Reevaluation of the parton distribution of strange quarks in the nucleon

Abstract

An earlier extraction from the HERMES experiment of the polarization-averaged parton distribution of strange quarks in the nucleon has been reevaluated using final data on the multiplicities of charged kaons in semi-inclusive deep-inelastic scattering obtained with a kinematically more comprehensive method of correcting for experimental effects. General features of the distribution are confirmed, but the rise at low x is less pronounced than previously reported.

Figures (4)

• Figure 1: The multiplicity of charged kaons in semi-inclusive DIS from a deuterium target, as a function of Bjorken $x$. The continuous curve is calculated from the strange-quark contribution taken from the fit in Fig. \ref{['fig:sxdz']}, together with the nonstrange contribution as extracted from the high-$x$ multiplicity data (see text). The green dashed (magenta dash-dotted) curve shows separately the nonstrange- (strange-) quark contribution to the multiplicity for that fit. The blue-dotted curve is the LO prediction obtained with CTEQ6L PDFs and fragmentation functions from deFlorian:2007aj. The values of $\langle {\mathrm{Q}}^2 \rangle$ for each $x$ bin are shown in the lower panel. The band represents systematic uncertainties.
• Figure 2: The strange-parton distribution $xS(x,{\mathrm{Q}}^2)$ from the measured HERMES multiplicity for charged kaons evolved to ${\mathrm{Q}}^2=2.5$ GeV$^2$ using the compilation of fragmentation functions (FFs) from deFlorian:2007aj (DSS). The solid curve is a three-parameter fit to the data, the dashed curve gives $xS(x)$ from CTEQ6L, and the dot-dash curve is the sum of light antiquarks from CTEQ6L. The dotted curve is from CTEQ6.5S-0, a PDF reference set Lai:2007dq in which the shape of $xS(x)$ has not been constrained. The band containing the experimental points represents the fully correlated systematic uncertainties arising from the imprecision of ${\int{\cal D}^{K}_{Q}(z,{\mathrm{Q}}^2)dz}$.
• Figure 3: The product, $S(x,{\mathrm{Q}}^2){\int{\cal D}^{K}_{S}(z,{\mathrm{Q}}^2)dz}$, of the strange-quark PDF and the integral of the fragmentation function for strange quarks (squares) obtained from the measured HERMES multiplicity for charged kaons at the $\langle {\mathrm{Q}}^2 \rangle$ for each bin. The solid curve is a least-squares fit with the result $f(x)=x^{-0.834\pm 0.019 }e^{-x/(0.0337\pm 0.0014)}(1-x)$. The band represents propagated experimental systematic uncertainties. The open points and the dashed curve show the data and fit published previously in Ref. Airapetian:2008qf.
• Figure 4: The strange-parton distribution $xS(x,{\mathrm{Q}}^2)$ from the measured HERMES multiplicity for charged kaons evolved to ${\mathrm{Q}}^2=2.5$ GeV$^2$ assuming ${\int{\cal D}^{K}_{S}(z,{\mathrm{Q}}^2)dz}=1.27$. The solid curve is a two-parameter fit with $S(x)=[\int \mathcal{D}_S^K(z,{\mathrm{Q}}^2)dz]^{-1}\times x^{-0.867\pm 0.019}e^{-x/(0.0331\pm 0.0014)}(1-x)$. The dashed, dot-dash, dotted curves are as given in Fig. \ref{['fig:dss']}. The broad band is the $\pm 1\sigma$ zone of allowed values predicted by the neural network (NNPDF2.3) reference set Ball:2011uy. The band at the bottom represents the propagated experimental systematic uncertainties. A scale uncertainty of approximately 10% coming from the precision of ${\int{\cal D}^{K}_{S}(z,{\mathrm{Q}}^2)dz}$ is not shown.