Constraining Sea Quark Distributions Through $W^{\pm}$ Cross Section Ratios Measured at STAR

TL;DR

This paper reports on precise measurements of the $W^+/W^-$ cross section ratio at STAR, providing new constraints on unpolarized sea quark distributions near Bjorken-$x$ of 0.1, using high-statistics data from recent RHIC runs.

Contribution

It presents an updated, high-precision measurement of the $W$ cross section ratio at STAR, improving constraints on sea quark distributions at mid-rapidity.

Findings

Higher precision $W^+/W^-$ ratio measurement achieved.

First measurement of $ ext{eta}$ dependence at mid-rapidity.

Enhanced understanding of sea quark distributions near Bjorken-$x$ of 0.1.

Abstract

Over the past several years the STAR experiment at RHIC has been contributing to our understanding of the proton structure. Through its instrumentation, STAR is well equipped to measure $W \rightarrow \nu + e$ in $\sqrt{s}$ = 500/510 GeV proton-proton collisions at mid-rapidity (-1.1 $\le \eta \le $ 1.1) . The $W$ cross section ratio ($W^+/W^-$) is 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 RHIC runs in 2011, 2012 and 2013 at $\sqrt{s}$ = 500/510 GeV saw a significant increase in delivered luminosity from previous years. This resulted in a total data sample being collected of about 352 pb$^{-1}$ of integrated luminosity. The increased statistics will lead to a higher precision measurement of the $W^+/W^-$ cross section ratio than was previously measured by STAR's 2009 run, as well as allow for a measurement of its $\eta$ dependence at mid-rapidity. Presented here is an update of the $W$ cross section ratio analysis from the STAR 2011, 2012 and 2013 runs.