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

Apr 13, 2016, 12:20 PM
SR4a (DESY Hamburg)


DESY Hamburg

Spin Physics WG6 Spin Physics


Prof. Bernd Surrow (Temple University)


The STAR experiment at RHIC has provided significant contributions to our understanding of the structure of the proton. The STAR experiment is well equipped to measure $W^\pm \rightarrow e^\pm + \nu$ in $\sqrt{s}=510$ GeV longitudinally polarized $p+p$ collisions at mid-rapidity ($|\eta|<1$). W single-spin asymmetries, $A_L$, measured as a function of decay positron (electron) pseudo-rapidity $\eta$ for $W^+ (W^-)$ are sensitive to the individual helicity polarizations of $u$ and $\bar{d}$ ($d$ and $\bar{u}$) quarks. Due to maximal violation of parity, during the production, $W$ bosons couple to left-handed quarks and right-handed anti-quarks and hence offer direct probes of their respective helicity distributions in the nucleon. The published STAR $A_L$ results (combination of 2011 and 2012 data) have been used by several theoretical analyses suggesting a significant impact in constraining the helicity distributions of anti-$u$ and anti-$d$ quarks. In 2013 STAR collected a large sample of data at $\sqrt{s}=510$ GeV with a total integrated luminosity of $\sim$300 pb$^{-1}$ with an average beam polarization of $\sim$54\%. This resulted in an increase of a factor 3 in the figure of merit compared to the dataset used for previous analyses. The status of the analysis of the STAR 2013 W $A_L$ will be presented along with the future plans for final W $A_L$ results by combining both STAR 2012 and 2013 data of total integrated luminosity of about $\sim$400 pb$^{-1}$. \\ W cross section ratio ($W^+ / W^-$) measurements at STAR are sensitive to unpolarized u, d, $\bar{u}$, and $\bar{d}$ quark distributions. At these kinematics, STAR is able to measure the quark distributions near Bjorken-x values of 0.1. The increased statistics will lead to a higher precision measurement of the $W^+ / W^-$ cross section ratio as well as allow for a measurement of its $\eta$ dependence at mid-rapidity. An update of the W cross section ratio analysis from the STAR 2011, 2012 and 2013 runs is presented.

Primary author

Ms Devika Gunarathne (Temple University)

Presentation materials