Annual Review of Nuclear and Particle Science - Volume 65, 2015

This is the story of a culture and its evolution and legacy. Beginning with the invention of the cyclotron at Berkeley, the path of further accelerator development at Cornell via the Los Alamos experience of the primary actors is described. The science done with the accelerators and on the accelerators and beams themselves is reviewed and brought up to the current time. The evolution of the user community and the sources of support for accelerators and science done with them are discussed at the appropriate places in the story.

The discovery of the Higgs boson is a major milestone in our progress toward understanding the natural world. A particular aim of this review is to show how diverse ideas came together in the conception of electroweak symmetry breaking that led up to the discovery. I also survey what we know now that we did not know before, what properties of the Higgs boson remain to be established, and what new questions we may now hope to address.

Naturalness has for many years been a guiding principle in the search for physics beyond the Standard Model, particularly for understanding the physics of electroweak symmetry breaking. However, the discovery of the Higgs particle at 125 GeV, accompanied by the exclusion of many types of new physics expected in natural models, has called the principle into question. In addition, apart from the scale of weak interactions, there are other quantities in nature that appear unnaturally small and for which we have no proposal for a natural explanation. I first review the principle, then discuss some of the conjectures it has spawned. I then turn to some of the challenges to the naturalness idea and consider alternatives.

Studying the violation of parity and time-reversal invariance in atomic systems has proven to be a very effective means of testing the electroweak theory at low energy and searching for physics beyond it. Recent developments in both atomic theory and experimental methods have led to the ability to make extremely precise theoretical calculations and experimental measurements of these effects. Such studies are complementary to direct high-energy searches, and can be performed for only a fraction of the cost. We review the recent progress in the field of parity and time-reversal violation in atoms, molecules, and nuclei, and examine the implications for physics beyond the Standard Model, with an emphasis on possible areas for development in the near future.

The radiative capture reactions of greatest importance in nuclear astrophysics are identified and placed in their stellar contexts. Recent experimental efforts to estimate their thermally averaged rates are surveyed.

With the completion of Run I of the CERN Large Hadron Collider, particle physics has entered a new era. The production of unprecedented numbers of heavy-flavored hadrons in high-energy proton–proton collisions allows detailed studies of flavor-changing processes. The increasingly precise measurements allow the Standard Model to be tested with a new level of accuracy. Rare b hadron decays provide some of the most promising approaches for such tests because there are several observables that can be cleanly interpreted from a theoretical viewpoint. In this article, we review the status and prospects in this field, with a focus on precision measurements and null tests.

This article reviews the current status of the nuSTORM facility and shows how it can be utilized to perform the next step on the path toward the realization of a μ⁺μ⁻ collider. This review includes the physics motivation behind nuSTORM, a detailed description of the facility and the neutrino beams it can produce, and a summary of the short-baseline neutrino oscillation physics program that can be carried out at the facility. The basic idea for nuSTORM (the production of neutrino beams from the decay of muons in a racetrack-like decay ring) was discussed in the literature more than 30 years ago in the context of searching for noninteracting (sterile) neutrinos. However, only in the past 5 years has the concept been fully developed, motivated in large part by the facility's unmatched reach in addressing the evolving data on oscillations involving sterile neutrinos. Finally, this article reviews the basics of the μ⁺μ⁻ collider concept and describes how nuSTORM provides a platform to test advanced concepts for six-dimensional muon ionization cooling.

The unknown neutrino mass hierarchy—whether the ν3 mass eigenstate is the heaviest or the lightest—represents a major gap in our knowledge of neutrino properties. Determining the hierarchy is a critical step toward further precision measurements in the neutrino sector. The hierarchy is also central to interpreting the next generation of neutrinoless double-β decay results, plays a role in numerous cosmological and astrophysical questions, and serves as a powerful model discriminant for theories of neutrino mass generation and unification. Various current and planned experiments claim sensitivity for establishing the neutrino mass hierarchy. I review the most promising of these here, paying special attention to points of concern and consolidating the projected sensitivities into an outlook for the years ahead.

Gauge theories with compact symmetry groups possess topologically nontrivial configurations of gauge field. This characteristic has dramatic implications for the vacuum structure of quantum chromodynamics (QCD) and for the behavior of QCD plasma, as well as for condensed matter systems with chiral quasi-particles. I review the current status of this problem with an emphasis both on the interplay between chirality and a background magnetic field and on the observable manifestations of topology in heavy-ion collisions, Dirac semimetals, neutron stars, and the early Universe.

Thanks to the unnaturally small value of the QCD vacuum angle , time–reversal violation () offers a window into physics beyond the Standard Model (SM) of particle physics. We review the effective field theory framework that establishes a clean connection between (a) mechanisms, which can be represented by higher-dimensional operators involving SM fields and symmetries, and (b) hadronic interactions, which allow for controlled calculations of low-energy observables involving strong interactions. The chiral properties of mechanisms lead to a pattern that should be identifiable in measurements of the electric dipole moments of the nucleon and light nuclei.

In recent years, observational γ-ray astronomy has witnessed a remarkable range of exciting new results in the high-energy and very high-energy regimes. Coupled with extensive theoretical and phenomenological studies of nonthermal processes in the Universe, these observations have provided deep insights into a number of fundamental problems of high-energy astrophysics and astroparticle physics. Although the main motivations of γ-ray astronomy remain unchanged, recent observational results have contributed significantly toward our understanding of many related phenomena. This article aims to review the most important results in the young and rapidly developing field of γ-ray astrophysics.

Searches for magnetic monopoles (s) are a fascinating interdisciplinary area with implications for fundamental theories, particle physics, astrophysics, and cosmology. The quantum theory of s, and its consistency with electrodynamics, was originally derived by Dirac, marking the beginning of the search for classical monopoles at every new accelerator, including the Large Hadron Collider. s are required by Grand Unification Theories (GUTs), but unlike classical monopoles they would be incredibly massive, out of reach for any conceivable accelerator. Significant effort has been made to search for s in the cosmic radiation as relic particles from the early Universe in the widest experimentally available range of mass and velocity. In this review, we discuss the status of the searches for classical s at accelerators for GUTs, superheavy s in the penetrating cosmic radiation, and intermediate-mass s at high altitudes, emphasizing the most recent results and future perspectives.

Neutron matter is an intriguing nuclear system with multiple connections to condensed matter and astrophysics. Considerable progress has been made over the past 20 years in exploring the properties of pure neutron fluids. We begin by reviewing research exploring the behavior of very low density neutron matter, which forms a strongly paired superfluid and is thus similar to cold Fermi atoms, although at energy scales that differ by many orders of magnitude. We then review the behavior of higher-density neutron matter, discussing research that ties the study of neutron matter to the determination of the properties of neutron-rich nuclei and neutron star crusts. Finally, we review the impact that neutron matter at even higher densities has on the mass–radius relation of neutron stars, thereby making contact with astrophysical observations.

In the paradigm of three generations, neutrino mixing is described by three angles and a CP-violating phase. With the recent observation of electron antineutrino disappearance at baselines of 1 to 2 km from groups of reactors, the smallest neutrino mixing angle, θ13, has been unambiguously determined. The rather large value of θ13 enables the pursuit of CP violation and mass hierarchy in the neutrino sector. This review summarizes the progress of measurements of θ13 and highlights future prospects.

One of the possible next steps for high-energy physics research relies on a high-energy hadron or muon collider. The energy of a circular collider is limited by the strength of bending dipoles, and its maximum luminosity is determined by the strength of final focus quadrupoles. For this reason, the high-energy physics and accelerator communities have shown much interest in higher-field and higher-gradient superconducting accelerator magnets. The maximum field of NbTi magnets used in all present high-energy machines, including the LHC, is limited to ∼10 T at 1.9 K. Fields above 10 T became possible with the use of Nb3Sn superconductors. Nb3Sn accelerator magnets can provide operating fields up to ∼15 T and can significantly increase the coil temperature margin. Accelerator magnets with operating fields above 15 T require high-temperature superconductors. This review discusses the status and main results of Nb3Sn accelerator magnet research and development and work toward 20-T magnets.

Over the past few years, new physics methods and algorithms as well as the latest supercomputers have enabled the study of the QCD thermodynamic phase transition using lattice gauge theory numerical simulations and allowing unprecedented control over systematic errors. This progress is largely a consequence of the ability to perform continuum extrapolations with physical quark masses. We review recent progress in lattice QCD thermodynamics, focusing mainly on results that benefit from the use of physical quark masses: the crossover temperature, the equation of state, and fluctuations of the quark number susceptibilities. In addition, we emphasize calculations that are directly relevant to the study of relativistic heavy-ion collisions at RHIC and the LHC.

The violation of CP symmetry between matter and antimatter in the neutral K and B meson systems is well established, with a high degree of consistency between all available experimental measurements and with the Standard Model of particle physics. On the basis of the up-to-now-unbroken CPT symmetry, the violation of CP symmetry strongly suggests that the behavior of these particles under weak interactions must also be asymmetric under time reversal T. Many searches for T violation have been performed and proposed using different observables and experimental approaches. These include T-odd observables, such as triple products in weak decays, and genuine observables, such as permanent electric dipole moments of nondegenerate stationary states and the breaking of the reciprocity relation. We discuss the conceptual basis of the required exchange of initial and final states with unstable particles, using quantum entanglement and the decay as a filtering measurement, for the case of neutral B and K mesons. Using this method, the BaBar experiment at SLAC has clearly observed T violation in B mesons.

We review the current status and future perspectives of theory and experiments of transverse spin phenomena in high-energy scattering processes off nucleon targets and related issues in nucleon structure and QCD. Systematic exploration of transverse spin effects requires measurements in polarized deep-inelastic scattering, polarized pp collisions, and e⁺e⁻ annihilations. Sophisticated QCD-based techniques are also needed to analyze the experimental data sets.

We review the impact of nuclear forces on matter at neutron-rich extremes. Recent results have shown that neutron-rich nuclei become increasingly sensitive to three-nucleon forces, which are at the forefront of theoretical developments based on effective field theories of quantum chromodynamics. These developments include the formation of shell structure, the spectroscopy of exotic nuclei, and the location of the neutron drip line. Nuclear forces also constrain the properties of neutron-rich matter, including the neutron skin, the symmetry energy, and the structure of neutron stars. First, we review our understanding of three-nucleon forces and show how chiral effective field theory makes unique predictions for many-body forces. Then, we survey results with three-nucleon forces in neutron-rich oxygen and calcium isotopes and neutron-rich matter, which have been explored with a range of many-body methods. Three-nucleon forces therefore provide an exciting link between theoretical, experimental, and observational nuclear physics frontiers.

Four decades after its prediction, the axion remains the most compelling solution to the strong-CP problem and a well-motivated dark matter candidate, inspiring a host of elegant and ultrasensitive experiments based on axion–photon mixing. This article reviews the experimental situation on several fronts. The microwave cavity experiment is making excellent progress in the search for dark matter axions in the μeV range and may plausibly be extended up to 100 μeV. Within the past several years, however, researchers have realized that axions are pervasive throughout string theories, but with masses that fall naturally in the neV range, for which an NMR-based search is under development. Both searches for axions emitted from the Sun's burning core and purely laboratory experiments based on photon regeneration have recently made great progress, with ambitious projects proposed for the coming decade. Each of these campaigns has pushed the state of the art in technology, enabling large gains in sensitivity and mass reach. Furthermore, each modality has been exploited in order to search for more generalized axion-like particles, which we also discuss in this review. We are hopeful, even optimistic, that the next review of the subject will concern the discovery of the axion, its properties, and its exploitation as a probe of early universe cosmology and structure formation.