Differential cross section and photon beam asymmetry for the gamma p -> pi+ n reaction at forward pi+ angles at Egamma=1.5-2.95 GeV

TL;DR

This study measures differential cross sections and photon beam asymmetries for gamma p -> pi+ n at forward angles and energies 1.5-2.95 GeV, revealing resonance effects and t-channel contributions in the reaction mechanism.

Contribution

First detailed measurement of cross sections and asymmetries at forward angles in this energy range, highlighting resonance and exchange contributions in the reaction.

Findings

Cross sections decrease with energy for 0.6<cos(theta)<0.9.

Different energy dependence for 0.9<cos(theta)<1 suggests resonance effects.

Positive asymmetries consistent with rho-exchange in the t-channel.

Abstract

Differential cross sections and photon beam asymmetries for the gamma p -> pi+ n reaction have been measured for 0.6<cos(theta)<1 and Egamma=1.5-2.95 GeV at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for 0.6<cos(theta)<0.9. However, the energy dependence of the cross sections for 0.9<cos(theta)<1 and Egamma=1.5-2.2 GeV (W=1.9-2.2 GeV) is different, which may be due to a nucleon or Delta 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 rho-exchange in the t-channel. Large positive asymmetries in the small |t| region, where the rho-exchange contribution becomes small, could be explained by introducing pi-exchange interference with the s-channel.