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 (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_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 μ ^2= ⟨ Q^2 ⟩ = 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 < x_g < 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.