Annual Review of Nuclear and Particle Science - Volume 49, 1999

We provide an up-to-date account of progress toward understanding the origin of the observed baryon asymmetry of the universe. We provide a pedagogical introduction to the primary areas of research in this field, giving a detailed description of the different scenarios. The very recent developments in Grand-Unified-Theory baryogenesis, leptogenesis, electroweak baryogenesis, and the Affleck-Dine mechanism are presented. In particular, we focus on specific particle physics implementations, mostly in the context of supersymmetry, which lead to specific testable predictions.

In forthcoming years, connections between cosmology and particle physics will become increasingly important with the advent of a new generation of cosmic microwave background (CMB) experiments. Here, we review a number of these links. Our primary focus is on new CMB tests of inflation. We explain how the inflationary predictions for the geometry of the Universe and primordial density perturbations will be tested by CMB temperature fluctuations, and how the gravitational waves predicted by inflation can be pursued with the CMB polarization. The CMB signatures of topological defects and primordial magnetic fields from cosmological phase transitions are also discussed. Furthermore, we review current and future CMB constraints on various types of dark matter (e.g. massive neutrinos, weakly interacting massive particles, axions, vacuum energy), decaying particles, the baryon asymmetry of the Universe, ultra-high-energy cosmic rays, exotic cosmological topologies, and other new physics.

Measurements of transverse beam size are tremendously important to the performance of e⁺e⁻ linear colliders. In this paper we review the traditional technologies used to make such measurements, such as profile monitors and wire scanners, and their limitations. We introduce a new technique for electron beam-size measurement: Compton-scattered laser light, which may be used as an unbreakable “wire” or in the form of an interferometer beam-size monitor (BSM). We describe the use of such an interferometer BSM, noting both the general issues related to its design and operation and the specific experiences with such an interferometer at the Final Focus Test Beam at SLAC. We conclude by considering the ultimate limitations of the laser-interferometer BSM.

Low-mass particles, such as neutrinos, axions, other Nambu-Goldstone bosons, and gravitons, are produced in the hot and dense interior of stars. Therefore, astrophysical arguments constrain the properties of these particles in ways that are often complementary to cosmological arguments and to laboratory experiments. This review provides an update on the most important stellar-evolution limits and discusses them in the context of other information from cosmology and laboratory experiments.

We review the production of high-mass lepton pairs in fixed-target experiments, including both Drell-Yan (DY) and heavy quarkonium production [J/ψ, ψ′, ϒ(1S), ϒ(2S), and ϒ(3S)]. In recent years, DY data have become standard input to the determination of parton density distributions. DY data have recently yielded the first measurement of the x dependence of the , asymmetry of the proton. Similar to the observations in deeply inelastic scattering, precision measurements of the nuclear dependence of the proton-induced DY process exhibit shadowing at small target momentum fraction, x2. There is, however, no evidence of enhanced DY production from nuclear targets. Mean transverse momenta of DY pairs are observed to increase with target mass. These data, analyzed within a new theoretical framework, provide an estimation of the energy loss of fast quarks in nuclear matter. In contrast to the DY process, there are large nuclear effects in the production of all quarkonia. These effects show strong dependence on pt and xF but do not scale with x2. Statistically less precise measurements of D meson (open-charm) production at small xF are consistent with no nuclear dependence.

Data and their interpretations are reviewed for J/ψ and ψ′ production in heavy-ion collisions with particular emphasis on the recently reported anomaly in Pb-Pb reactions. The observed suppressions in pA collisions are discussed in terms of final-state absorption of a pre-meson and the pT distributions due to parton scattering in the initial state. The experimental setup is described and the various results for J/ψ and ψ′ suppressions, including their pT-dependence, are presented. The data, in particular the observed anomalous suppression in Pb-Pb collisions, are discussed in the light of the proposed ideas, among them quark-gluon plasma formation and suppression via co-movers.

We review the study of the internal spin structure of the proton and neutron. High-energy scattering of polarized leptons by polarized protons, neutrons, and deuterons provides a measurement of the nucleon spin structure functions. These structure functions give information on the polarized quark contributions to the spin of the proton and the neutron and allow tests of the quark-parton model and quantum chromodynamics. We discuss the formalism of deep inelastic scattering of polarized leptons on polarized nucleons, the past decade of experimental progress, and future programs to measure the polarized gluon contribution to the proton spin.

Introduced in 1988, microstrip gas chambers perform considerably better than classic multiwire detectors. Excellent localization, high rate capability, and good granularity make them attractive for charged-particle tracking at high-luminosity colliders, among other applications. The technology continues to improve; for example, substrates have been developed that prevent charge accumulations. Some problems persist, namely the slow degradation under sustained irradiation (aging) and the serious damage that can result from accidental discharges. New types of detectors aim at improving on these points; the microdot, micromegas, and gas electron multiplier detectors are promising examples. They are generally more reliable and cheaper.

An unexpected excess of events with high Q² reported by the H1 and ZEUS collaborations at HERA in early 1997 provoked speculation that the production of first-generation leptoquarks had been observed. In this article we review the anomalous HERA events, give a brief summary of the motivation for leptoquarks, and discuss the most recent searches at HERA and the Tevatron for first-, second-, and third-generation leptoquarks as well as generation-changing leptoquarks. We conclude with a short discussion of future prospects for leptoquark searches.

We review the direct measurements of the top quark mass Mt using the sample of events collected by the DØ and CDF experiments at Fermilab. Measurements using events in the lepton plus jets, dilepton, and all-hadronic decay modes are reviewed, as is the combination of the results to yield the current world average Mt = 174.3 ± 3.2 (stat.) ± 4.0 (syst.) GeV/c². We close by estimating the precision attainable with future data sets at Fermilab and the Large Hadron Collider.

The question of neutrino mass is one of the major riddles in particle physics. Recently, strong evidence that neutrinos have nonzero masses has been found. Though tiny, these masses could be large enough to contribute significantly to the mass density of the universe. The evidence for nonvanishing neutrino masses is based on the apparent observation of neutrino oscillation—the transformation of a neutrino from one type or “flavor” to another. We explain the physics of neutrino oscillation and review and weigh the evidence that it actually occurs in nature. We discuss the constraints on neutrino mass from cosmology and from experiments with negative results. After presenting illustrative neutrino mass spectra suggested by the present data, we consider how experiments in the near and distant future can further illuminate the nature of neutrinos and their masses.

Two-particle momentum correlations between pairs of identical particles produced in relativistic heavy-ion reactions can be analyzed to extract the space-time structure of the collision fireball. We review recent progress in the application of this method, based on newly developed theoretical tools and new high-quality data from heavy-ion collision experiments. Implications for our understanding of the collision dynamics and for the search for the quark-gluon plasma are discussed.

This chapter presents an experimental overview of collective flow phenomena in heavy-ion collisions in the incident energy regime from 100 AMeV to 160 AGeV. The characteristic features of various types of flow are described. The discussion of common dependences on the collision system and bombarding energy focuses on the different time scales of the underlying physics processes. Special emphasis is placed on the presence of structures in the excitation functions. The sensitivity of the available data to changes of nuclear-matter properties, e.g. equation of state, medium effects, and phase transitions, is discussed.

High-energy jet distributions measured since 1992 at the Fermilab Tevatron proton-antiproton collider are presented and compared with theoretical predictions. The statistical uncertainties on these measurements are significantly reduced relative to previous results. The systematic uncertainties are comparable in size to the uncertainty in the theoretical predictions. Although some discrepancies between theory and measurements are noted, the inclusive jet and dijet cross sections can be described by quantum chromodynamics. Prospects for reducing the uncertainty in the theoretical predictions by incorporating Tevatron measurements into the proton parton distributions are discussed. Dijet distributions, in excellent agreement with quantum chromodynamics, set a 2.5-TeV limit on the mass scale of quark constituents.