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
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.
Kohri, H.; Shiu, S. H.; Chang, W. C.; Yanai, Y.; Ahn, D. S.; Ahn, J. K.; Chen, J. Y.; Daté, S.; Ejiri, H.; Fujimura, H.; Fujiwara, M.; Fukui, S.; Gohn, W.; Hicks, K.; Hosaka, A.; Hotta, T.; Hwang, S. H.; Imai, K.; Ishikawa, T.; Joo, K.; Kato, Y.; Kon, Y.; Lee, H. S.; Maeda, Y.; Mibe, T.; Miyabe, M.; Morino, Y.; Muramatsu, N.; Nakano, T.; Nakatsugawa, Y.; Nam, S. I.; and Niiyama, M., "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" (2018). Physics & Astronomy Open Access Publications. 105.