Probing hard/soft factorization via beam-spin asymmetry in exclusive pion electroproduction from the proton

TL;DR

This study investigates the transition to the hard/soft factorization regime in exclusive pion electroproduction by measuring beam-spin asymmetries and cross section ratios, comparing results with theoretical models to assess factorization validity.

Contribution

The paper provides new experimental data on the $Q^2$-dependence of the cross section ratio $\sigma_{LT'}/\sigma_0$ in exclusive pion production, testing the onset of factorization.

Findings

$\sigma_{LT'}/\sigma_0$ shows little $Q^2$ dependence in the measured range.

Regge models better predict the observed asymmetries than GPD-based models.

Results suggest the hard/soft factorization regime is not yet fully reached.

Abstract

Deep exclusive meson production (DEMP) reactions, such as $p(\vec{e},e'\pi^+)n$, provide opportunities to study the three-dimensional structure of the nucleon through differential cross section and beam- and target-spin asymmetry measurements. This work aims to probe the onset of the hard/soft factorization regime through the exclusive $p(\vec{e},e'\pi^+)n$ reaction, as measured in the KaonLT experiment at Jefferson Lab Hall C. A 10.6 GeV longitudinally polarized electron beam was incident on an unpolarized liquid hydrogen target, and the scattered electron and produced meson were detected in two magnetic focusing spectrometers, enabling precision cross section measurements. The cross section ratio $\sigma_{LT'}/\sigma_0$ was extracted from the beam-spin asymmetry $A_{LU}$. The $t$-dependence of $\sigma_{LT'}/\sigma_0$ was determined at fixed $Q^2$ and $x_B$ over a range of kinematics from $2<Q^2<6$ GeV$^2$ above the resonance region ($W>2$ GeV). Furthermore, these data are combined with recent results from CLAS/CLAS12 to determine the $Q^2$-dependence of $\sigma_{LT'}/\sigma_0$ at two ($x_B$, $t$) settings. This was fairly flat, with $Q^2$ not having a measurable effect on the value of $\sigma_{LT'}/\sigma_0$ in the range explored. Results are compared to predictions from the generalized parton distribution (GPD) formalism, which relies explicitly on hard/soft factorization, and Regge formalism. The Regge models better predict $\sigma_{LT'}/\sigma_0$, which suggests that the factorization regime is not yet reached.