Measurement of the left-right asymmetry in pi -p--> gamma n from 301 to 625 MeV/c at backward angles.

Abstract

The left-right asymmetry of ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\gamma}n$ has been measured using a transversely polarized target at seven pion momenta from 301 to 625 MeV/c, mostly at photon angles of 90\ifmmode^\circ\else\textdegree\fi{} and 110\ifmmode^\circ\else\textdegree\fi{} c.m. The final-state $\ensuremath{\gamma}$ and neutron were detected in coincidence. Neutrons were recorded in two arrays of plastic scintillators and the $\ensuremath{\gamma}'\mathrm{s}$ in two matching sets of lead-glass counters. The results are compared with the predictions from the two most recent single-pion photoproduction partial-wave analyses. The agreement with the analysis of Arai and Fujii is poor, casting some doubt on the correctness of their values for the radiative decay amplitude of the neutral Roper resonance which are used widely. The agreement is much better with the results of the VPI analysis. Also, a comparison is made with the recoil-proton polarization data from the inverse reaction measured at 90\ifmmode^\circ\else\textdegree\fi{} with a deuterium target. It reveals substantial discrepancies, indicating the shortcomings of the deuterium experiments for neutron target experiments. Our data are also compared with several bag-model calculations.