Annual Review of Nuclear and Particle Science - Volume 47, 1997

High-energy physics and the World Wide Web (WWW) share a rich history. The Web, developed at CERN as a collaboration tool and quickly adopted by the Internet community, has become a communications phenomenon. This article reviews early WWW development and its basic technology. I also summarize some significant applications of Web technology, past and present, and discuss prospects for future use.

The measurement of nuclear masses far from stability provides a fundamental test for our understanding of nuclear structure. Over the last decade new experimental techniques for the production and measurement of the masses of exotic nuclei have been developed. Predictions of nuclear models, however, exhibit significant disaccord as one progresses away from β stability. As a result, nuclear masses remain a field of experimental and theoretical challenge.

The development, in the early 1960s, of the dynamic nuclear polarization process in solid diamagnetic materials, doped with paramagnetic radicals, led to the use of solid polarized targets in numerous nuclear and particle physics experiments. Since then steady progress has been made in all contributing subsystems so that proton polarizations near 100% and deuteron polarizations higher than 50% have been achieved in various materials. More radiation-resistant materials, such as ammonia, have made it possible to perform experiments with high beam intensities and experiments that benefit from ⁴He cooling at 1K and high magnetic fields. The development of dilution refrigerators have allowed frozen spin operation so that experiments with large angular acceptance for the scattered particles have become routine. Many experiments have taken advantage of these developments and many more are being planned, especially with electromagnetic probes.

This review is devoted to presenting the status of the experimental search of gravitational radiation. After a short history of the development of detectors, we present the state of the art for both resonant antennas and interferometers. The final section is devoted to possible future developments of both techniques.

The use of feedback to stabilize the beam and improve the performance of accelerators is becoming more common. The methods used to design the feedback algorithms are introduced and some practical implementation details are described. The design of a PID loop using classical control techniques is covered as is the design of an optimal controller using modern control theory. Some adaptive control techniques are also briefly described. Examples are given of multiple-input-multiple-output loops and of how to handle systems of many interacting feedback loops.

The electromagnetic form factors of hadrons at large momentum transfer have been the subject of intense theoretical and experimental scrutiny over the past two decades, yet there is still not a universally accepted framework for their description. This review is a synopsis of their current status. The basic theoretical approaches to form factors at large momentum transfer are developed, emphasizing the valence quark and Feynman (soft) pictures. The discussion includes the relation of these descriptions to the parton model, as well as the roles of factorization, evolution, Sudakov resummation and QCD sum rules. This is followed by a discussion of the experimental status of pion and nucleon elastic form factors and resonance production amplitudes in the light of recent data; the successes and shortcomings of various theoretical proposals are highlighted.

Cooled beams in storage rings, circulating through internal targets, offer new experimental possibilities. This review summarizes the technical innovations that were required to exploit this novel environment, as well as advances in nuclear physics that are a result of it. Ongoing research and future trends are discussed as well.

A new generation of imaging atmospheric Čerenkov telescope arrays with energy thresholds as low as 30 GeV, angular resolutions of a few arc minutes, energy resolutions better than 20%, and energy flux sensitivities ∼10⁻¹³ erg cm⁻² s⁻¹ will elevate ground-based gamma-ray astronomy to a level comparable to that of research at longer wavelengths. The recent exciting observational results as well as theoretical predictions supply a strong rationale for the systematic study of primary γ-radiation in the very-high-energy (VHE) domain and the investigation of ultrarelativistic processes in astrophysical settings on both galactic and extragalactic scales. Such efforts will provide crucial insight into a number of fundamental problems of modern astrophysics and cosmology. This review discusses the current status of the field, the technical improvements that can be achieved, and the astrophysical topics that will be investigated.

In recent years a new generation of electron-positron colliders that will have 10–100 times the luminosity of today's machines has started construction. In a similar time frame, a new generation of high-brightness synchrotron light sources has become operational. The scientific goals for these “particle factories” and light sources are explained briefly, and the machine parameters required to achieve them are indicated. Both the accelerator physics and technology issues that result from the required parameters are discussed to show the challenges involved in such significant extensions of accelerator performance. Examples taken from representative projects are used to illustrate the types of solutions adopted to meet these challenges.

We review recent progress in understanding the importance of instanton effects in QCD. Instantons provide a mechanism for quark and gluon condensation, explain the U(1)A anomaly and the appearance of a non-perturbative vacuum energy density. In the framework of the instanton liquid model, a large number of hadronic correlation functions were calculated. The results are in good agreement with both experimental data and lattice simulations. We also review recent results on the phase structure of QCD-like theories. Instantons provide a mechanism for chiral symmetry restoration at finite temperature (or for a large number of quark flavors) connected with the formation of instanton–anti-instanton molecules.

The length scales for QCD and for nuclear physics are about the same, both about 1 fermi, and this directly motivates a search for a QCD explanation of nuclear phenomena, beginning with the deuteron. We first review the theoretical tools and results relevant to nuclear QCD, and then discuss data for several reactions. In particular, the remarkable data on deuteron photodisintegration agree with the naive perturbative QCD predictions for their scaling behavior and disagree with predictions from traditional approaches. This may be the first distinctive signature wherein considering the quark substructure in a nucleus is crucial in explaining some phenomenon.

Light forces acting on radioactive atoms confine them in cold samples of large phase space density. We describe the processes of radioactive atom production, reduction of their velocity, and capture. The trapped atoms form excellent sources for the study of atomic parity non-conservation, β decay and α decay asymmetries, and other weak interactions.

We review results obtained using Shell-Model Monte Carlo (SMMC) techniques. These methods reduce the imaginary-time many-body evolution operator to a coherent superposition of one-body evolutions in fluctuating one-body fields; the resultant path integral is evaluated stochastically. After a brief review of the methods, we discuss a variety of nuclear-physics applications. These include studies of the ground-state properties of pf-shell nuclei, Gamow-Teller strength distributions, thermal and rotational pairing properties of nuclei near N = Z, γ-soft nuclei, and ββ-decay in ⁷⁶Ge. Several other illustrative calculations are also reviewed. Finally, we discuss prospects for further progress in SMMC and related calculations.

This review is devoted to discussion of hadron properties in the nuclear medium and their relationship to the partial restoration of chiral symmetry. Special attention is given to the difference between in-medium effects caused by conventional many-body interactions and those caused by change of the chiral condensate. In particular, we discuss medium effects on the Goldstone bosons (pion, kaon, and η), the vector mesons (ρ, ω, and ϕ), and the nucleon. Also, for each proposed in-medium effect the experimental consequences and results are reviewed.

The proton drip-line defines the limit at which nuclei become unbound to the emission of a proton from their ground states. Low-Z nuclei lying beyond this limit only exist as short-lived resonances and cannot be detected directly. The location of the drip-line constrains the path of nucleosynthesis in explosive astrophysical scenarios such as novae and X-ray bursters, and consequently controls the rate of energy generation. In higher-Z regions of the drip-line, the potential energy barrier resulting from the mutual electrostatic interaction between the unbound proton and the core can cause nuclei to survive long enough to be detected. This review describes the recent major advances in the study of these exotic nuclei. Particular emphasis is placed on understanding the phenomenon of proton radioactivity and the unique insights it offers into the structure of nuclei lying beyond one of nature's fundamental limits to stability.

Recent achievements in heavy-quark theory are critically reviewed. The emphasis is put on those aspects that either did not attract enough attention or cause heated debates in the current literature. Among other topics we discuss (a) basic parameters of the heavy-quark theory; (b) a class of exact QCD inequalities; (c) new heavy quark sum rules; (d) the virial theorem; (e) applications (|Vcb| from the total semileptonic width and from the B → D* transition at zero recoil). In some instances new derivations of previously known results are given, or new aspects addressed. In particular, we dwell on the exact QCD inequalities. Furthermore, a toy model is considered that may shed light on the controversy regarding the value of the kinetic energy of heavy quarks obtained by different methods.

We provide an overview of collective flow phenomena observed in heavy ion collisions from the Fermi energy range up to CERN Super Proton Synchrotron (SPS) energies. We summarize the experimental data in terms of the various observed aspects of flow, namely directed flow in the reaction plane, elliptic flow in- and out-of-plane, and azimuthally symmetric radial flow originating from the expansion of the hot and compressed reaction zone. Also reviewed are the theoretical concepts developed to simulate the complex reactions with the aim of extracting fundamental properties of hot and compressed nuclear matter.