Spin in Particle Physics Elliot Leader Cambridge U. Press, New York, 2001. $130.00 (500 pp.). ISBN 0-521-35281-9
Elliot Leader is a particle theorist who has made significant contributions to what we now call the standard model of elementary particle physics. He is the author, with Enrico Predazzi, of a two-volume book on gauge theory and the standard model (Introduction to Gauge Theory and Modern Particle Physics , Cambridge U. Press, 1996). In Spin in Particle Physics , Leader applies his extensive knowledge and broad judgment to this subject as well. At a time when much of the effort in particle physics goes to issues beyond the standard model—and even to trans-Planckian physics, with an attendant degree of speculation—Leader’s book offers a fine discussion of the concrete topic of spin.
The book’s preface establishes the case that spin is an essential complication in the physics of elementary particles—as a counter to the oft-stated dismissal of spin as “an inessential complication.” As Leader states, he has three aims in the book, to give a simple pedagogical exposition of spin in relativistic physics, to describe the experimental side of spin physics, and to show the relevance of spin-dependence both in testing quantum chromodynamics (QCD) and in elucidating the structure of the electroweak interactions.
For this review I divide the chapters of the book into three groups corresponding to the three aims. Leader spends the first group (five chapters) discussing the theoretical basis of spin in quantum theory. Those chapters include discussions of spin in both nonrelativistic and relativistic quantum theory and in the relativistic case for both massive and massless particles. The book’s preface defines the notation clearly. Leader uses the Bjorken–Drell gamma matrices, introducing the Weyl representation in only one of twelve appendices that provide definitions and examples for some of the topics discussed in the text. He derives the action of the Lorentz group on both massive and massless particles (using both Wick and Wigner rotations) and the action of the discrete symmetries—parity, time reversal, and charge conjugation. He explains the Thomas precession in two different ways, first using a simplified argument and later deriving it as a case of the general Bargmann-Michel-Telegdi equation. He also discusses the spin density matrix, transition amplitudes, and observables of reactions, giving specific examples for the various cases involving particles of spin 0 and 1/2. The five chapters serve as a fine reference for the quantum theory of spin.
Leader devotes the second group (three chapters) to experimental issues in the production of polarized hadrons, electrons, and positrons and to the analysis of polarized states. He surveys various types of polarized sources and targets, including the “Siberian snake” method of avoiding depolarization of beams by resonances during the acceleration process. He discusses the conditions under which the Sokolov–Ternov effect leads to a large “natural” polarization of electrons in a perfect storage ring. These chapters provide much more detail than one would expect in a book on spin and will be useful as a starting point for physicists entering the field.
The final group (six chapters) starts with a succinct introduction to QCD and the electroweak interactions. Next Leader describes what has been learned, what can still be learned, and what topics are still completely mysterious in particle physics within the standard model. After a review of deep inelastic lepton–hadron scattering, he emphasizes issues related to the spin-dependent scaling functions—from the standpoint of the parton model and its field theory justification [using the Feynman diagram approach]. He discusses the “spin crisis” in the context of attempts to use data to find the carriers of spin in the nucleon, QCD corrections, inclusive and semi-inclusive reactions, and elastic scattering, among other topics, always emphasizing issues connected to spin dependence. The production of polarized hyperons and antihyperons is one such issue still shrouded in mystery.
I am enthusiastic about this book. It is carefully and clearly written. Each of the three sections will be useful to a specific audience. The book as a whole should be in every university and research institution library; however, it is not suitable as a text for a graduate course in particle physics, nor will many individuals find it a necessary part of their personal libraries.
