Soft Contributions to Hard Pion Photoproduction

TL;DR

This paper estimates soft contributions to high transverse momentum pion photoproduction, helping to identify the energy region where perturbative QCD processes are dominant, and discusses implications for measuring polarized gluon distributions.

Contribution

It introduces a method to estimate soft contributions using vector meson dominance and data-driven parameterizations, clarifying the transition to perturbative dominance in pion photoproduction.

Findings

Soft processes dominate above 2 GeV transverse momentum.

Polarization asymmetry data aligns with soft process dilution.

Feasibility of polarized gluon distribution measurement at few hundred GeV.

Abstract

Hard, or high transverse momentum, pion photoproduction can be a tool for probing the parton structure of the beam and target. We estimate the soft contributions to this process, with an eye toward delineating the region where perturbatively calculable processes dominate. Our soft process estimate is based on vector meson dominance and data based parameterizations of semiexclusive hadronic cross sections. We find that soft processes dominate in single pion photoproduction somewhat past 2 GeV transverse momentum at a few times 10 GeV incoming energy. The recent polarization asymmetry data is consistent with the perturbative asymmetry being diluted by polarization insensitive soft processes. Determining the polarized gluon distribution using hard pion photoproduction appears feasible with a few hundred GeV incoming energy (in the target rest frame).