Differential Cross Section and Photon-Beam Asymmetry for the γpπΔ++(1232) Reaction at Forward π Angles for Eγ=1.52.95GeV

H. Kohri et al. (LEPS Collaboration)
Phys. Rev. Lett. 120, 202004 – Published 18 May 2018

Abstract

Differential cross sections and photon-beam asymmetries for the γpπΔ++(1232) reaction have been measured for 0.7<cosθπc.m.<1 and Eγ=1.52.95GeV 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 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 Eγ=1.51.8GeV. 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 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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  • Received 24 January 2018
  • Revised 11 March 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.202004

© 2018 American Physical Society

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Vol. 120, Iss. 20 — 18 May 2018

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