Spin physics at COMPASS
Physik Department, Technische Universität München, Germany
Accepted: 30 November 2006
Published online: 15 March 2007
Results for the spin structure of the nucleon from the COMPASS data taking periods 2002 to 2004 are presented. The quark contribution to the nucleon spin, following from a QCD fit to the new data, turns out to be significantly larger than it was derived from the previous world data. The new data favour, on the other side, a comparatively small gluon polarisation in the range x g ≈ 0.1. In the data taken with the deuteron target polarised transversely the related asymmetries are found to be small on the level of accuracy reached so far, indicating a cancellation of the proton and neutron contributions. This is in agreement, for both the Collins and the Sivers asymmetry, with recent theoretical calculations. Also, a step towards the understanding of angular-momentum contributions with COMPASS is taken by the evaluation of asymmetries in exclusive vector meson production.
PACS: 25.30.Mr Muon scattering (including the EMC effect) – / 21.10.Hw Spin, parity, and isobaric spin – / 24.85.+p Quarks, gluons, and QCD in nuclei and nuclear processes – / 13.60.-r Photon and charged-lepton interactions with hadrons –
© Società Italiana di Fisica and Springer-Verlag, 2007