Abstract
The COMPASS experiment at the CERN SPS has taken data for deep inelastic scattering of polarised muons off a polarised NH3 target in 2007 and 2011 and off a polarised 6LiD target in 2002–2004 and 2006.We present our new results on the longitudinal double spin asymmetry A1p and the spin-dependent structure function g1p obtained from the 2011 data set. These results are used in a NLO QCD fit to the world data to obtain the polarised parton distributions. Also an update on our results on the Bjorken sum rule, connecting the integral of the non-singlet spin-dependent structure function with the ratio of the weak coupling constants, is given.The gluon polarisation can be accessed directly via the photon gluon fusion process in semi-inclusive deep inelastic scattering. This process is studied using the pT dependence of charged hadron asymmetries. The latest results indicate a positive gluon polarisation in the kinematic region of COMPASS.
Highlights
One of the physics goals of the COMPASS experiment at CERN is the determination of the different contributions to the nucleon spin
The longitudinal photon-nucleon double-spin asymmetry is extracted from the measurement of DIS events from the oppositely polarised target cells
In order to obtain the asymmetry A1(xBj, Q2) the raw asymmetry has to be corrected for the target and beam polarisation together with the dilution and depolarisation factor
Summary
One of the physics goals of the COMPASS experiment at CERN is the determination of the different contributions to the nucleon spin. In case of the 2011 data the beam energy has been increased from 160 GeV to 200 GeV to measure lower values of the Bjorken scaling variable xBj and higher values of the photon virtuality Q2. From these data the spin-dependent structure function g1 can be determined. This structure function is of special interest as it can be used in a global QCD analysis to extract the helicity distributions of quarks and gluons. In case of the gluon polarisation a different approach can be used to extract this quantity directly in leading order
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