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

29 Apr 2015, 14:50


WG6 Spin Physics WG6 Spin Physics




Devika Gunarathne for the STAR Collaboration Temple University, Philadelphia, PA, USA Email: $W^\pm$ 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 maximal violation of parity, $W$ 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 decay lepton (positron) pseudo-rapidity $\eta$. The STAR experiment is well equipped to measure $A_L$ for $W^{-(+)}$ boson production between $|\eta|<1$. 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 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.

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