The Standard Model is the basis of our understanding of the fundamental interactions. At the present time, it remains in excellent agreement with experiment. It is clear that any further progress in the field will need to build on a solid understanding of the Standard Model. Since the first edition was written in 1992 there have been major discoveries in neutrino physics, in CP violation, the discoveries of the top quark and the Higgs boson, and a dramatic increase in precision in both electroweak physics and in QCD. We feel that the present is a good moment to update our book, as the Standard Model seems largely complete.

The opportunity to revise our book at this time has also enabled us to survey the progress since the first edition went to print. Besides the experimental discoveries that have taken place during these two decades, we have been impressed by the increase in theoretical sophistication. Many of the topics which were novel at the time of the first edition have now been extensively developed. Perturbative treatments have progressed to higher orders and new techniques have been developed. To cover all of these completely would require the expansion of many chapters into book-length treatments. Indeed, in many cases, entire new books dedicated to specialized topics have been published. Our revision is meant as a coherent pedagogic introduction to these topics, providing the reader with the basic background to pursue more detailed studies when appropriate.

A gauge theory involves two kinds of particles, those which carry ‘charge’ and those which ‘mediate’ interactions between currents by coupling directly to charge. In the former class are the fundamental fermions and nonabelian gauge bosons, whereas the latter consists solely of gauge bosons, both abelian and nonabelian. The physical nature of charge depends on the specific theory. Three such kinds of charge, called color, weak isospin, and weak hypercharge, appear in the Standard Model. The values of these charges are not predicted from the gauge symmetry, but must rather be determined experimentally for each particle. The strength of coupling between a gauge boson and a particle is determined by the particle's charge, e.g., the electron–photon coupling constant is −e, whereas the u-quark and photon couple with strength 2e/3. Because nonabelian gauge bosons are both charge carriers and mediators, they undergo self-interactions. These produce substantial nonlinearities and make the solution of nonabelian gauge theories a formidable mathematical problem. Gauge symmetry does not generally determine particle masses. Although gauge-boson mass would seem to be at odds with the principle of gauge symmetry, the Weinberg–Salam model contains a dynamical procedure, the Higgs mechanism, for generating mass for both gauge bosons and fermions alike.

Quantum Electrodynamics

Historically, the first of the gauge field theories was electrodynamics. Its modern version, Quantum Electrodynamics (QED), is the most thoroughly verified physical theory yet constructed. QED represents the best introduction to the Standard Model, which both incorporates and extends it.

Heavy quarks provide a valuable guide to the study of weak interactions. Measurements of decay lifetimes and of semileptonic decay spectra of heavy, flavored mesons yield information on individual elements of the CKM matrix, as does the observation of heavy-meson particle–antiparticle transitions such as Bd–B̄d mixing. Long anticipated data involving detection of CP-violating signals have been found to be in accord with expectations of the Standard Model and have played a crucial role in constraining the sole complex phase in the CKM matrix.

Heavy-quark mass

At the level of the Standard Model lagrangian, the six quark masses are equivalent; they are all just input parameters that must each be determined experimentally. In the real world of particle phenomenology, quark mass divides into two sectors, ‘light’ (u, d, s) and ‘heavy’ (c, b, t). It is a hallmark of light-quark spectroscopy that hadron mass is not a direct reflection of quark mass. However, for hadrons which contain a heavy quark, the energy scale is set by the mass of the heavy quark. In the following, we discuss topics of special relevance to heavy-quark mass.

Running quark mass

Heretofore we have described the renormalization of quark mass in terms of the mass shift δm = m−m0, where m0 is the bare mass. We can also, for convenience, employ a multiplicative mass renormalization constant Zm with m0 = Zmm.

This book is about the Standard Model of elementary particle physics. If we set the beginning of the modern era of particle physics in 1947, the year the pion was discovered, then the ensuing years of research have revealed the existence of a consistent, self-contained layer of reality. The energy range which defines this layer of reality extends up to about 1 TeV or, in terms of length, down to distances of order 10−17 cm. The Standard Model is a field-theoretic description of strong and electroweak interactions at these energies. It requires the input of as many as 28 independent parameters. These parameters are not explained by the Standard Model; their presence implies the need for an understanding of Nature at an even deeper level. Nonetheless, processes described by the Standard Model possess a remarkable insulation from signals of such New Physics. Although the strong interactions remain a calculational challenge, the Standard Model (generalized from its original form to include neutrino mass) would appear to have sufficient content to describe all existing data. Thus far, it is a theoretical structure which has worked splendidly.

Quarks and leptons

The Standard Model is an SU(3) × SU(2) × U(1) gauge theory which is spontaneously broken by the Higgs potential. Table I–1 displays mass determinations [RPP 12] of the Z0 and W± gauge bosons, the Higgs boson H0, and the existing mass limit on the photon γ.