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 Q2>1(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 ⁶LiD target. By analysing the full range in hadron transverse momentum p_T, the different p_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 Δ g/ g is evaluated at leading order in pQCD at a hard scale of μ²= ⟨ Q²⟩ = 3 (GeV / c) ². It is determined in three intervals of the nucleon momentum fraction carried by gluons, x_g, covering the range 0.04<xg<0.28 and does not exhibit a significant dependence on x_g. The average over the three intervals, ⟨ Δ g/ g⟩ = 0.113 ± 0. 038 _(stat.)± 0. 036 _(syst.) at ⟨ x_g⟩ ≈ 0.10 , suggests that the gluon polarisation is positive in the measured x_g range.