Extraction of Beam-Spin Asymmetries from the Hard Exclusive ${\ensuremath{\pi}}^{+}$ Channel off Protons in a Wide Range of Kinematics

Extraction of Beam-Spin Asymmetries from the Hard Exclusive $π^{+}$ Channel off Protons in a Wide Range of Kinematics

S. Diehl et al. (The CLAS Collaboration)

Phys. Rev. Lett. 125, 182001 – Published 28 October 2020

Abstract

We have measured beam-spin asymmetries to extract the $\sin ϕ$ moment $A_{L U}^{\sin ϕ}$ from the hard exclusive $\vec{e} p \to e^{'} n π^{+}$ reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center of mass. The $A_{L U}^{\sin ϕ}$ moment has been measured up to $6.6 {GeV}^{2}$ in $- t$ , covering the kinematic regimes of generalized parton distributions (GPD) and baryon-to-meson transition distribution amplitudes (TDA) at the same time. The experimental results in very forward kinematics demonstrate the sensitivity to chiral-odd and chiral-even GPDs. In very backward kinematics where the TDA framework is applicable, we found $A_{L U}^{\sin ϕ}$ to be negative, while a sign change was observed near 90° in the center of mass. The unique results presented in this Letter will provide critical constraints to establish reaction mechanisms that can help to further develop the GPD and TDA frameworks.