Leading-order determination of the gluon polarisation from semi-inclusive deep inelastic scattering data

Abstract

Using a novel analysis technique, the gluon polarisation in the nucleon is re-evaluated using the longitudinal double-spin asymmetry measured in the cross section of semi-inclusive single-hadron muoproduction with photon virtuality $Q^2>1(\mathrm{GeV}/c{)}^2$. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/$c$ polarised muon beam impinging on a polarised ${}^6$LiD target. By analysing the full range in hadron transverse momentum $p_{\mathrm{T}}$, the different $p_{\mathrm{T}}$-dependences of the underlying processes are separated using a neural-network approach. In the absence of pQCD calculations at next-to-leading order in the selected kinematic domain, the gluon polarisation $\hat{I}”g/g$ is evaluated at leading order in pQCD at a hard scale of $\hat{I}{¼}^2=⟨Q^2⟩=3(\mathrm{GeV}/c{)}^2$. It is determined in three intervals of the nucleon momentum fraction carried by gluons, $x_{\mathrm{g}}$, covering the range $0.04<x_{\mathrm{g}}<0.28$~ and does not exhibit a significant dependence on $x_{\mathrm{g}}$. The average over the three intervals, $⟨\hat{I}”g/g⟩=0.113\pm 0.038_{(stat.)}\pm 0.036_{(syst.)}$ at $⟨x_{\mathrm{g}}⟩\approx 0.10$, suggests that the gluon polarisation is positive in the measured $x_{\mathrm{g}}$ range.

14 pages, 6 figures