Recent STAR results and prospects of $W^{-(+)}$ boson production in polarized $\vec{p}+\vec{p}$ collisions at RHIC

TL;DR

This paper reports on the first measurements of W boson production in polarized proton collisions at RHIC, highlighting experimental techniques, initial results, and future prospects with upgraded detectors.

Contribution

It presents the first STAR results on W boson production in polarized proton collisions and discusses detector upgrades for improved measurements.

Findings

Initial cross section measurements for W boson production

First single spin asymmetry results for W bosons

Demonstration of background suppression techniques

Abstract

In 2009, the RHIC spin physics program completed the first data taking period of polarized $\vec{p}+\vec{p}$ collisions at $\sqrt{s} = 500$ GeV, accumulating ~12 pb$^{-1}$ with ~39% beam polarization. This opens a new era in the study of the spin-flavor structure of the proton based on the production of $W^{-(+)}$ bosons. $W^{-(+)}$ bosons are produced in $\bar{u}+d (\bar{d}+u)$ collisions and can be detected through their leptonic decays, $e^-+\bar{\nu_e}(e^++\nu_e)$, where only the respective charged lepton is measured. The discrimination of $\bar{u}+d$ and $\bar{d}+u$ quark combinations requires distinguishing the charge sign of high $p_T$ electrons and positrons, which in turn requires precise tracking information. At mid-rapidity, STAR relies on the Time Projection Chamber. At forward rapidity, new tracking capabilities will be provided by the Forward GEM Tracker, consisting of six triple-GEM detectors which are under construction. The suppression of QCD background over W boson signal events by several orders of magnitude is accomplished by using the highly segmented STAR Electromagnetic Calorimeters to impose isolation criteria suppressing jet events, and vetoing dijet events based on the measured away side energy. The status of the STAR results on the first measurements of the cross section and single spin asymmetry for $W^{-(+)}$ boson production in polarized $\vec{p}+\vec{p}$ collisions will be presented along with a discussion of prospects involving the STAR Forward GEM Tracker.