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, $\Delta u_v(x)$, is positive and the polarisation increases with $x$. The polarised $d$ valence quark distribution, $\Delta d_v(x)$, is negative and the non-strange sea distribution, $\Delta \bar{q}(x)$, is consistent with zero over the measured range of $x$. We find for the first moments ${\int }_0^1\Delta u_v(x)dx=0.77\pm 0.10\pm 0.08$, ${\int }_0^1\Delta d_v(x)dx=-0.52\pm 0.14\pm 0.09$ and ${\int }_0^1\Delta \bar{q}(x)dx=0.01\pm 0.04\pm 0.03$, where we assumed $\Delta \bar{u}(x)=\Delta \bar{d}(x)$. We also determine for the first time the second moments of the valence distributions ${\int }_0^{1}x\Delta q_v(x)dx$.

17 pages

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