Constraining the Sea Quark Distributions Through W$^\pm$ Cross Section Ratio Measurements at STAR

TL;DR

This paper presents measurements of W$^ ext{±}$ cross section ratios at STAR to better constrain sea quark distributions, especially $ar{d}/ar{u}$, in the proton at high Bjorken-x, addressing uncertainties in global parton distribution functions.

Contribution

It provides new experimental data on W cross section ratios at STAR, extending sea quark sensitivity to higher x and improving constraints on PDFs in the valence region.

Findings

Preliminary W$^+$/$^-$ ratio results from 2011-2012 data.

Updated analysis of 2013 data on W cross section ratios.

Enhanced understanding of sea quark distributions at high x.

Abstract

Over the past several years, parton distribution functions (PDFs) have become more precise. However there are still kinematic regions where more data are needed to help constrain global PDF extractions, such as the sea quark distributions $\bar{d}$/$\bar{u}$ near the valence region (Bjorken-x $\approx$ 0.1 - 0.3).~Current measurements appear to suggest different high-x behaviors of these distributions, leading to large uncertainties in global fits. The charged W cross section ratio (W$^+$/W$^-$) is sensitive to the unpolarized $u,\;d,\;\bar{u},$ and $\bar{d}$ quark distributions at large $Q^2$ set by the $W$ mass and could help shed light on this discrepancy. The STAR experiment at RHIC is well equipped to measure the leptonic decays of W bosons, in the mid-rapidity range $\left(|\eta| \leq 1 \right)$, produced in proton+proton collisions at $\sqrt{s}$ = 500/510 GeV. At these kinematics STAR is sensitive to quark distributions near Bjorken-x of 0.16. STAR can also measure the W cross section ratio in a more forward bin ranging from 1.1 $< \eta <$ 2.0, which extends the sea quark sensitivity to higher x. RHIC runs from 2011 through 2013 have collected about 350 pb$^{-1}$ of integrated luminosity, and a 2017 run is expected to provide an additional 400 pb$^{-1}$. Presented here are preliminary results for the 2011-2012 charged W cross section ratios ($\sim$100pb$^{-1}$) and an update on the 2013 charged W cross section analysis ($\sim$250 pb$^{-1}$).