Recent measurement of Delta G/G at COMPASS

TL;DR

This paper reports a preliminary measurement of the gluon polarization in the nucleon using spin asymmetry data from the COMPASS experiment, finding results consistent with existing QCD fits and indicating a small gluon polarization.

Contribution

First measurement of gluon polarization Delta G/G at COMPASS using high transverse momentum hadron pairs in polarized muon scattering.

Findings

Delta G/G(x_g=0.095, mu^2=3 GeV^2)=0.024 +- 0.089(stat) +- 0.057(syst)

Helicity asymmetry A_||/D = 0.002 +- 0.019(stat) +- 0.003(syst)

Results are consistent with QCD fits and suggest a small gluon polarization.

Abstract

We present a preliminary measurement of the gluon polarization Delta G/G in the nucleon, based on the spin asymmetry of quasi-real photoproduction events for which a pair of large transverse momentum hadrons is produced. The data were obtained by the COMPASS experiment at CERN using a 160 GeV polarized muon beam scattered on a large polarized 6LiD target. The preliminary helicity asymmetry for the selected events is A_||/D = 0.002 +- 0.019(stat) +- 0.003(syst). From this value, a leading order analysis based on the model of PYTHIA leads to the gluon polarization in the nucleon Delta G/G(x_g=0.095, mu^2=3 GeV^2)=0.024 +- 0.089(stat) +- 0.057(syst). This value is consistent with parameterizations obtained from QCD fits to the g_1 data, with a first moment Delta G lower than 1, at the same scale.