Measurement of the Spin Asymmetry in the Photoproduction of Pairs of High-pT Hadrons at HERMES

TL;DR

This paper reports a measurement of the spin asymmetry in high-pT hadron pair photoproduction at HERMES, revealing a negative asymmetry that suggests a positive gluon polarization, contrasting with previous deep inelastic scattering results.

Contribution

First measurement of the longitudinal spin asymmetry in high-pT hadron pair photoproduction, providing new insights into gluon polarization in protons.

Findings

Measured asymmetry A_|| = -0.28 +/- 0.12 (stat.) +/- 0.02 (syst.)

Negative asymmetry indicates positive gluon polarization

Contrasts with positive asymmetries in deep inelastic scattering

Abstract

We present a measurement of the longitudinal spin asymmetry A_|| in photoproduction of pairs of hadrons with high transverse momentum p_T. Data were accumulated by the HERMES experiment using a 27.5 GeV polarized positron beam and a polarized hydrogen target internal to the HERA storage ring. For h+h- pairs with p_T^h_1 > 1.5 GeV/c and p_T^h_2 > 1.0 GeV/c, the measured asymmetry is A_|| = -0.28 +/- 0.12 (stat.) +/- 0.02 (syst.). This negative value is in contrast to the positive asymmetries typically measured in deep inelastic scattering from protons, and is interpreted to arise from a positive gluon polarization.