Measurement of $W^{\pm}$ single spin asymmetries and $W$ cross section ratio in polarized $p+p$ collisions at $\sqrt{s}=510$ GeV at STAR

Abstract

We present the preliminary results of measurements of single spin asymmetries, $A_L$ for $W^\pm$ boson production in longitudinally polarized p+p collisions at $\sqrt{s}=510$ GeV and measurements of cross section ratios, $σ_{W^+} / σ_{W^-}$, for $W^+,W^-$ boson production in $p+p$ collisions at $\sqrt{s}$ = 500 and 510 GeV. The asymmetry measurements are based on 246.2 pb$^{-1}$ of data taken in the RHIC 2013 run and the cross section ratio measurements are based on 102 pb$^{-1}$ of data taken during RHIC 2011 and 2012 runs by the STAR experiment. While the asymmetry results are shown as a function of the decay lepton pseudorapidity, $η_e$, the cross section results are shown as a function of both $η_e$ and W boson rapidity, $y_W$ in the mid-rapidity region ($|η_e|<1$). At this kinematics, $W^\pm$ single spin asymmetries provide a theoretically clean probe of the proton's polarized quark and antiquark distributions and the W cross section ratio provides sensitivity to unpolarized sea quark distributions at the scale of the W mass. The asymmetry results are consistent with recently published STAR $A^{W^\pm}_L$ results based on the data collected during RHIC 2011 and 2012 runs which showed a preference for a sizable, positive up antiquark polarization in the range $0.05 < x < 0.2$. The new preliminary results can be considered as the most precise results of $A^{W^\pm}_L$ in the world to date, with uncertainties reduced by 40\% in comparison to the published results.