Recoil Proton Polarization from 225-Mevπ−−pScattering

Abstract

${\pi }^{-}$ mesons of energy 225 Mev were scattered from liquid hydrogen. The polarization of the recoil proton has been measured at two angles. For analyzing the polarization, a counter controlled cloud chamber was used. Recoil protons, which experienced scatterings in a carbon plate of the cloud chamber, were photographed stereoscopically. The pictures were later projected, and the necessary measurements of the scattering were made directly in three dimensions. The computed polarizations are -0.13±0.16 at a laboratory recoil angle of 15°, and +0.36±0.29 at 31°. The positive sign is for polarization in the direction of the vector cross product of the incident pion momentum and the recoil proton momentum. A comparison of the data is made with various sets of scattering phase shifts which represent the differential cross-section data equally well. The results favor the Orear type of Fermi set in which the $S$-wave ${\alpha }_1$ phase shift is positive.