The induced pseudoscalar coupling $g_p$ is the least well known of the weak couplings of the proton’s charged-current interaction. Its size is dictated by chiral symmetry arguments, and its measurement represents an important test of quantum chromodynamics at low energies. Experiments over the past decade have produced a large body of new data relevant to the coupling $g_p.$ These data include measurements of radiative and nonradiative muon capture on targets ranging from hydrogen and few-nucleon systems to complex nuclei. The authors review the theoretical underpinnings of $g_p,$ the experimental studies, and the procedures and uncertainties in extracting the coupling from data. They also discuss current puzzles and future opportunities.

Although the origin of matter-antimatter asymmetry remains unknown, continuing advances in theory and improved experimental limits have ruled out some scenarios for baryogenesis, for example, sphaleron baryogenesis at the electroweak phase transition in the Standard Model. At the same time, the success of cosmological inflation and the prospects for discovering supersymmetry at the Large Hadron Collider have put some other models in sharper focus. We review the current state of our understanding of baryogenesis with emphasis on those scenarios that we consider most plausible.

The edges of quantum Hall fluids behave as one-dimensional conductors. This article reviews electron transport into these edge states, covering both the theory based on the chiral Luttinger liquid and the experimental findings using electron tunneling as the probe. The first part of the review presents a basic description of this theory, including a derivation of the density of states, to provide a framework and language for discussing the experimental observations. The signature of the chiral Luttinger liquid is a power-law behavior for the density of states and the tunneling conductances. Experimentally, two techniques have been applied to study the tunneling conductance, using a gated point contact between two quantum Hall edges, or using a cleaved-edge barrier between an edge and a normal conductor. The point-contact method exhibits resonant tunneling, which appears to show some aspects of the Luttinger liquid, and the cleaved-edge method has yielded clear power-law dependences in the off-resonance conductances. Power-law behavior over many orders of magnitude is observed, confirming the Luttinger-liquid character of the edge states. However, the power-law exponents, while in agreement with finite-size numerical calculations, can differ from the universal values predicted by the Chern-Simon field theory. This disagreement is still not well understood. The review concludes with a brief survey of other one-dimensional conductors that have been studied to look for characteristics of the nonchiral Tomonaga-Luttinger liquid.

Revolutionary advances in both theory and technology have launched cosmology into its most exciting period of discovery yet. Unanticipated components of the universe have been identified, promising ideas for understanding the basic features of the universe are being tested, and deep connections between physics on the smallest scales and on the largest scales are being revealed.

During the past five decades, the development of accelerator science and technology sustained exponential growth in the energy and intensity of proton beams. Combined with an increasing repetition rate, the use of high-powered proton beams has extended from nuclear and high-energy physics to modern applications, including spallation-neutron production, kaon factories, nuclear transmutation, neutrino-factory drivers, and, in future, energy amplification and muon-collider drivers. This paper surveys the design and operational experience of existing and proposed proton facilities, summarizes physical and engineering issues limiting the efficiency of high-intensity synchrotrons and accumulators, reviews beam-dynamics topics pertaining to high-intensity performance, and discusses future applications and outlook.

The Falicov-Kimball model was introduced in 1969 as a statistical model for metal-insulator transitions; it includes itinerant and localized electrons that mutually interact with a local Coulomb interaction and is the simplest model of electron correlations. It can be solved exactly with dynamical mean-field theory in the limit of large spatial dimensions, which provides an interesting benchmark for the physics of locally correlated systems. In this review, the authors develop the formalism for solving the Falicov-Kimball model from a path-integral perspective and provide a number of expressions for single- and two-particle properties. Many important theoretical results are examined that show the absence of Fermi-liquid features and provide a detailed description of the static and dynamic correlation functions and of transport properties. The parameter space is rich and one finds a variety of many-body features like metal-insulator transitions, classical valence fluctuating transitions, metamagnetic transitions, charge-density-wave order-disorder transitions, and phase separation. At the same time, a number of experimental systems have been discovered that show anomalies related to Falicov-Kimball physics [including ${\mathrm{YbInCu}}_4,$ ${\mathrm{EuNi}}_2({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x{)}_2,$ ${\mathrm{NiI}}_2,$ and ${\mathrm{Ta}}_x\mathrm{N}].$

Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods—scanning tunneling microscopy and scanning force microscopy—in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface—the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors’ own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation.

Calorimetry has become a well-understood, powerful, and versatile measurement method. Besides perfecting this technique to match increasingly demanding operation at high-energy particle accelerators, physicists are developing low-temperature calorimeters to extend detection down to ever lower energies, and atmospheric and deep-sea calorimeters to scrutinize the universe up to the highest energies. The authors summarize the state of the art, with emphasis on the physics of the detectors and innovative technologies.

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems—an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

Most inclusive rare $B$ decays are important modes of flavor physics due to the small hadronic uncertainties. In this article, the author gives a status report on such decays, highlighting recent developments and open problems. The focus is on the decay modes $\overrightarrow{B}{X}_{s,d}\gamma ,$ $\overrightarrow{B}{X}_s{\mathcal{l}}^{+}{\mathcal{l}}^{-},$ and $\overrightarrow{B}{X}_s\nu \overline{\nu }$ and on their role as laboratories in the search for new physics. The experimental data already available from CLEO and the $B$ factories BABAR and BELLE are collected and discussed. The article then reviews the next-to-leading-log (NLL) and next-to-next-to-leading log (NNLL) QCD calculations of the inclusive decay rates that were recently completed and discusses future prospects, especially the issue of the charm-mass-scheme ambiguity. The phenomenological impact of these decay modes, in particular, on the CKM phenomenology and on the indirect search for supersymmetry, is analyzed. Direct $\mathrm{CP}$ violation in inclusive rare $B$ decays is briefly treated, as are the rare kaon decays $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ and $K_L\to {\pi }^0\nu \overline{\nu },$ which offer complementary, theoretically clean information.

This paper reviews progress on the fractional quantum Hall effect (FQHE) based on what we term Hamiltonian theories, i.e., theories that proceed from the microscopic electronic Hamiltonian to the final solution via a sequence of transformations and approximations, in either the Hamiltonian or path-integral approach, as compared with theories based on exact diagonalization or trial wave functions. The authors focus on the Chern-Simons approach, in which electrons are converted to Chern-Simons fermions or bosons that carry along flux tubes, and on their own extended Hamiltonian theory, in which electrons are paired with pseudovortices to form composite fermions whose properties are a lot closer to the ultimate low-energy quasiparticles. The article addresses a variety of qualitative and quantitative questions: In what sense do electrons really bind to vortices? What is the internal structure of the composite fermion and what does it mean? What exactly is the dipole picture? What degree of freedom carries the Hall current when the quasiparticles are localized or neutral or both? How exactly is the kinetic energy quenched in the lowest Landau level and resurrected by interactions? How well does the composite-fermion picture work at and near $\nu =1/2?$ Is the system compressible at $\nu =1/2?$ If so, how can composite fermions be dipolar at $\nu =1/2\mathrm{}$ and still be compressible? How is compressibility demonstrated experimentally? How does the charge of the excitation get renormalized from that of the electron to that of the composite fermion in an operator treatment? Why do composite fermions sometimes appear to be free when they are not? How does one compute (approximate) transport gaps, zero-temperature magnetic transitions, the temperature-dependent polarizations of gapped and gapless states, the NMR relaxation rate ${1/T}_1$ in gapless states, and gaps in inhomogeneous states? It is seen that though the Chern-Simons and extended Hamiltonian approaches agree whenever a comparison is possible, results that are transparent in one approach are typically opaque in the other, making them truly complementary.

Resistivity saturation is observed in many metallic systems with large resistivities—when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations—the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. This colloquium first reviews experimental results and early theories of resistivity saturation. It then describes more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular, the authors show how the (semiclassical) Ioffe-Regel condition can be derived quantum mechanically under certain assumptions about the system and why these assumptions are violated for high-$T_c$ cuprates and alkali-doped fullerides.

The mass of the nucleus, through its binding energy, continues to be of capital importance not only for various aspects of nuclear physics, but also for other branches of physics, notably weak-interaction studies and astrophysics. The authors first describe the modern experimental techniques dedicated to the particularly challenging task of measuring the mass of exotic nuclides and make detailed comparisons. Though tremendous progress in these and the associated production techniques has been made, allowing access to nuclides very far from stability, it is still not yet possible to produce many nuclides involved in stellar nucleosynthesis, especially the $r$ process, leaving no choice but to resort to theory. The review thus goes on to describe and critically compare the various modern mass formulas that may be used to extrapolate from the data towards the neutron drip line. Special attention is devoted to the crucial interplay between theory and experiment, showing how new measurements far from stability can considerably reduce the ambiguity in extrapolations to nuclides even further away.

This article reviews the progress of atomic force microscopy in ultrahigh vacuum, starting with its invention and covering most of the recent developments. Today, dynamic force microscopy allows us to image surfaces of conductors and insulators in vacuum with atomic resolution. The most widely used technique for atomic-resolution force microscopy in vacuum is frequency-modulation atomic force microscopy (FM-AFM). This technique, as well as other dynamic methods, is explained in detail in this article. In the last few years many groups have expanded the empirical knowledge and deepened our theoretical understanding of frequency-modulation atomic force microscopy. Consequently spatial resolution and ease of use have been increased dramatically. Vacuum atomic force microscopy opens up new classes of experiments, ranging from imaging of insulators with true atomic resolution to the measurement of forces between individual atoms.

Quantum corrals are two-dimensional structures built atom by atom on an atomically clean metallic surface using a scanning tunneling microscope (STM). These two-dimensional structures “corral” electrons in the surface states of noble metals, leading to standing-wave patterns in the electron density inside the quantum corral. The authors review the physics of quantum corrals and relate the signal of the STM to the scattering properties of substrate electrons from atomic impurities supported on the surface. The theory includes the effects of incoherent surface-state electron scattering at the impurities and quantitively describes nearly all of the current STM data on quantum corrals, including the recent quantum mirage experiments with Kondo effect. The physics underlying the recent mirage experiments is discussed, as are some of the outstanding questions regarding the Kondo effect from impurities in nanoscale structures on metallic surfaces. The authors also summarize recent work on variations of “quantum” corrals: Optical corrals and acoustical corrals.

It is now widely accepted that the cuprate superconductors are characterized by a long-range order similar to that present in the Bardeen-Cooper-Schrieffer (BCS) theory, that associated with the condensation of Cooper pairs. The author argues that many physical properties of the cuprates require interplay with additional order parameters associated with a proximate Mott insulator. A classification of Mott insulators in two dimensions is proposed. Experimental evidence so far shows that the class appropriate to the cuprates has collinear spin correlations, bond order, and confinement of neutral, spin $S=1/2\mathrm{}$ excitations. Proximity to second-order quantum phase transitions associated with these orders, and with the pairing order of BCS, has led to systematic predictions for many physical properties. In this context the author reviews the results of recent neutron scattering, fluxoid detection, nuclear magnetic resonance, and scanning tunneling microscopy experiments.