Measurement of longitudinal single-spin asymmetries for $W^{\pm}$ boson production in polarized $p+p$ collisions at $\sqrt{s}=510$ GeV at STAR

Abstract

$W^\pm$ boson production in longitudinally polarized $p+p$ collisions provides unique and clean access to the individual helicity polarizations of $u$ / $d$ quarks and anti-quarks. Due to the maximal violation of parity in the coupling, $W$ bosons couple to left-handed quarks and right-handed anti-quarks and hence offer direct probes of their respective helicity distributions in the nucleon. These can be extracted from measured parity-violating longitudinal single-spin asymmetries, $A_L$, for $W^{+(-)}$ boson production as a function of the decay lepton (positron) pseudo-rapidity $\eta$. The STAR experiment is well equipped to measure $A_L$ for $W^\pm$ boson production for $|\eta|<1$. The published STAR $A_L$ results (2011 and 2012 data combined) 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 the STAR experiment has collected a large data sample of $\sim$250 pb$^{-1}$ which is more than 3 times larger than the total integrated luminosity in 2012, at $\sqrt{s}=510$ GeV with an average beam polarization of $\sim$54\%, comparable to run 2012. The status of the 2013 $A_L$ analysis will be discussed along with an overview of future plans.