Valence quark helisity distribution from COMPASS

The evaluation of the polarized valence quark distribution $\Delta u_{v}(x) +\Delta d_{v}(x)$ from the COMPASS experiment (CERN/SPS) is presented. The analysis is based on the "difference asymmetry", $A^{h^{+}-h^{-}} (x)$, for hadrons of opposite charges. The data were collected in the years 2002 - 2004 using a 160 GeV polarized muon beam scattered off a large polarized $^{6}$LiD target in the kinematic range 0.006 $< x <$ 0.7 and 1 $< Q^{2} <$ 100 (GeV/c)$^{2}$. In the leading order (LO) QCD the first moment of $\Delta u_{v}(x) +\Delta d_{v}(x)$ was found to be equal to 0.04 $\pm$ 0.07(stat) $\pm$ 0.06(syst). By combining it with the first moment of the structure function $g^{d}_{1}(x)$ an estimate on the light sea quark contribution is obtained. The present results disfavor the assumption of a flavor symmetric polarized sea within two standard deviations.