Measurement of the longitudinal spin structure of the proton by COMPASS

The inclusive $A_{1,p}$ and hadron double-spin asymmetries $A_{p}^{\pi+}, A_{p}^{\pi-}, A_{p}^{K+}, A_{p}^{K-}$ measured at COMPASS (CERN SPS) in deep-inelastic scattering of a polarized muon beam off a polarized $NH_3$ solid target are presented. The results have been obtained with the full statistics collected in 2007 for the longitudinal target polarization. Proton asymmetries have been combined with the published deuteron ones. An evaluation of the non-singlet spin-dependent structure function $g_{1}^{NS}(x,Q^{2})$ and its first moment, which confirms the validity of the Bjorken sum-rule, is presented. A LO evaluation of polarized quark densities is also presented. The use of the proton data allows to perform a full flavor separation and to extract individual helicity densities of u, d, anti-u, anti-d and s quarks. All sea quark densities are found to be compatible with zero in the full range of the measurements.