# XXIV International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS16)

Apr 11 – 15, 2016
DESY Hamburg
Europe/Berlin timezone

## \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
15m
SR4a (DESY Hamburg)

Spin Physics

### Speaker

Prof. Bernd Surrow (Temple University)

### Description

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)

 Slides