Outlook

Abstract

Based on the spontaneous breaking of chiral symmetry, chiral perturbation theory (ChPT) is believed to approximate confinement scale QCD. Dedicated and increasingly accurate experiments and improving lattice calculations are confirming this belief, and we are entering a new era in which we can test confinement scale QCD in some well chosen reactions. This is demonstrated with an overview of low energy experimental tests of ChPT predictions of ππ scattering, pion properties, πN scattering and electromagnetic pion production. These predictions have been shown to be consistent with QCD in the meson sector by increasingly accurate lattice calculations. At present there is good agreement between experiment and ChPT calculations, including the ππ and πN s wave scattering lengths and the π 0 lifetime. Recent, accurate pionic atom data are in agreement with chiral calculations once isospin breaking effects due to the mass difference of the up and down quarks are taken into account, as was required to extract the ππ scattering lengths. In addition to tests of the theory, comparisons between ππ and πN interactions based on general chiral principles are discussed. Lattice calculations are now providing results for the fundamental, long and inconclusively studied, πN σ term and the contribution of the strange quark to the mass of the proton. Increasingly accurate experiments in electromagnetic pion production experiments from the proton which test ChPT calculations (and their energy region of validity) are presented. These experiments are also beginning to measure the final state πN interaction.

On decays of light unflavoured pseudoscalar mesons

“…The spread in the data basis of the PDG, summarized in the previous paragraph, shows, however, that the quoted errors seem to be underestimated [17]. The present situation fortunately looks more optimistic as the world average accuracy of 8 % is planned to be improved to the level of one or two percents in ongoing experiment PrimEx at JLab.…”