First data are presented for the polarized-target asymmetry in the reaction ${\mathrm{\pi }}^{+}$p→${\mathrm{\pi }}^{+}$pγ at an incident pion energy of 298 MeV. The geometry was chosen to maximize the sensitivity to the radiation of the magnetic dipole moment ${\mathrm{\mu }}_{\mathrm{\Delta }}$ of the ${\mathrm{\Delta }}^{++}$(1232 MeV). A fit of the asymmetry in the cross section ${\mathit{d}}^5$σ/d${\mathrm{\Omega }}_{\mathrm{\pi }}$ d${\mathrm{\Omega }}_{\gamma }$ dk as a function of the photon energy k to predictions from a recent isobar-model calculation with ${\mathrm{\mu }}_{\mathrm{\Delta }}$ as the only free parameter yields ${\mathrm{\mu }}_{\mathrm{\Delta }}$=1.64(±0.19expΔ,±0.14 theor)${\mathrm{\mu }}_{\mathit{p}}$. Though this value agrees with bag-model corrections to the SU(6) prediction ${\mathrm{\mu }}_{\mathrm{\Delta }}$=2${\mathrm{\mu }}_{\mathit{p}}$, further clarifications on the model dependence of the result are needed, in particular since the isobar model fails to describe both the cross section and the asymmetry at the highest photon energies.

• Received 28 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.64.2619