Pion Scattering from HELIUM-4.

Data were taken for both the elastic and inelastic scattering of (pi)('+) and (pi)('-) from ('4)He at the Clinton P. Anderson Meson Physics Faculty (LAMPF). Elastic (pi)('+) - ('4)He angular distributions were taken for 32(DEGREES) (LESSTHEQ) (theta)(,cm) (LESSTHEQ) 114(DEGREES) at five pion energies: T(,(pi)) = 90, 110, 130, 150, and 180 MeV. At T(,(pi)) = 180 Mev (pi)('-) - ('4)He elastic data were also taken. Inelastic data were taken for both (pi)('+) and (pi)('-) at T(,(pi)) = 180 MeV at (theta)(,lab) = 20(DEGREES), 30(DEGREES), and 40(DEGREES). The inelastic data provide important new information with regard to isospin mixing in the J('(pi)) = 1('-) levels. We find an isospin mixing matrix element completely consistent with Coulomb mixing ((TURN)150 keV). This stands in sharp contrast to ('4)He photoproton and photoneutron disintegration data, which indicate a mixing much larger ((TURN)450 keV) than that attributable to the Coulomb force. Recoil-corrected continuum shell model (RCCSM) calculations, using Coulomb mixing, reproduce the (pi) - ('4)He inelastic double-differential cross-sections (d(sigma)/d(OMEGA)dE) very well. A phase shift analysis of the elastic scattering data is presented. The phase shifts for the L (GREATERTHEQ) 2 partial waves are fixed to the predictions of an optical model program. Our results for the L = 0 and L = 1 partial waves are compared to earlier phase shift analyses of (pi) - ('4)He elastic scattering.