Search Results

This chapter reviews the determination of the strong coupling. Apart from the quark masses, the strong coupling is the only free parameter of QCD. Frequently employed methods to infer the strength of the strong coupling are presented, as well as a global average of the individual results. Experimental inputs cover inclusive measurements, decay rates, lattice calculations, scaling violations, and event shape measurements. The results cover reactions with time-like and space-like momentum transfers and lepton-lepton, lepton-hadron, and hadron-hadron collisions. All results are consistent with a single value, which evolves with energy according to the QCD expectation.

The advantages and disadvantages of various methods of determining the strong coupling α_s from DIS data are considered including the use of moments of the measured structure functions and the conventional method in which α_s is determined in an NLO QCD fit along with the parton momentum densities. The estimation of experimental and theoretical uncertainties is discussed in detail. Various possible extensions to the theoretical framework are summarised. The remaining sections consider jet production at HERA. The importance of using the Breit frame is covered and a brief outline given of the QCD calculation of the dijet cross-section. Jet measures, including the cone and k_T algorithms, which provide the link between the final state hadrons and the underlying partonic structure, are outlined. It is shown that NLO QCD gives a good description of dijet data. Then measurement of α_s and the running of α_s from jet production rates at large Q² are discussed.

The aim of this chapter is to provide a pedagogical introduction to the application of Chiral Perturbation Theory to Lattice QCD. Topics covered include a general introduction to Chiral Perturbation Theory, the inclusion of scaling violations in chiral perturbation theory, partial quenching and mixed actions, chiral perturbation theory with heavy kaons, and the effects of finite volume, both in the p- and epsilon-regimes.

This chapter presents the determination of structure functions and parton distributions from deep inelastic lepton-nucleon scattering, both for the case of charged lepton-nucleon and the case of neutrino-nucleon scattering. The focus is on the results obtained by the HERA experiments. The direct measurements as well as the indirect extraction of the gluon density from scaling violations are discussed. Information obtained from sum rules and from hadron-hadron scattering is briefly summarized, followed by a discussion of global QCD fits of the parton densities based on all available information.

This chapter discusses how Parton Distribution Functions (parton momentum densities) are extracted from data. It starts with an outline of the general framework based on NLO QCD. Next, the concept of the global fit is introduced and the choice of reactions and data sets discussed. The following section gives a detailed account of some global fits. Particular attention is paid to the extraction of the gluon distribution since most cross-sections at hadron colliders are dominated by gluon induced processes. Sources of information on PDFs from non-DIS processes are considered. Next, uncertainties arising from the theoretical framework and from the model assumptions are considered. Experimental sources of uncertainty are examined, together with the problems of defining a reasonable measure of the error on a parton density and how to estimate it. The final two sections cover the ‘dynamically generated’ partons of the GRV group and prospects for improving our knowledge of the gluon distribution.

This chapter reviews and puts into context the experiments which lead to the formulation of QCD. Starting from the static quark model, the phenomenology of deep inelastic scattering processes is introduced and used to derive and explain the quark parton model (QPM). Discussing in detail the conceptual problems of the QPM, the arguments that lead to the theory of strong interactions based on an SU(3) gauge symmetry are explained. The chapter concludes with the lagrangian of QCD.