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Abstract
Almost30yearsagotheEMCexperimentdiscoveredthatthequarkhelicitycontributiontothe
nucleonspinissmall.Afterwardsthisresultwasconfirmedbyseveralotherexperiments.Itcameas
asurprise,asitcontradictstheexpectationfromsimplemodelswhichotherwisewerequitesuccessful
indescribingthenucleonproperties.Apossiblesolutionforthis“spinpuzzle”isthatgluonsare
polarised.Therefore,gluonpolarisationmeasurementsareofgreatimportanceandmotivatedthe
flagshipmeasurementoftheCOMPASSproposal.
Inthismonographdetailsaboutthedirectmeasurementofthegluonpolarisationinthenucleon,
extractedfromthedataoftheCOMPASSexperimentatCERNarepresented.Anovelmethod,the
so-calledall-pTmethod,wasdevelopedbytheauthorwhowasalsoresponsibleforthecorresponding
analysisinCOMPASS.
Theproposedmethodallowssimultaneousextractionofthegluonpolarisationandspin-depen-
dentasymmetryA1,resultinginaconsiderablereductionofstatisticalandsystematicuncertainties
byafactorof1.6and1.8,respectively,comparedwiththepreviouslyusedmethod.Theanalysed
datacoverthekinematicregionofQ2>1GeV2,whichallowstheuseofperturbativeQCD.The
obtainedresultofthegluonpolarisationatLOpQCDis∆g/g=0.113±0.038±0.036foraverage
nucleonmomentumfractioncarriedbythegluonabout0.10andaveragehardscaleof3GeV2.The
obtainedresultssuggestthatthegluonpolarisationinthenucleonispositive.Thisobservationisin
linewithrecentNLOQCDfits,whichincludeRHICppdata.
