CHIRAL SYMMETRY
G. Ecker (Wien)

In the first part of these lectures, the chiral symmetry of QCD and its spontaneous and explicit breaking are discussed. The current status of our knowledge of the light quark masses is reviewed. The spontaneous breaking of chiral symmetry is the starting point for constructing the effective field theory of the Standard Model at low energies. The systematic treatment of this effective field theory is called chiral perturbation theory. The basic ingredients are effective chiral Lagrangians and the loop expansion for a nonrenormalizable quantum field theory.

The second part of the lectures deals with recent applications of chiral perturbation theory. In the purely mesonic sector, several processes have now been analysed to two-loop accuracy corresponding to in the low-energy expansion. I discuss the general two-loop functional, elastic pion-pion scattering and nonleptonic kaon decays. Baryons are included in the framework of heavy baryon chiral perturbation theory where amplitudes are systematically expanded in inverse powers of the nucleon mass. After a general introduction to this approach, recent work in elastic pion-nucleon scattering and attempts to include the resonance as an explicit degree of freedom in chiral Lagrangians are reviewed.

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