Quantum Chromodynamics

After a brief historical review of the emergence of QCD as the quantum field theory of strong interactions, the basic notions of colour and gauge invariance are introduced leading to the QCD Lagrangian. The second lecture is devoted to perturbative QCD, from tree-level processes to higher-order corrections in renormalized perturbation theory, including jet production in e + e − annihilation , hadronic τ decays and deep inelastic scattering. The final two lectures treat various aspects of QCD beyond perturbation theory. The main theme is effective field theories, from heavy quarks to the light quark sector where the spontaneously broken chiral symmetry of QCD plays a crucial role. 1 Introduction Why do we still study QCD after more than 30 years? • By decision of the Nobel Prize Committee in 2004 [1], QCD is the correct theory of the strong interactions. • The parameters of QCD, the coupling strength α s and the quark masses, need to be measured as precisely as possible. • Electroweak processes of hadrons necessarily involve the strong interactions. • In searches for new physics at present and future accelerators, the " QCD background " must be understood quantitatively. • Although QCD is under control for high-energy processes, many open questions remain in the nonperturbative domain (confinement, chiral symmetry breaking, hadronization,. . .) • Last but not least, QCD is a fascinating part of modern physics. The lectures will therefore start with a brief historical review of the developments in particle physics in the sixties and early sev-enties of the last century. The following lectures were given to an audience of young experimental particle physicists. Although the lectures emphasize some of the theoretical aspects of QCD, the mathematical level was kept reasonably low. The first two lectures cover the basics of QCD, from the concepts of colour and gauge invariance to some applications of perturbative QCD. The last two lectures treat aspects of QCD beyond perturbation theory. The main theme is effective field theories, from heavy quarks to the light quark sector where the spontaneously broken chiral symmetry plays a crucial role. Particle physics in the early sixties of the last century was not in a very satisfactory state. Only for the electromagnetic interactions of leptons a full-fledged quantum field theory (QFT) was available. Quantum electrodynamics (QED) produced increasingly precise predictions that were confirmed experimentally. Nevertheless, the methodology of renormalization, an essential aspect of the perturbative treatment …