Differential Cross Section and Photon-Beam Asymmetry for the γ → p → π- Δ++ (1232) Reaction at Forward π- Angles for Eγ=1.5-2.95 GeV

Differential cross sections and photon-beam asymmetries for the γ→p→π-Δ++(1232) reaction have been measured for 0.7<cosθπc.m.<1 and Eγ=1.5-2.95 GeV at SPring-8/LEPS. The first-ever high statistics cross-section data are obtained in this kinematical region, and the asymmetry data for 1.5<Eγ(GeV)<2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure uū quark pair in the final state from the proton. Although there is no distinct peak structure in the cross sections, a non-negligible excess over the theoretical predictions is observed at Eγ=1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of π exchange in the t channel. The negative asymmetries at forward meson production angles are different from the asymmetries previously measured for the photoproduction reactions producing a dd or an ss quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural-parity exchanges are needed to reproduce the present data.