Gluon Polarization Measurement at COMPASS

The determination of the gluon helicity contribution, $\Delta G/G$, to the nucleon spin is one of the main goals in the COMPASS spin physics program. The polarisation of gluons carrying a fraction $x_{G}$ of the nucleon momentum is obtained from the cross-section asymmetry of the photon-gluon fusion reaction in the scattering of polarised 160 GeV muons on a polarised deuteron target. The analysed data have been collected by the COMPASS experiment at CERN in the years 2002-2004. Two approaches have been used for direct $\Delta G/G$ measurements: the "open charm" method and the "high $p_{T}$ hadron pairs" one. The results of QCD fits on all $g_{1}$ data, which provide two solutions with the gluon spin distribution function $\Delta G$ positive or negative, are also presented. Our measurements show that the helicity contribution of gluons at $x_{G} \approx$ 0.1 is small and is in agreement with the results of the $g_{1}$ global QCD analysis fits.