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

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Differential cross sections and photon-beam asymmetries for the →γp→π−Δ++(1232) reaction have been measured for 0.7 < cosθc.m.π < 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 u¯u 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 d¯d or an s¯s quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural-parity exchanges are needed to reproduce the present data.