Pion Physics at Low Energy and High Accuracy

The role of the quark condensate for the low energy structure of QCD is discussed in some detail. In particular, the dependence of M_pi on m_u and m_d and the low energy theorems for the pi-pi scattering amplitude are reviewed. The new data on Ke4 decay beautifully confirm the standard picture, according to which the quark condensate is the leading order parameter of the spontaneously broken chiral symmetry.Comment: Talk given at the Workshop QCD@Work, Martina Franca, June 200

The Masses of the Light Quarks

Talk given at the Conference on Fundamental Interactions of Elementary Particles, ITEP, Moscow, Oct. 1995. The paper reviews the current status of knowledge concerning m_u, m_d and m_s. Qualitative aspects of the resulting picture for the breaking of isospin and eightfold way symmetries are discussed. At a more quantitative level, the review focuses on the chiral perturbation theory results for the masses of the Goldstone bosons. The corresponding bounds and estimates for the ratios m_u/m_d and m_s/m_d are described in some detail.Comment: 23 pages, 3 figure

Light Quark Effective Theory

Lecture Notes, Summer School on Effective Theories and Fundamental Interactions, Erice, 1996. The application of effective field theory methods to the low energy structure of QCD is discussed. I emphasize the universal structure of the corresponding effective Lagrangian and then discuss the physics of the effective coupling constants. The implications of the effective theory for the masses of the light quarks are analyzed in some detail.Comment: 30 pages, 3 postscript figure

On the dispersion theory of ππ\pi\pi scattering

Recent developments in low energy pion physics are reviewed, emphasizing the strength of dispersion theory in this context. As an illustration of the method, I discuss some consequences of the forward dispersion relation obeyed by the isoscalar component of the scattering amplitude.Comment: Contribution to the proceedings of the workshop "Quark Confinement and the Hadron Spectrum VII", Ponta Delgada, Azores Islands, Portugal, September 200