Analyzing power in pion-proton bremsstrahlung, and the Delta ++(1232) magnetic moment.

Abstract

We report on a first measurement of the polarized-target asymmetry of the pion-proton bremsstrahlung cross section (${\ensuremath{\pi}}^{+}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}p\ensuremath{\gamma}$). As in previous cross section measurements the pion energy (298 MeV) and the detector geometry for this experiment was chosen to optimize the sensitivity to the radiation from the magnetic dipole moment of the ${\ensuremath{\Delta}}^{++}(1232)$ resonance ${\ensuremath{\mu}}_{\ensuremath{\Delta}}$. Comparison to a recent isobar model for pion-nucleon bremsstrahlung yields ${\ensuremath{\mu}}_{\ensuremath{\Delta}}=(1.62\ifmmode\pm\else\textpm\fi{}0.18) {\ensuremath{\mu}}_{p}$, where ${\ensuremath{\mu}}_{p}$ is the proton magnetic moment. Since the asymmetry depends less than the cross section on the choice of the other input parameters for the model, their uncertainties affect this analysis by less than the experimental error. However the theory fails to represent both the cross section and the asymmetry data at the highest photon energies. Hence further improvements in the calculations are needed before the model dependence of the magnetic moment analysis can be fully assessed. The present result agrees with bag-model corrections to the SU(6) prediction ${\ensuremath{\mu}}_{\ensuremath{\Delta}}=2{\ensuremath{\mu}}_{p}$. As a by-product, the analyzing power for elastic ${\ensuremath{\pi}}^{+}p$ scattering at 415 MeV/c was also measured. This second result is in good agreement with phase shift calculations.