Optical Analysis of the 900-MeVπ−−pResonance

Abstract

A model for elastic scattering near the 900-MeV ${\pi }^{-}-p$ resonance is proposed, consisting of a Breit-Wigner term together with an optical background. The optical background is chosen to fit experimental data beyond the 900-MeV resonance; the spin of the Breit-Wigner term is then adjusted to give an elastic cross section in agreement with experiment. A distorted-wave Born-approximation calculation shows a spin-$\frac{5}{2}$ resonance term to be in agreement with the data, in accord with the usual spin assignment. Parity of the Breit-Wigner term cannot be decided from the elastic energy spectra and angular distributions alone; however, if the elastic resonance is identified with the observed $\Lambda -K$ resonance near 900 MeV, one finds a suitable branching ratio only for the $D_{\frac{5}{2}}$ choice. The resulting $\Lambda -K$ angular distributions, as well as the production cross section as a function of energy, agree generally with experiment. It is therefore proposed that the 900-MeV ${\pi }^{-}-p$ resonance has negative parity relative to the proton.