Differential cross section and photon-beam asymmetry for the gamma p -> pi- Delta++(1232) reaction at forward pi- angles for Egamma=1.5-2.95 GeV

TL;DR

This study provides the first high-statistics measurements of differential cross sections and photon-beam asymmetries for the gamma p -> pi- Delta++(1232) reaction at forward angles and energies 1.5-2.95 GeV, revealing insights into production mechanisms and exchange processes.

Contribution

It presents the first high-statistics data on cross sections and asymmetries for this reaction in the specified kinematic region, highlighting the dominance of pi-exchange and the need for advanced models.

Findings

Negative asymmetries suggest pi-exchange dominance.

No distinct peak in cross sections but excess over theory at 1.5-1.8 GeV.

Asymmetries differ from reactions producing other quark pairs.

Abstract

Differential cross sections and photon-beam asymmetries for the gamma p -> pi- Delta++(1232) reaction have been measured for 0.7<cos(theta)<1 and Egamma=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<Egamma(GeV)<2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure $u\bar{u}$ 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 Egamma=1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of pi-exchange in the t-channel. The negative asymmetries at forward meson production angles are different from the asymmetries previously measured for the photoproduction reactions which produce a $d\bar{d}$ or an $s\bar{s}$ quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural parity exchanges are needed to reproduce the present data.