Leading-Order Determination of the Gluon Polarization from high-$p_T$ Hadron Electroproduction

TL;DR

The study uses LO hadron electroproduction at HERMES to constrain the gluon polarization in the nucleon by measuring longitudinal double-spin asymmetries at high transverse momentum and interpreting them with a PYTHIA-based Monte Carlo that separates signal (gluon-initiated) from background processes. By combining three event categories and performing a detailed treatment of subprocess fractions, asymmetries, and kinematics, the authors extract $Δg/g$ as a function of $x$ and provide an averaged value around $⟨x⟩\approx 0.22$. The results, which favor a small gluon polarization in the probed $x$-range, are consistent with contemporary NLO fits and RHIC findings, and they highlight sizable model-dependent systematic uncertainties that accompany LO extractions. Overall, the work offers a LO determination of $Δg/g$ in lepton-nucleon scattering and provides a framework for combining hadron-level observables with Monte Carlo modeling to access gluon spin contributions.

Abstract

Longitudinal double-spin asymmetries of charged hadrons with high transverse momentum $p_T$ have been measured in electroproduction using the \hermes\ detector at \hera. Processes involving gluons in the nucleon have been enhanced relative to others by selecting hadrons with $p_T$ typically above 1 GeV. In this kinematic domain the gluon polarization has been extracted in leading order making use of the model embedded in the Monte Carlo Generator \Pythia\ 6.2. The gluon polarization obtained from single inclusive hadrons in the $p_T$ range 1 GeV $< p_T <$ 2.5 GeV using a deuterium target is $\frac{Δg}{g}(\langle x\rangle, \langle μ^2\rangle)=0.049\pm 0.034 (stat)\pm 0.010 (sys\textrm{-}exp)^{+0.126}_{-0.099}(sys\textrm{-}models)$ at a scale $\laμ^2\ra=1.35~{\rm GeV}^2$ and $\langle x\rangle = 0.22$. For different final states and kinematic domains, consistent values of \DGG\ have been found within statistical uncertainties using hydrogen and deuterium targets.