Measurement ofπ−pElastic Scattering at 180°

Abstract

We have measured the differential cross section for ${\ensuremath{\pi}}^{\ensuremath{-}}p$ elastic scattering at 180\ifmmode^\circ\else\textdegree\fi{} in steps of 0.10 GeV/c or less in the region ${P}_{0}=1.6 \mathrm{to} 5.3$ GeV/c. We detected elastic scattering events, from protons in a liquid ${\mathrm{H}}_{2}$ target, with a double spectrometer consisting of magnets and scintillation counters in coincidence. The incident ${\ensuremath{\pi}}^{\ensuremath{-}}$ beam was counted by scintillation counters. The cross section was found to have considerable structure. This may be interpreted as interference between the resonant amplitudes and the nonresonant or background amplitude. Very strong destructive interference occurs around ${P}_{0}=2.15$ GeV/c, where the cross section drops almost two orders of magnitude in passing through the ${N}^{*}(2190)$. Another interesting feature of the data is a large narrow peak in the cross section at ${P}_{0}=5.12$ GeV/c, providing firm evidence for the existence of a nucleon resonance with a mass of 3245\ifmmode\pm\else\textpm\fi{}10 MeV. This ${N}^{*}(3245)$ has a full width of less than 35 MeV, which is about 1% of its mass. From this experiment we were able to determine the parity and the quantity $\ensuremath{\chi}(J+\frac{1}{2})$ for each ${N}^{*}$ resonance, where $\ensuremath{\chi}$ is the elasticity and $J$ is the spin of the resonance.