Flavor Decomposition of the Polarized Quark Distributions in the Nucleon from Inclusive and Semi-inclusive Deep-inelastic Scattering

Abstract

Spin asymmetries of semi-inclusive cross sections for the production of positively and negatively charged hadrons have been measured in deep-inelastic scattering of polarized positrons on polarized hydrogen and 3He targets, in the kinematic range 0.023<x<0.6 and 1 GeV^2<Q^2<10 GeV^2. Polarized quark distributions are extracted as a function of x for up $(u+u_bar) and down (d+d_bar) flavors. The up quark polarization is positive and the down quark polarization is negative in the measured range. The polarization of the sea is compatible with zero. The first moments of the polarized quark distributions are presented. The isospin non-singlet combination Delta_q_3 is consistent with the prediction based on the Bjorken sum rule. The moments of the polarized quark distributions are compared to predictions based on SU(3)_f flavor symmetry and to a prediction from lattice QCD.

Figures (6)

• Figure 1: The inclusive (a) and semi-inclusive asymmetries for positively (b) and negatively (c) charged hadrons on the proton (top) and $^3$He (bottom) target. The inclusive asymmetries are compared to SLAC results for $g_1/F_1$ (open triangles). The hadron asymmetries on the proton are compared to SMC results (open squares) truncated to the HERMES $x$-range. The data points are given for the measured mean $Q^2$ at each value of $x$, which is different for the different experiments. The error bars of the HERMES (SLAC and SMC) data are statistical (total) uncertainties and the bands are systematic uncertainties of the HERMES data.
• Figure 2: The flavor decomposition $(\Delta u(x)+\Delta \bar{u}(x))/$$(u(x)+\bar{u}(x))$, $(\Delta d(x)+\Delta \bar{d}(x))/(d(x)+\bar{d}(x))$ and ${\Delta q_{s}(x)}/{q_{s}(x)}$ of the quark polarization as a function of $x$, derived from the HERMES inclusive and semi-inclusive asymmetries. The sea polarization is assumed to be flavor symmetric in this analysis. The error bars shown are the statistical and the bands represent the systematic uncertainties.
• Figure 3: The quark spin distributions at $Q^2=2.5$ GeV$^2$ separately for $x(\Delta u(x)+\Delta \bar{u}(x))$ and $x(\Delta d(x)+\Delta \bar{d}(x))$ as a function of $x$. They are compared to different sets of parametrizations which correspond to the following publications: De Florian et al. ($0.1<\Delta G<0.8$, LO) Flori, Gehrmann and Stirling ('Gluon A', LO) GER96, and Glück et al. ('Standard Scenario', LO) GLU96. The De Florian and Glück parametrizations are corrected by a factor $(1+R)$ to allow for a direct comparison. The error bars shown are the statistical and the bands represent the systematic uncertainties.
• Figure 4: The spin distributions at $Q^2=2.5$ GeV$^2$ separately for the valence quarks ${x\Delta u_v (x)}$, ${x\Delta d_v(x)}$ and the sea quarks ${x\Delta \bar{u}(x)}$ as a function of $x$. The error bars shown are the statistical and the bands the systematic uncertainties. The distributions are compared to results from SMC, ex-tra-po-la-ted to $Q^2=2.5$ GeV$^2$. The error bars of the SMC result correspond to its total uncertainty. The solid lines indicate the positivity limit and the dashed lines are the parametrization from Gehrmann and Stirling ('Gluon A', LO) GER96.
• Figure 5: The non-singlet contribution $x\Delta q^{NS}(x)$ at $Q^2=2.5$ GeV$^2$. The result is compared to the same sets of parametrizations as in Figure \ref{['fig:spindistr']}. The error bars are the statistical and the band the systematic uncertainties.