Introductory remarks on the study of the pion-nucleon interaction

Abstract

Our knowledge of the scattering interactions in the pion-nucleon system has been developing rapidly during the past few years. For the π + p and π - p systems, total cross-section measurements have been extended upwards in energy to 20 GeV/c laboratory momentum. As Galbraith will report at this meeting, measurements of high precision have recently been carried out for π + p and π - p total cross-sections between 2·5 and 7GeV/c by Citron et al. (1964,1965) at Brookhaven. From these and earlier data, about ten excited nucleon states have become established, with mass values up to about 3200 MeV. These excited nucleon states are listed in table 1. In the resonance energy range, detailed angular distribution measurements have now been made up to the region of the fifth excited state ( N * 1/2 (2190)), and many of these new measurements will be reported at this meeting. The recent use of polarized proton targets has allowed a very direct means for the systematic and accurate study of the polarization properties of pion-nucleon scattering, and this technique has recently been exploited at the Nimrod accelerator of the Rutherford High Energy Laboratory at Chilton by the Murphy-Thresher group (Atkinson et al. 1966) for the determination of the spin and parity of the third and fourth resonance states, N * 1/2 (1688) and N * 1/2 (1920).