Pions: Experimental Tests of Chiral Symmetry Breaking

Aron Bernstein (MIT)

We are entering a new era in which we can test confinement scale QCD in some well chosen reactions such as pion-pion scattering, pion properties, pion-nucleon scattering, and electromagnetic pion production from the nucleon.  Based on the spontaneous breaking of chiral symmetry, chiral perturbation theory (ChPT) is believed to approximate confinement scale QCD as demonstrated by increasingly accurate lattice calculations, particularly in pion-pion scattering.  At present there is good agreement between experiment and ChPT calculations, including the ππ and πN s-wave scattering lengths and the π⁰ 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 will be discussed.  Increasingly accurate experiments in electromagnetic pion production experiments from the proton, which test ChPT calculations (and their energy region of validity), will be presented.  These experiments are also beginning to measure the final state πN interaction.

(Seminarraum II)