Annual Review of Nuclear and Particle Science - Volume 59, 2009

This article follows the scientific life of John Norris Bahcall, including his tenacious pursuit of the solar neutrino problem, his contributions to our understanding of galaxies, quasars, and their emissions, and his leadership of and advocacy for astronomy and astrophysics.

Neutrino astronomy, the observation of neutrinos from extraterrestrial sources, began in 1966, when Raymond Davis, Jr. turned on his deep-underground chlorine-based neutrino detector. Over the next three decades, the lower-than-predicted solar neutrino flux that Davis observed confused the scientific community. Was our understanding of energy generation in the core of stars flawed? Was there an unforeseen experimental error? Or were neutrinos more mysterious than we had anticipated? The scientific career of the remarkable scientist Raymond Davis played an integral role in unraveling the complex nature of neutrinos and in confirming our nuclear fusion model of energy generation in the core of the Sun.

Yoji Totsuka, a leading figure in the discovery of neutrino mass, died in 2008. His leadership of the Super-Kamiokande experiment and subsequent leadership of the High Energy Physics Laboratory (KEK) in Japan helped to lay the foundation for the current worldwide experimental and theoretical program in neutrinos.

Since the initial measurements of the electron charge were made a century ago, experimenters have faced the persistent question of the existence of elementary particles with charges that are fractional multiples of the electron charge. In this review, we discuss the results of recent searches for these fractionally charged particles.

We provide a pedagogical overview of inflation in string theory. We focus upon the sensitivity of inflation to Planck-scale physics, which we argue provides both the primary motivation and the central theoretical challenge for the subject. We illustrate these issues through two case studies of inflationary scenarios in string theory: warped d-brane inflation and axion monodromy inflation. Finally, we indicate how future observations can test scenarios of inflation in string theory.

The estimation of cosmological parameters from precision observables is an important industry with crucial ramifications for particle physics. In this article, I discuss the statistical methods presently used in cosmological data analysis, highlighting the main assumptions and uncertainties. The topics covered are parameter estimation, model selection, multimodel inference, and experimental design, all primarily from a Bayesian perspective.

We review photopion experiments on the nucleon in the near-threshold region. We compare the results with the predictions of the low-energy theorems of quantum chromodynamics calculated using chiral perturbation theory (ChPT), which is based on the spontaneous breaking of chiral symmetry as well as explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult to perform because the cross sections are small. Nevertheless, the field is now mature in terms of accuracy and sensitivity. We also discuss the accomplishments and limitations of past measurements, as well as future planned experiments at Mainz and HIγS. The technical developments required for the HIγS facility are emphasized. Finally, we show that future experiments will provide even more accurate tests of ChPT and will be sensitive to isospin-breaking dynamics due to the mass difference between the up and down quarks.

We survey recent progress in relating string theory to quantum chromodynamics (QCD), focusing on (a) holographic models of gauge theories that are similar to QCD and (b) applications to heavy-ion collisions.

We review the theory of hadronic atoms in the framework of quantum chromodynamics, with photons and electrons included (QCD+QED). We illustrate the method by means of π⁺π⁻ atoms and briefly consider the application of the same approach to several other hadronic atoms.

We present a comprehensive overview of an extension of the Standard Model that contains three right-handed (sterile) neutrinos with masses below the electroweak scale [the Neutrino Minimal Standard Model (νMSM)]. We consider the history of the Universe from the inflationary era through today and demonstrate that most of the observed phenomena beyond the Standard Model can be explained within the framework of this model. We review the mechanism of baryon asymmetry of the Universe in the νMSM and discuss a dark matter candidate that can be warm or cold and that satisfies all existing constraints. From the viewpoint of particle physics, the model provides an explanation for neutrino flavor oscillations. Verification of the νMSM is possible with existing experimental techniques.

We give an overview of the experimental measurements and the theoretical understanding of the branching fractions and CP-violating asymmetries of charmless B meson decays. Most experimental results are from the BaBar and Belle Collaborations’ data, accumulated during the past decade. The global features of these experimental results are typically well described by quantum chromodynamics (QCD)-motivated theories such as QCD factorization, perturbative QCD, and soft-collinear effective theory. The agreement between theory and experiment is generally satisfactory, although there remain some unsolved puzzles that pose a great challenge to both theorists and experimentalists.

We review the main theoretical motivations and observational constraints on Planck scale–suppressed violations of Lorentz invariance. After introducing the problems related to the phenomenological study of quantum gravitational effects, we discuss the main theoretical frameworks within which possible departures from Lorentz invariance can be described. In particular, we focus on the framework of effective field theory, describing several possible ways of including Lorentz violation therein and discussing their theoretical viability. We review the main low-energy effects that are expected in this framework. We discuss the current observational constraints on such a framework, focusing on those achievable through high-energy astrophysics observations. In this context, we present a summary of the most recent and strongest constraints on quantum electrodynamics with Lorentz-violating nonrenormalizable operators. Finally, we discuss the status of the field and its future perspectives.

We review recent progress in determining the effects of d-brane instantons in supersymmetric compactifications of Type II string theory to four dimensions. We describe the abstract d-brane instanton calculus for holomorphic couplings such as the superpotential, the gauge kinetic function, and higher fermionic F-terms, and we briefly discuss the implications of background fluxes for the instanton sector. We then summarize the concrete consequences of stringy d-brane instantons for the construction of semirealistic models of particle physics or supersymmetry breaking in compact and noncompact geometries.

We review 30 years of Cornell Electron Storage Ring (CESR) history, beginning with a brief account of the early CUSB and CLEO accomplishments and following with details on some of the more recent work by CLEO.

The search for the origin of cosmic rays with energies above 10¹⁸ eV has made considerable progress during the past few years. With new data from the High Resolution Fly's Eye experiment in Utah and the Pierre Auger Observatory in Argentina, a suppression of the cosmic ray flux above 5 × 10¹⁹ eV consistent with the predicted suppression arising from interaction with the cosmic microwave background (the so-called GZK suppression) has been observed. Recent results from Auger seem to indicate that the arrival direction distribution of cosmic rays with energies above the GZK suppression is not isotropic. Progress toward a better understanding of the chemical composition of the cosmic rays is under way. A new generation of detectors is providing data of unprecedented quality and quantity. In this article, we review these recent developments and briefly outline how our view of the highest-energy cosmic rays has evolved over the past several years.

Over the past decade, there has been significant progress in developing the concepts and technologies needed to produce, capture, and accelerate O(10²¹) muons per year. These developments have paved the way for a new type of neutrino source (neutrino factory) and a new type of very high energy lepton-antilepton collider (muon collider). This article reviews the motivation, design, and research and development for future neutrino factories and muon colliders.

We emphasize the importance of including radiative corrections when extracting physics from colliders such as the Tevatron Run II at Fermilab, the Large Hadron Collider (LHC) at CERN, and a future linear collider (LC). We review both well-tested methods and recent advances for calculating these corrections within the Standard Model of electroweak and strong interactions. We discuss the demands on theory and the prospects for matching or exceeding the anticipated experimental precision.

The discovery that the cosmic expansion is accelerating has been followed by an intense theoretical and experimental response in physics and astronomy. The discovery implies that our most basic notions about how gravity works are violated on cosmological distance scales. A simple fix is to introduce a cosmological constant into the field equations for general relativity. However, the extremely small value of the cosmological constant, relative to theoretical expectations, has led theorists to explore numerous alternative explanations that involve the introduction of an exotic negative-pressure fluid or a modification of general relativity. Here we review the evidence for cosmic acceleration. We then survey some of the theoretical attempts to account for it, including the cosmological constant, quintessence and its variants, mass-varying neutrinos, and modifications of general relativity. We discuss experimental and observational tests that may allow us to distinguish among some of the theoretical ideas that have been proposed.

The solar neutrino problem arose when the first measurements of the flux of neutrinos from the Sun, taken by Raymond Davis, Jr. with a Cl-Ar radiochemical detector, fell substantially below the value predicted theoretically by John Bahcall. Bahcall's prediction came from a detailed model of the nuclear reactions powering the Sun. Resolution of the problem came three decades later with the observation of nonelectron flavors of neutrinos in the solar flux. The use of heavy water in the Sudbury Neutrino Observatory (SNO) experiment provided a means to measure both electron and nonelectron components, and the presence of the latter showed that neutrino flavor conversion was taking place—a hallmark of neutrino oscillation and mass. The solar models were vindicated, and the Standard Model of elementary particles and fields had to be revised. Here we present an account of the SNO project, its conclusions to date, and its ongoing analysis.