interacting quantum systems are reviewed, and the use of this platform for optimization and other tasks is discussed. The use of such quantum simulators for studying spin models may inform our understanding of exotic quantum materials and shed light on the behavior of interacting quantum systems that cannot be modeled with conventional computers.

Coupling a quantum many-body system to an external environment dramatically changes its dynamics and offers novel possibilities not found in closed systems. Of special interest are the properties of the steady state of such open quantum many-body systems, as well as the relaxation dynamics toward the steady state. However, new computational tools are required to simulate open quantum many-body systems, as methods developed for closed systems cannot be readily applied. Several approaches to simulating open many-body systems are reviewed, and advances made in recent years toward the simulation of large system sizes are pointed out.

A general classification of flavor symmetries is provided according to their interplay with the proper Poincaré and gauge groups and to their linear or nonlinear action in field space. The focus is on the lepton sector, and the different types of symmetries describing neutrino masses and the lepton mixing matrix are reviewed. Several illustrative examples are presented for each type of symmetry, and specific strengths and limitations are discussed.

The state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers is surveyed. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally driven two-dimensional structural transformations. Additionally, these 2D materials display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry (such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies) is provided as well. The physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in nonvolatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D material toolbox.

Since the initial development of charged particle colliders in the middle of the 20th century, these advanced scientific instruments have been at the forefront of scientific discoveries in high-energy physics. Collider accelerator technology and beam physics have progressed immensely and modern facilities now operate at energies and luminosities many orders of magnitude greater than the pioneering colliders of the early 1960s. In addition, the field of colliders remains extremely dynamic and continues to develop many innovative approaches. Indeed, several novel concepts are currently being considered for designing and constructing even more powerful future colliders. The colliding-beam method and the history of colliders are first reviewed. Then, the major achievements of operational machines and the key features of near-term collider projects that are currently under development are presented. The review concludes with an analysis of numerous proposals and studies for distant-future colliders. The evaluation of their respective potentials reveals promising prospects for further significant breakthroughs in the collider field.

The current understanding of the role of topology in non-Hermitian (NH) systems and its far-reaching physical consequences observable in a range of dissipative settings are reviewed. In particular, how the paramount and genuinely NH concept of exceptional degeneracies, at which both eigenvalues and eigenvectors coalesce, leads to phenomena drastically distinct from the familiar Hermitian realm is discussed. An immediate consequence is the ubiquitous occurrence of nodal NH topological phases with concomitant open Fermi-Seifert surfaces, where conventional band-touching points are replaced by the aforementioned exceptional degeneracies. Furthermore, new notions of gapped phases including topological phases in single-band systems are detailed, and the manner in which a given physical context may affect the symmetry-based topological classification is clarified. A unique property of NH systems with relevance beyond the field of topological phases consists of the anomalous relation between bulk and boundary physics, stemming from the striking sensitivity of NH matrices to boundary conditions. Unifying several complementary insights recently reported in this context, a picture of intriguing phenomena such as the NH bulk-boundary correspondence and the NH skin effect is put together. Finally, applications of NH topology in both classical systems including optical setups with gain and loss, electric circuits, and mechanical systems and genuine quantum systems such as electronic transport settings at material junctions and dissipative cold-atom setups are reviewed.

In the late 1970s, the axion was proposed as a solution to the strong $CP$ problem, i.e., the puzzle why the strong interactions conserve parity $P$ and the product $CP$ of charge conjugation and parity in spite of the fact that the standard model of elementary particles as a whole violates those symmetries. The original axion was soon ruled out by laboratory experiments and astrophysical considerations, but a new version was invented that is much more weakly coupled and that evades the laboratory and astrophysical constraints. It was named the “invisible” axion. However, the axion cannot be arbitrarily weakly coupled because it is overproduced in the early Universe by vacuum realignment in the limit of vanishing coupling. The axions produced by vacuum realignment are a form of cold dark matter today. The axion provides a solution then not only to the strong $CP$ problem but also to the dark matter problem. Various methods have been proposed to search for dark matter axions and for axions emitted by the Sun. Their implementation and improvement has led to significant constraints on the notion of an invisible axion. Even purely laboratory methods may place significant constraints on invisible axions or axionlike particles. This review discusses the various methods that have been proposed and provides theoretical derivations of their signals.

The Novel Probes Project, an initiative to advance the field of astrophysical tests of the dark sector by creating a forum that connects observers and theorists, is introduced. This review focuses on tests of gravity and is intended to be of use primarily to observers, as well as theorists with an interest in the development of experimental tests. It is twinned with a separate upcoming review on dark matter self-interactions. The review focuses on astrophysical tests of gravity in the weak-field regime, ranging from stars to quasilinear cosmological scales. This regime is complementary to both strong-field tests of gravity and background and linear probes in cosmology. In particular, the nonlinear screening mechanisms that are an integral part of viable modified-gravity models lead to characteristic signatures, specifically on astrophysical scales. The potential of these probes is not limited by cosmic variance but comes with the challenge of building robust theoretical models of the nonlinear dynamics of stars, galaxies, and large-scale structure. The groundwork is laid for a thorough exploration of the weak-field, nonlinear regime, with an eye to using the current and next generation of observations for tests of gravity. The scene is set by showing how gravitational theories beyond general relativity are expected to behave, focusing primarily on screening mechanisms. Analytic and numerical techniques for exploring the relevant astrophysical regime are described, as are the pertinent observational signals. With these in hand a range of astrophysical tests of gravity are presented, and prospects for future measurements and theoretical developments are discussed.

The production of about half of the heavy elements found in nature is assigned to a specific astrophysical nucleosynthesis process: the rapid neutron-capture process ($r$ process). Although this idea was postulated more than six decades ago, the full understanding faces two types of uncertainties or open questions: (a) The nucleosynthesis path in the nuclear chart runs close to the neutron-drip line, where presently only limited experimental information is available, and one has to rely strongly on theoretical predictions for nuclear properties. (b) While for many years the occurrence of the $r$ process has been associated with supernovae, where the innermost ejecta close to the central neutron star were supposed to be neutron rich, more recent studies have cast substantial doubts on this environment. Possibly only a weak $r$ process, with no or negligible production of the third $r$-process peak, can be accounted for, while much more neutron-rich conditions, including an $r$-process path with fission cycling, are likely responsible for the majority of the heavy $r$-process elements. Such conditions could result during the ejection of initially highly neutron-rich matter, as found in neutron stars, or during the fast ejection of matter that has previously experienced strong electron captures at high densities. Possible scenarios are the mergers of neutron stars, neutron-star–black hole mergers, but also include rare classes of supernovae as well as hypernovae or collapsars with polar jet ejecta, and possibly also accretion disk outflows related to the collapse of fast rotating massive stars. The composition of the ejecta from each event determines the temporal evolution of the $r$-process abundances during the “chemical” evolution of the Galaxy. Stellar $r$-process abundance observations have provided insight into and constraints on the frequency of and conditions in the responsible stellar production sites. One of them, neutron-star mergers, was just identified thanks to the observation of the $r$-process kilonova electromagnetic transient following the gravitational wave event GW170817. These observations, which are becoming increasingly precise due to improved experimental atomic data and high-resolution observations, have been particularly important in defining the heavy element abundance patterns of the old halo stars, and thus in determining the extent and nature of the earliest nucleosynthesis in the Galaxy. Combining new results and important breakthroughs in the related nuclear, atomic, and astronomical fields of science, this review attempts to answer the question “How were the elements from iron to uranium made?”

Yearslong time series of high-precision brightness measurements have been assembled for thousands of stars with telescopes operating in space. Such data have allowed astronomers to measure the physics of stellar interiors via nonradial oscillations, opening a new avenue to study the stars in the Universe. Asteroseismology, the interpretation of the characteristics of oscillation modes in terms of the physical properties of the stellar interior, brought entirely new insights in how stars rotate and how they build up their chemistry throughout their evolution. Data-driven space asteroseismology has delivered a drastic increase in the reliability of computer models mimicking the evolution of stars born with a variety of masses and metallicities. Such models are critical ingredients for modern physics as a whole because they are used throughout various contemporary and multidisciplinary research fields in space science, including the search for life outside the Solar System, archaeological studies of the Milky Way, and the study of single and binary supernova progenitors, among which are future gravitational wave sources. The specific role and potential of asteroseismology for those modern research fields are illustrated. The review concludes with current limitations of asteroseismology and highlights how they can be overcome with ongoing and future large infrastructures for survey astronomy combined with new theoretical research in the era of high-performance computing. This review presents results obtained through major community efforts over the past decade. These breakthroughs were achieved in a collaborative and inclusive spirit that is characteristic of the asteroseismology community. The review’s aim is to make this research field accessible to graduate students and readers coming from other fields of physics, with incentives to enjoy and join future applications in this domain of astrophysics.

Double-beta processes play a key role in the exploration of neutrino and weak interaction properties, and in the searches for effects beyond the standard model. During the last half century many attempts were undertaken to search for double-beta decay with emission of two electrons, especially for its neutrinoless mode $0\nu 2{\beta }^{-}$, the latter having still not been observed. Double-electron capture (2EC) was not yet in focus because of its in general lower transition probability. However, the rate of neutrinoless double-electron capture $0\nu 2\mathrm{EC}$ can experience a resonance enhancement by many orders of magnitude when the initial and final states are energetically degenerate. In the resonant case, the sensitivity of the $0\nu 2\mathrm{EC}$ process can approach the sensitivity of the $0\nu 2{\beta }^{-}$ decay in the search for the Majorana mass of neutrinos, right-handed currents, and other new physics. An overview of the main experimental and theoretical results obtained during the last decade in this field is presented. The experimental part outlines search results of 2EC processes and measurements of the decay energies for possible resonant $0\nu 2\mathrm{EC}$ transitions. An unprecedented precision in the determination of decay energies with Penning traps has allowed one to refine the values of the degeneracy parameter for all previously known near-resonant decays and has reduced the rather large uncertainties in the estimate of the $0\nu 2\mathrm{EC}$ half-lives. The theoretical part contains an updated analysis of the electron shell effects and an overview of the nuclear-structure models, in which the nuclear matrix elements of the $0\nu 2\mathrm{EC}$ decays are calculated. One can conclude that the decay probability of $0\nu 2\mathrm{EC}$ can experience a significant enhancement in several nuclides.