Low Energy Studies of the Pion Nucleon System

Two topics in pion-nucleon physics will be experimentally investigated at the TRIUMF meson facility. In one, we will make a complete kinematical study of electron-positron pair production following capture of negative pions by the proton. At the low energies of this experiment, the virtual photon produced by pion capture probes the axial currents of the nucleon. By measuring the electron and positron momenta, the complete kinematics of the virtual photon are determined. This allows the cross section to be decomposed into the longitudinal, transverse, and various interference response functions characteristic of pion electroproduction experiments. In the present experiment, however, the axial structure of the nucleon is uniquely determined at small time-like momenta from the measured response functions. Recent claims of a violation of isospin symmetry in the low energy pion-nucleon interaction will be investigated experimentally. Under isospin symmetry, the amplitude for elastic pion-nucleon scattering determines the pion charge-exchange cross section. We will make new low energy measurements of the forward-angle neutral pion yield following negative pion interactions in liquid hydrogen. The pion yield and angular distribution are determined from the measured decay gamma-ray energy spectrum, which will be measured in a new high resolution sodium-iodide spectrometer.