Flavour separation of helicity distributions from deep inelasticmuon–deuteron scattering

We present a LO evaluation of helicity densities of valence, Delta u_v + Delta d_v , non-strange sea, Delta bar(u) + Delta bar(d), and strange quarks, Delta s (assumed to be equal to Delta bar(s)). They have been obtained from the inclusive asymmetry A1,d and the semi-inclusive asymmetries Aπ^+_(1,d) , Aπ^−_(1,d) , AK^+_(1,d) , AK^−_(1,d) measured in polarised deep inelastic muon–deuteron scattering. The full deuteron statistics of COMPASS (years 2002–2004 and 2006) has been used. The data cover the range Q^2 > 1 (GeV/c)^2 and 0.004 < x < 0.3. Both non-strange densities are found to be in a good agreement with previous measurements. The distribution of s(x) is compatible with zero in the whole measured range, in contrast to the shape of the strange quark helicity distribution obtained in most LO and NLO QCD fits. The sensitivity of the values of Delta s(x) upon the choice of fragmentation functions used in the derivation is discussed