Differential Cross Section and Photon-Beam Asymmetry for the gamma(->)p -> Pi(-)Delta(+ +) (1232) Reaction at Forward Pi(-) Angles for E-gamma=1.5-2.95 GeV

초록

Differential cross sections and photon-beam asymmetries for the gamma(->)p -> Pi(-)Delta(++)(1232) reaction have been measured for 0.7 < cos Theta(c.m.)(Pi) < 1 and E-gamma= 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 < E-gamma(GeV) < 2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure uu 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 E-gamma= 1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of n 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.

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