Polarised quark distributions in the nucleon from semi-inclusive spin asymmetries

We present a measurement of semi-inclusive spin asymmetries for positively and negatively charged hadrons from deep inelastic scattering of polarised muons on polarised protons and deuterons in the range 0.0031 GeV2. Compared to our previous publication on this subject, with the new data the statistical errors have been reduced by nearly a factor of two. From these asymmetries and our inclusive spin asymmetries we determine the polarised quark distributions of valence quarks and non-strange sea quarks at Q2=10 GeV2. The polarised u valence quark distribution, Δuv(x), is positive and the polarisation increases with x. The polarised d valence quark distribution, Δdv(x), is negative and the non-strange sea distribution, Δq̄(x), is consistent with zero over the measured range of x. We find for the first moments ∫01Δuv(x)dx=0.77±0.10±0.08, ∫01Δdv(x)dx=−0.52±0.14±0.09 and ∫01Δq̄(x)dx=0.01±0.04±0.03, where we assumed Δū(x)=Δd̄(x). We also determine for the first time the second moments of the valence distributions ∫01xΔqv(x)dx.