Differential cross section and photon-beam asymmetry for the γ - p→π+n reaction at forward π+ angles at Eγ=1.5 -2.95 GeV

Differential cross sections and photon-beam asymmetries for the γ - p→π+n reaction have been measured for 0.6<cosθπ<1 and Eγ=1.5-2.95GeV at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for 0.6<cosθπ<0.9. However, the energy dependence of the cross sections for 0.9<cosθπ<1 and Eγ=1.5-2.2GeV (W=1.9-2.2GeV) is different, which may be due to a nucleon or Δ resonance. The present cross sections agree well with the previous cross sections measured by other groups and show forward peaking, suggesting significant t-channel contributions in this kinematical region. The asymmetries are found to be positive, which can be explained by ρ exchange in the t channel. Large positive asymmetries in the small-|t| region, where the ρ-exchange contribution becomes small, could be explained by introducing π-exchange interference with the s channel.