Chiral Effective Action of QCD: Precision tests, Questions and Electroweak extensions

A major implication of confinement in QCD is to generate an ordered structure for the ground state1. As a consequence, at low energies, the non-perturbative QCD dynamics can be described by an effective theory (called ChPT)2,3,4. In this approach, an expansion in either two or three of the light quark masses is performed and it is important to verify its accuracy. This is now becoming possible as many observables have been computed at NNLO and impressive progress has also been achieved on the experimental side. A summary of the most interesting results concerning the SU(2) expansion are presented by Irinel Caprini at this conference. The SU(3) chiral expansion is of considerable practical interest as it embodies the whole field of kaon physics. The difference beween the SU(2) and the SU(3) expansions is not simply that the expansion parameter is larger for SU(3). It reflects also a difference between the respective chiral vacuums, which is an OZI suppressed dynamical effect. This topic is discussed in sec. 2 below. The computation of radiative corrections is important for precision tests and for properly dealing with isospin breaking and this has motivated an extension of ChPT. Some aspects and applications of this extension are presented in sec.3.