The polarised valence quark distribution from semi-inclusive DIS -: COMPASS Collaboration

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 (LiD)-Li-6 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 Delta u(v) + Delta 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 Delta(u) over bar = -Delta(d) over bar at a confidence level of two standard deviations, in contrast to the often assumed symmetric scenario Delta(u) over bar = Delta(d) over bar = Delta(s) over bar = Delta s. (c) 2008 Elsevier B.V. All rights reserved.