Two days after the Cosmic Background Explorer (COBE) satellite was launched, my wife heard me answer a 4:00am phone call with the words “So we've lost the mission?”. COBE had lost a gyro and we didn't know how well we would recover. Needless to say I got up, only an hour after getting home, to see what could be done.

Gamma-ray bursts (GRBs) were discovered with four US military spacecraft: Vela 5A, 5B, 6A, and 6B, launched in the late 1960s to monitor Soviet compliance with the nuclear test ban treaty. While first bursts were recorded in July of 1969, it took several years to develop proper software to uncover them from a huge volume of data, and the discovery paper by Ray W. Klebesadel, Ian B. Strong and Roy A. Olson of the Los Alamos Scientific Laboratory was published in The Astrophysical Journal on June 1, 1973. This became instant headline news for the astronomical community.

Accelerators

All accelerators employ electric fields to accelerate stable charged particles (electrons, protons, or heavier ions) to high energies. The simplest machine would be a d.c. high-voltage source (called a Van der Graaff accelerator), which can, however, only achieve beam energies of about 20 MeV. To do better, one has to employ a high frequency a.c. voltage and carefully time a bunch of particles to obtain a succession of accelerating kicks. This is done in the linear accelerator, with a succession of accelerating elements (called drift tubes) in line, or by arranging for the particles to traverse a single (radio-frequency) voltage source repeatedly, as in the cyclic accelerator.

Linear accelerators (linacs)

Figure 11.1 shows a sketch of a proton linac. It consists of an evacuated pipe containing a set of metal drift tubes, with alternate tubes attached to either side of a radio-frequency voltage. The proton (hydrogen ion) source is continuous, but only those protons inside a certain time bunch will be accelerated. Such protons traverse the gap between successive tubes when the field is from left to right, and are inside a tube (therefore in a field-free region) when the voltage changes sign. If the increase in length of each tube along the accelerator is correctly chosen then as the proton velocity increases under acceleration the protons in a bunch receive a continuous acceleration. Typical fields are a few MeV per metre of length. Such proton linacs, reaching energies of 50 MeV or so, are used as injectors for the later stages of cyclic accelerators.

Introduction

As indicated in Chapters 1 and 2, we are faced in nature with several types of fundamental interaction or field between particles. Each field has its distinct characteristics, such as space–time transformation properties (vector, tensor, scalar etc.), a particular set of conservation rules that are obeyed by the interaction and a characteristic coupling constant that determines the magnitude of the collision cross-sections or decay rates mediated by the interaction.

The fact that the strength of the gravitational interaction between two protons, for example, is only 10−38 of their electrical interaction has always been a puzzle and a challenge, and many attempts have been made to understand the interrelation between the different fields. In the last decades, the belief has grown that the strong, weak, electromagnetic and gravitational interactions are but different aspects of a single universal interaction, which would be manifested at some colossally high energy. At the everyday energies met with in laboratory studies in particle physics, it is necessary to assume that this symmetry is badly broken, at these mass or energy scales which are puny relative to the unification energy.

The first successful attempt to unify two apparently different interactions was achieved by Clerk Maxwell in 1865. He showed that electricity and magnetism could be unified into a single theory involving a vector field (the electromagnetic field) interacting between electric charges and currents.

The main object in writing this book has been to present the subject of elementary particle physics at a level suitable for advanced physics undergraduates or to serve as an introductory text for graduate students.

Since the first edition of this book was produced over 25 years ago, and the third edition over 10 years ago, there have been many revolutionary developments in the subject, and this has necessitated a complete rewriting of the text in order to reflect these changes in direction and emphasis. In comparison with the third edition, the main changes have been in the removal of much of the material on hadron–hadron interactions as well as most of the mathematical appendices, and the inclusion of much more detail on the experimental verification of the Standard Model of particle physics, with emphasis on the basic quark and lepton interactions. Although much of the material is presented from the viewpoint of the Standard Model, one extra chapter has been devoted to physics outside of the Standard Model and another to the role of particle physics in cosmology and astrophysics.

Many – indeed most – texts on this subject place particular emphasis on the power and beauty of the theoretical description of high energy processes. However, progress in this field has in fact depended crucially on the close interplay of theory and experiment. Theoretical predictions have challenged the ingenuity of experimentalists to confirm or refute them, and equally there have been long periods when unexpected experimental discoveries have challenged our theoretical description of high energy phenomena.

Preamble

The subject of elementary particle physics may be said to have begun with the discovery of the electron 100 years ago. In the following 50 years, one new particle after another was discovered, mostly as a result of experiments with cosmic rays, the only source of very high energy particles then available. However, the subject really blossomed after 1950, following the discovery of new elementary particles in cosmic rays; this stimulated the development of high energy accelerators, providing intense and controlled beams of known energy that were finally to reveal the quark substructure of matter and put the subject on a sound quantitative basis.

Why high energies?

Particle physics deals with the study of the elementary constituents of matter. The word ‘elementary’ is used in the sense that such particles have no known structure, i.e. they are pointlike. How pointlike is pointlike? This depends on the spatial resolution of the ‘probe’ used to investigate possible structure. The resolution is Δr if two points in an object can just be resolved as separate when they are a distance Δr apart. Assuming the probing beam itself consists of pointlike particles, the resolution is limited by the de Broglie wavelength of these particles, which is λ = h/p where p is the beam momentum and h is Planck's constant. Thus beams of high momentum have short wavelengths and can have high resolution.

A very important concept in physics is the symmetry or invariance of the equations describing a physical system under an operation – which might be, for example, a translation or rotation in space. Intimately connected with such invariance properties are conservation laws – in the above cases, conservation of linear and angular momentum. Such conservation laws and the invariance principles and symmetries underlying them are the very backbone of particle physics. However, one must remember that their credibility rests entirely on experimental verification. A conservation law can be assumed to be absolute if there is no observational evidence to the contrary, but this assumption has to be accompanied by a limit set on possible violations by experiment.

The transformations to be considered can be either continuous or discrete. A translation or rotation in space is an example of a continuous transformation, while spatial reflection through the origin of coordinates (the parity operation) is a discrete transformation. The associated conservation laws are additive and multiplicative, respectively.

Translation and rotation operators

In an isolated physical system, free of any external forces, the total energy must be invariant under translations of the whole system in space. Since there are no external forces, the rate of change of momentum is zero and the momentum is constant. So invariance of the energy of a system under space translations corresponds to conservation of linear momentum. Similarly, invariance of the energy of a system under spatial rotations corresponds to conservation of angular momentum.