Polarised Quark Distributions in the Nucleon from Semi-Inclusive Spin Asymmetries

Abstract

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 $0.003<x<0.7$ and $Q^2>$1 GeV$^2$. 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 $Q^2$=10 GeV$^2$. The polarised $u$ valence quark distribution, $Δu_v(x)$, is positive and the polarisation increases with $x$. The polarised $d$ valence quark distribution, $Δd_v(x)$, is negative and the non-strange sea distribution, $Δ\bar q(x)$, is consistent with zero over the measured range of $x$. We find for the first moments $\int_0^1 Δu_v(x) dx = 0.77 \pm 0.10 \pm 0.08$, $\int_0^1 Δd_v(x) dx = -0.52 \pm 0.14 \pm 0.09$ and $\int_0^1 Δ\bar q(x) dx= 0.01 \pm 0.04 \pm 0.03$, where we assumed $Δ\bar u(x) = Δ\bar d(x)$. We also determine for the first time the second moments of the valence distributions $\int_0^1 x Δq_v(x) dx$.