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

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 $\Delta g/g$ is evaluated at leading order in pQCD at a hard scale of $\mu ^2= \langle Q^2 \rangle = 3 (\mathrm{GeV}/c)^2$ . 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, $\langle \Delta g/g \rangle = 0.113 \pm 0.038_\mathrm{(stat.)}\pm 0.036_\mathrm{(syst.)}$ at $\langle x_\mathrm{g} \rangle \approx 0.10$ , suggests that the gluon polarisation is positive in the measured $x_\mathrm{g}$ range.