Pion Induced Proton Knockout from Polarized Lithium -7 at 240 Mev.

Measurements of vector analyzing power, A _{rm y}, are reported for positive pion induced proton knockout using a polarized ^7Li target and an incident pion energy of 240 MeV: ^7Li( pi^+,pi^{+^{ '}}p)^6He(g.s. + 1.8 MeV); the lowest two states in ^6He were not resolved. The experiment was performed at the Paul Scherrer Institute (PSI). The scattered pion was analyzed using the SUSI magnetic spectrometer, while the proton was observed in an array of 7 plastic Delta E-E scintillation detectors. Measurements were made in coplanar geometry at Theta_ {pi} = 60^circ , 85^circ, and 108 ^circ (lab); with the proton detector array centered on the corresponding quasi-free angle. The target consisted of dynamically polarized LiH in a dilution refrigerator cryostat; polarizations in excess of 35% for Li and 50% for H were achieved. Both elastic and quasi-free scattering asymmetries were observed concurrently. The analyzing powers for vec p(pi^+,pi^+p), obtained by two independent methods, are in very good agreement with previous data. The motivation for this experiment was to study the effects of distortion and medium modification on the elementary piN interaction using a zero-range impulse approximation framework that employs free piN amplitudes and optical model distorted wave potentials for the incident and emitted particles. Whereas the DWIA calculations predict large asymmetries for the proton knockout from a ^7 Li target, the experimental asymmetries are consistent with zero. We speculate that spin dependent Delta N interactions during the propagation of an intermediate Delta resonance, neglected in the present DWIA model, may be responsible for the discrepancy between theory and experiment.