COMPASS results on inclusive and semi-inclusive polarised DIS

The COMPASS experiment at the CERN SPS has an extensive experimental program focused on the nucleon structure and on hadron spectroscopy. A main topic of investigation is the spin structure of the nucleon via deep-inelastic scattering of 160 GeV polarised muons on polarised nucleons. Results obtained in the kinematic ranges Q$^{2} <$ 1 (GeV/c)$^{2}$ and 0.0005 $< x <$ 0.02, as well as 1 $< Q^{2} <$ 100 GeV$^{2}$ and 0.004 $< x <$ 0.7 are shown. The results of a global QCD fit at NLO to the world $g_{1}$ data are discussed. Then, the evaluation of the polarised valence quark distributions $u_{v}(x) + \Delta d_{v}(x)$ is presented. The analysis is based on the difference asymmetry, A$^{(h^{+}−h^{−})}$, for hadrons of opposite charges. This approach gives direct access to the valence quark helicity distributions, as the fragmentation functions do cancel out in LO QCD. The results derived provide information about the contribution of the sea quarks to the nucleon spin. Comparison with SMC and HERMES results is also shown.