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.

1897 Discovery of electron

1900 α, β and γ radioactivity

1905 Photon identified as quantum of electromagnetic field

1911 Discovery of atomic nucleus

1912 Discovery of cosmic rays

Invention of cloud chamber

1913 Bohr model of atom

1919 Discovery of proton

1923 de Broglie wave–particle duality

1925 Introduction of electron spin

1926 Wave mechanics

1927 Uncertainty Principle

1928 Dirac wave equation

1930 Neutrino hypothesis

1931 Operation of first cyclotron and of Van der Graaff accelerator

1932 Discovery of positron

Discovery of neutron

1933 Discovery of electromagnetic showers

1934 Theory of beta decay

Discovery of Čerenkov effect

1935 Yukawa theory of nuclear forces

1936 Breit–Wigner resonance formula

1937 First evidence for mesotron (= muon)

1939 Observation of mesotron (= muon) decay

1940 Spin-statistics theorem

1945 Phase stability in accelerators (synchrotron principle)

Classification

The weak interactions between quarks and leptons are those mediated by the socalled intermediate bosons W± and Z0, with a coupling which we generically label g. As indicated in Figure 7.1, there is a strong similarity between the Feynman diagrams for electromagnetic interactions mediated by photon exchange and weak interactions mediated by the intermediate bosons.

A familiar interaction is that between two straight wires hanging close together when electric currents – i.e. a flow of electrons and ions – pass through them. So, we can view the interaction of Figure 7.1(a) as that of two conserved electric currents jem, carried in this case by the electron e and the quark Q. Similarly, the interactions shown in Figures 7.1(b) and (c) can be viewed as between weak currents jweak. The difference is that these currents consist of a flow of conserved weak charge g rather than electric charge, e. In either case, these currents will contain the product of the (normalised) wavefunctions of the ‘in’ and ‘out’ particles that occur in the matrix element (2.16) for the interaction. Thus j ∝ ψ* ψ, where ψ* denotes an incoming and ψ an outgoing amplitude.

In Figure 7.1(b) one sees that the electric charges associated with the weak current actually change in the interaction, while in Figure 7.1(c) they do not. Rather inaccurately, these two are referred to as charged-current weak interactions (those in which the electric charge of each weak current changes) and neutral-current weak interactions (those in which the electric charges do not change).

Classical and quantum pictures of interactions

Classically, interaction at a distance, e.g. in electromagnetism, is described in terms of a potential or field due to one charged particle acting on another, the field permeating the whole of space around the source charge. A conceptually more appealing idea, perhaps, is that of an exchange interaction, where the object exchanged carries momentum from one charge to the other, the rate of exchange of momentum providing the force. A frequently quoted everyday example is of two ice-skaters travelling side by side, initially moving parallel, who start to diverge as they repel one another by throwing a ball back and forth.

In quantum theory, action at a distance is indeed viewed in terms of an exchange interaction, the exchange being of a specific quantum (a boson) associated with the particular type of interaction. Since the quantum carries momentum and energy, say ΔE, the conservation laws can only be satisfied if the process takes place within a timescale Δt limited by the Uncertainty Principle, i.e. ΔE Δt ≃ ħ. Such transient quanta are said to be virtual.

The quantum concept of an electromagnetic interaction is of the continual exchange (emission and absorption) of virtual photons by the charges, which is no more or less fictitious than the classical concept of an all-pervading field surrounding a source charge. Neither the field nor the virtual quanta are directly observable; it is the force that is the measured quantity.