Abstract
We present a determination of the gluon polarisation Δg/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with Q2>1 (GeV/c)2 including a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised LiD6 target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction xg covering the range 0.04<xg<0.27. The values obtained at leading order in QCD do not show any significant dependence on xg. Their average is Δg/g=0.125±0.060 (stat.)±0.063 (syst.) at average xg=0.09 and a scale of μ2=3 (GeV/c)2.
Highlights
The spin structure of the nucleon has been studied in polarised Deep Inelastic lepton–nucleon Scattering (DIS) for many years
A different situation occurs for QCD Compton scattering processes (QCDC) and Photon–Gluon Fusion (PGF) processes, in which hadrons mainly acquire transverse momentum from the partons produced in the hard process
While the LP probability reduces with increasing pT, QCDC and PGF become the dominant contributions rising with similar strength
Summary
The spin structure of the nucleon has been studied in polarised Deep Inelastic lepton–nucleon Scattering (DIS) for many years. The cleanest way to tag the PGF process is via open charm production, i.e. by selecting charmed mesons in the final state [18] For this process the contribution from the LP is small because, in the COMPASS kinematic domain, the charm quark content in the nucleon is negligible. A way to overcome this limitation is to tag the PGF process leading to light quark pair production by detecting final state hadrons with large transverse momentum, pT , with respect to the virtual photon direction. A different situation occurs for QCDC and PGF processes, in which hadrons mainly acquire transverse momentum from the partons produced in the hard process For this reason the requirement of observing two hadrons with large transverse momentum enhances the contribution of the PGF process in the selected sample [23]. We present hereafter an analysis using this approach for the enhancement of PGF events in light quark production [24,25]
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