Leading and next-to-leading order gluon polarization in the nucleon and longitudinal double spin asymmetries from open charm muoproduction

The gluon polarization in the nucleon was measured using open charm production by scattering 160 GeV/c polarized muons off longitudinally polarized protons or deuterons. The data were taken by the COMPASS Collaboration between 2002 and 2007. A detailed account is given of the analysis method that includes the application of neural networks. Several decay channels of D⁰ mesons are investigated. Longitudinal spin asymmetries of the D meson production cross sections are extracted in bins of D⁰ transverse momentum and energy. At leading order QCD accuracy, the average gluon polarization is determined as ⟨Δg/g⟩LO=-0.06±0.21(stat.)±0.08(syst.) at the scale ⟨μ²⟩≈13 (GeV/c)² and an average gluon momentum fraction ⟨x⟩≈0.11. The average gluon polarization is also obtained at next-to-leading order QCD accuracy as ⟨Δg/g⟩ NLO=-0.13±0.15(stat.)±0.15(syst.) at the scale ⟨μ²⟩≈13 (GeV/c)² and ⟨x⟩≈0.20.