Determination of gluon polarisation in the nucleon from events with high-pT hadron pairs in COMPASS experiment

The main goal of the COMPASS experiment at CERN is the determination of the gluon spin contribution to the nucleon spin. It is measured using cross section spin asymmetries in Deep Inelastic Scattering of polarised muons off polarised nucleons. The COMPASS uses polarised muon beam of 160 GeV energy and a polarised 6LiD target. The gluons in the nucleon are accessed through a Photon Gluon Fusion process which is tagged by two signatures: an open charm production and pairs of hadrons with high transverse momenta. In this thesis we present the latter analysis performed for the Q2 > 1 GeV2 region and based on the data collected during 2002-2004 years. A novel method of G=G extraction based on Neural Networks is discussed. The result G=G = 0:08 0:1(stat:) 0:05(syst:) is consistent with zero within the measurement uncertainty. It is compatible with other lepton-nucleon scattering results (COMPASS, SMC, HERMES) and with results from the proton-proton interactions (STAR, PHENIX). The obtained result is currently one of the two most precise direct measurements of the gluon polarisation. iii