The polarised valence quark distribution from semi-inclusive DIS

Abstract The semi-inclusive difference asymmetry A h + − h − for hadrons of opposite charge has been measured by the COMPASS experiment at CERN. The data were collected in the years 2002–2004 using a 160 GeV polarised muon beam scattered off a large polarised 6 LiD target in the kinematic range 0.006 x 0.7 and 1 Q 2 100 ( GeV / c ) 2 . In leading order QCD (LO) the deuteron asymmetry A h + − h − measures the valence quark polarisation and provides an evaluation of the first moment of Δ u v + Δ d v which is found to be equal to 0.40 ± 0.07 ( stat. ) ± 0.06 ( syst. ) over the measured range of x at Q 2 = 10 ( GeV / c ) 2 . When combined with the first moment of g 1 d previously measured on the same data, this result favours a non-symmetric polarisation of light quarks Δ u ¯ = − Δ d ¯ at a confidence level of two standard deviations, in contrast to the often assumed symmetric scenario Δ u ¯ = Δ d ¯ = Δ s ¯ = Δ s .