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

Abstract

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 $$Q^2>1~(\mathrm{GeV}/c)^2$$ 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 $$^6$$ 6 LiD target. By analysing the full range in hadron transverse momentum $$p_\mathrm{T}$$ p T , the different $$p_\mathrm{T}$$ 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 $$\Delta g/g$$ Δ g / g is evaluated at leading order in pQCD at a hard scale of $$\mu ^2= \langle Q^2 \rangle = 3 (\mathrm{GeV}/c)^2$$ μ 2 = ⟨ Q 2 ⟩ = 3 ( GeV / c ) 2 . It is determined in three intervals of the nucleon momentum fraction carried by gluons, $$x_\mathrm{g}$$ x g , covering the range $$0.04 \!<\! x_{ \mathrm g}\! <\! 0.28$$ 0.04 < x g < 0.28 and does not exhibit a significant dependence on $$x_\mathrm{g}$$ x g . The average over the three intervals, $$\langle \Delta g/g \rangle = 0.113 \pm 0.038_\mathrm{(stat.)}\pm 0.036_\mathrm{(syst.)}$$ ⟨ Δ g / g ⟩ = 0.113 ± 0 . 038 ( stat . ) ± 0 . 036 ( syst . ) at $$\langle x_\mathrm{g} \rangle \approx 0.10$$ ⟨ x g ⟩ ≈ 0.10 , suggests that the gluon polarisation is positive in the measured $$x_\mathrm{g}$$ x g range.

Keywords

• Transverse Momentum
• Monte Carlo
• Systematic Uncertainty
• Parton Shower
• Monte Carlo Model