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

TL;DR

This paper reports the first measurement of beam-spin asymmetries in the exclusive $ o e' n \pi^+$ reaction over a wide kinematic range, providing new insights into GPDs and TDAs and their reaction mechanisms.

Contribution

It introduces the first comprehensive measurement of $A_{LU}^{ ext{sin}\phi}$ across a broad kinematic range, covering both GPD and TDA regimes, with implications for understanding nucleon structure.

Findings

$A_{LU}^{ ext{sin}\phi}$ is sensitive to GPDs in forward kinematics.

$A_{LU}^{ ext{sin}\phi}$ is negative in backward kinematics, indicating TDA effects.

Sign change of $A_{LU}^{ ext{sin}\phi}$ near 90° in the center-of-mass.

Abstract

We have measured beam-spin asymmetries to extract the $\sin\phi$ moment $A_{LU}^{\sin\phi}$ 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 $A_{LU}^{\sin\phi}$ 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 $A_{LU}^{\sin\phi}$ 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.