Beam Spin Asymmetry Measurements of Deeply Virtual $\pi^0$ Production with CLAS12

TL;DR

This paper reports new measurements of beam spin asymmetry in deeply virtual $_0$ production, revealing significant transverse photon contributions and sensitivity to chiral-odd GPDs, advancing understanding of nucleon transverse spin structure.

Contribution

It provides the first extensive data on beam spin asymmetry at high $Q^2$ for $_0$ production, highlighting the role of transverse virtual photons and GPD sensitivity.

Findings

Asymmetry values indicate significant transverse photon contributions.

Data show sensitivity to chiral-odd GPD _T.

Comparison with Regge model extends understanding at high $Q^2$.

Abstract

The new experimental measurements of beam spin asymmetry were performed for the deeply virtual exclusive $\pi^0$ production in a wide kinematic region with the photon virtualities $Q^2$ up to 8 GeV$^2$ and the Bjorken scaling variable $x_B$ in the valence regime. The data were collected by the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab with longitudinally polarized 10.6 GeV electrons scattered on an unpolarized liquid-hydrogen target. Sizable asymmetry values indicate a substantial contribution from transverse virtual photon amplitudes to the polarized structure functions.The interpretation of these measurements in terms of the Generalized Parton Distributions (GPDs) demonstrates their sensitivity to the chiral-odd GPD $\bar E_T$, which contains information on quark transverse spin densities in unpolarized and polarized nucleons and provides access to the proton's transverse anomalous magnetic moment. Additionally, the data were compared to a theoretical model based on a Regge formalism that was extended to the high photon virtualities.