Pion Absorption on Polarized Lithium-7 at 165 MeV

Pion absorption is not well understood. Cross section data alone does not provide a complete description since the cross section tends to be sensitive only to the spin-independent terms in the transition amplitude. Polarization observables are thus necessary to complete the picture. In this experiment, analyzing powers for the reaction 7→Li(π+,pp)5He at Tπ = 165 MeV were measured at the Paul Scherrer Institut, Switzerland. The emitted protons were detected with the SUSI spectrometer at 38° in coincidence with an array of five scintillator telescopes centered around the quasi-free angle for absorption on a nucleon pair. Theoretical predictions were made using a Distorted Wave Impulse Approximation (DWIA) and assuming that the absorption occurs on deuteron-like nucleon pairs. The measured analyzing powers were found to be consistent with zero in complete disagreement with the DWIA predictions. The reason for the discrepancy may be attributed to the reaction mechanism and the nuclear structure input to the DWIA, indicating a need for a more microscopic calculation.