Annual Review of Nuclear and Particle Science - Volume 51, 2001

Developments during the past eight years have refuted the folklore that chiral symmetries cannot be preserved on the lattice. The mechanism that permits chiral symmetry to coexist with the lattice is quite general and may work in nature as well. The reconciliation between chiral symmetry and the lattice is likely to revolutionize the field of numerical QCD.

Accurate quantum Monte Carlo calculations of ground states and low-lying excited states of light p-shell nuclei are now possible for realistic nuclear Hamiltonians that fit nucleon-nucleon scattering data. Results for more than 30 different (J^π;T) states, plus isobaric analogs, in A ≤ 8 nuclei have been obtained with an excellent reproduction of the experimental energy spectrum. These microscopic calculations show that nuclear structure, including both single-particle and clustering aspects, can be explained starting from elementary two- and three-nucleon interactions. Various density and momentum distributions, electromagnetic form factors, and spectroscopic factors have also been computed, as well as electroweak capture reactions of astrophysical interest.

Radioactive nuclei play an important role in a diverse range of astrophysical phenomena including the early universe, the sun, red giant stars, nova explosions, X-ray bursts, supernova explosions, and supermassive stars. Measurements of reactions with beams of short-lived radioactive nuclei can, for the first time, probe the nuclear reactions occurring in these cosmic phenomena. This article describes the astrophysical motivation for experiments with radioactive beams, the techniques to produce these beams and perform astrophysically relevant measurements, results from recent experiments, and plans for future facilities.

I review recent progress in our understanding of the color-superconducting phase of matter above nuclear density, giving particular emphasis to the effort to find observable signatures of the presence of this phase in compact stars.

CP violation plays a privileged role in our quest for new physics beyond the electroweak standard model. In the standard model, the violation of CP in the weak interactions has a single source: the phase of the quark mixing matrix (the CKM matrix, for Cabibbo-Kobayashi-Maskawa). Most extensions of the standard model exhibit new sources of CP violation. For instance, the truly minimal supersymmetric extension of the standard model has two new phases in addition to the CKM phase. Given that CP violation is so tiny in the kaon system, is still largely unexplored in B physics, and is negligibly small in the electric dipole moments, it is clear that new physics may have a good chance to manifest some departure from the standard model in this particularly challenging class of rare phenomena. On the other hand, it is also apparent that CP violation generally represents a major constraint on any attempt at model building beyond the standard model. In this review, we tackle these two sides of the relation between CP violation and new physics. Our focus will be on the potentialities to use CP violation as a probe on supersymmetric (SUSY) extensions of the standard model. We wish to clarify the extent to which such indirect signals for SUSY are linked to a fundamental theoretical issue: Is there a relation between the mechanism that originates the whole flavor structure and the mechanism that is responsible for the breaking of supersymmetry? Different ways to answer this question lead to quite different expectations for CP violation in B physics.

The measurement of parity violation in the helicity dependence of electron-nucleon scattering provides unique information about the basic quark structure of the nucleons. This review presents the general formalism of parity-violating electron scattering, with emphasis on elastic electron-nucleon scattering. The physics issues addressed by such experiments are discussed, and the major goals of the presently envisioned experimental program are identified. Results from a recent series of experiments are summarized and the future prospects of this program are discussed.

The conduction electrons in clusters of simple metal atoms are approximatively independent and free. Nucleons in nuclei also behave as delocalized and independent fermions. This generic behavior generates analogies between metal clusters and nuclei, such as the shell structure, the shapes, and the dipole vibration mode. However, there are also major differences that arise from the presence of ions in metal clusters. Fission of nuclei and clusters, and particle emission from them, reveal these differences.

Anapole moments are parity-odd, time-reversal-even moments of the E1 projection of the electromagnetic current. Although it was recognized, soon after the discovery of parity violation in the weak interaction, that elementary particles and composite systems such as nuclei must have anapole moments, it proved difficult to isolate this weak radiative correction. The first successful measurement, an extraction of the nuclear anapole moment of ¹³³Cs from the hyperfine dependence of the atomic parity violation, was obtained only recently. An important anapole moment bound in thallium also exists. We discuss these measurements and their significance as tests of the hadronic weak interaction, focusing on the mechanisms that operate within the nucleus to generate the anapole moment. The atomic results place new constraints on weak meson-nucleon couplings, constraints we compare to existing bounds from a variety of and nuclear tests of parity nonconservation.

Even the elusive neutrinos are trapped in matter, albeit transiently, in several astrophysical circumstances. Their interactions with the ambient matter not only reveal the properties of such exotic matter, but also shed light on the fundamental properties of the neutrinos. The physical sites of interest include the early universe, supernovae, and newly born neutron stars. Detection of neutrinos from these vastly different eras using the new generation of neutrino detectors holds great promise for enhancing our understanding of neutrino-matter interactions and astrophysical phenomena.

Starting in 1989, and continuing through the 1990s, high-energy physics witnessed a flowering of precision measurements in general and tests of the standard model in particular, led by e⁺e⁻ collider experiments operating at the Z⁰ resonance. Key contributions to this work came from the SLD collaboration at the SLAC Linear Collider. By exploiting the unique capabilities of this pioneering accelerator and the SLD detector, including a polarized electron beam, exceptionally small beam dimensions, and a CCD pixel vertex detector, SLD produced a broad array of electroweak, heavy-flavor, and QCD measurements. Many of these results are one of a kind or represent the world's standard in precision. This article reviews the highlights of the SLD physics program, with an eye toward associated advances in experimental technique, and the contribution of these measurements to our dramatically improved present understanding of the standard model and its possible extensions.

The Jefferson Laboratory's superconducting radiofrequency (srf) Continuous Electron Beam Accelerator Facility (CEBAF) provides multi-GeV continuous-wave (cw) beams for experiments at the nuclear and particle physics interface. CEBAF comprises two antiparallel linacs linked by nine recirculation beam lines for up to five passes. By the early 1990s, accelerator installation was proceeding in parallel with commissioning. By the mid-1990s, CEBAF was providing simultaneous beams at different but correlated energies up to 4 GeV to three experimental halls. By 2000, with srf development having raised the average cavity gradient to 7.5 MV/m, energies up to nearly 6 GeV were routine, at 1–150 μA for two halls and 1–100 nA for the other. Also routine are beams of >75% polarization. Physics results have led to new questions about the quark structure of nuclei, and therefore to user demand for a planned 12 GeV upgrade. CEBAF's enabling srf technology is also being applied in other projects.

The observation of muon flavor disappearance in the atmospheric neutrino flux provides compelling evidence for neutrino flavor oscillations and, by implication, for nonzero neutrino rest mass. We review recent results from Super-Kamiokande and other underground atmospheric neutrino experiments. These results, along with oscillation limits from reactor experiments, are evaluated in the context of neutrino three-state mixing. A unifying picture with νμ to ντ oscillation as the dominant mode emerges; however, certain ambiguities remain. Future directions for further experimentation are discussed.