Extraction of beam-spin asymmetries from the hard exclusive ${\pi }^{+}$ channel off protons in a wide range of kinematics

Abstract

We have measured beam-spin asymmetries to extract the $\sin \varphi$ moment $A_{LU}^{\sin \varphi }$ from the hard exclusive $\vec{e}p\to e^{\prime }n{\pi }^{+}$ reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center-of-mass. The $A_{LU}^{\sin \varphi }$ 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 $A_{LU}^{\sin \varphi }$ to be negative, while a sign change was observed near 90${}^{\circ }$ in the center-of-mass. The unique results presented in this paper will provide critical constraints to establish reaction mechanisms that can help to further develop the GPD and TDA frameworks.