Elliot Leader, Cambridge University Press, ISBN 0521352819, hbk £90/$130.
Elliot Leader’s book – in the series of Cambridge Monographs on Particle Physics, Nuclear Physics and Cosmology – is a thorough introduction to the theory and experimental study of high-energy spin physics.
Both theoretically and experimentally, spin physics has always been a challenge. In recent years there has been considerable growth in research activities related to spin phenomena and their theoretical interpretation. There is an extensive list of review papers but few books devoted to the subject.
However, Leader’s book provides a comprehensive introduction, with a pedagogical approach to high-energy spin physics. The novelty of the book is also in the rather detailed description of experimental techniques and apparatus, as well as the standard theoretical part. A large number of appendices with technical details and formalism are valuable for the pedagogical treatment of spin problems and make for quick reference.
A significant part of the monograph deals with the problem of nucleon spin structure – the topic widely discussed since 1988 when the results of the European Muon Collaboration showed that the spin of the proton is not the sum of the spins of its individual quarks.
The small-x behaviour of the polarized structure function is one of the unsolved problems en route to a final resolution of the overall nucleon spin puzzle and is discussed from different points of view. The gluon anomaly and a general perturbative QCD approach to the nucleon spin problem are discussed in detail, including the evolution, scheme-dependence and phenomenology of the polarized parton distributions.
Alongside these topics, an introduction to the parton model, the Standard Model, QCD and the general formalism of polarized deep-inelastic scattering is presented. The helicity structure of QCD interactions is considered thoroughly and fermion spin structure is analysed for the case of massive and massless spinors. Possibilities of testing Standard Model and perturbative QCD predictions in the two-spin and parity-violating single-spin asymmetries measured at large angles are listed. Such experiments are useful tools for the detection of gluon polarization, which is a possible solution of the nucleon spin problem.
Another outstanding problem of spin physics is the observation of significant single-spin transverse asymmetry. In the framework of perturbative QCD, the polarization of an individual quark in a hard subprocess should vanish because of the vector type of the QCD interaction, which leads to chirality conservation. However, experimentally there is a mass of data showing large asymmetries or large polarizations, in both elastic and semi-inclusive reactions.
The different mechanisms for producing non-zero, single-spin transverse asymmetries, including final-state interactions, are considered. All such explanations are in fact beyond the standard QCD parton model. The most prominent single-spin asymmetry was observed in inclusive hyperon production, where over two decades ago it was discovered that highly polarized , L hyperons are produced in the collision of unpolarized protons. Most dramatic is that despite the large hyperon polarization, it has no tendency to decrease with the transverse momentum of the produced hyperon, as could be expected from perturbative QCD mechanisms. This whole area of high-energy physics is a challenge for the theory of strong interactions. Only phenomenological models have had any success in the quantitative description of hyperon polarization data.
One chapter is devoted to spin effects in elastic scattering at high energy, which is a most fundamental type of reaction and where a lot of experimental data exist at low and medium energies. There are also high-energy data that demonstrate a rising dependence of analysing power in proton-proton scattering with transferred momentum. A conclusion made in the book clearly indicates that QCD demands the opposite behaviour of the analysing power in elastic proton-proton scattering, but there are no specific predictions for analysing power based on QCD and it does not provide an estimate for where the decreasing behaviour begins. The experimental study of this process is an indispensable source of knowledge on the nucleon wavefunction and a clear and unambiguous way to check perturbative QCD as well as the models based on the non-perturbative approaches to hadron dynamics.
Significant attention is devoted to technical problems, in particular the mechanisms of polarized hadron and electron production and acceleration. In particular, polarized proton sources, polarized targets, difficulties in the acceleration of polarized particles (including “Siberian snakes”) and polarization at LEP, HERA and SLC are described. This increases the potential reader audience. Besides these problems, the book treats in detail the polarimetry issues that are essential for modern experimental high-energy spin physics, especially for the experimental programme with polarized protons at RHIC.
The first five chapters of the book consider the basic formalism and definitions of spin, helicity, spin-density matrix, transition amplitudes and observables of a reaction. The properties of helicity states and wavefunctions under Lorentz and discrete transformations are described in a clear, pedagogical manner. For example, an intelligible derivation of the famous Thomas precession is presented.
In conclusion, it should be stressed that in the light of ongoing major experimental studies (for example, COMPASS at CERN, RHIC-SPIN at Brookhaven and SPIN@U70 at IHEP), this book is useful and timely, describing the state of the field and providing reference points for the interpretation of forthcoming experimental data in high-energy spin physics – a subject that underwent rapid growth during the last decades of the past century. The book is suitable for students at graduate level and will be of interest to the broad high-energy physics community.
