Measurement of the Longitudinal Single-Spin Asymmetry for $W$ Boson Production at STAR

Abstract

The production of $W^\pm$ bosons in longitudinally polarized $p+p$ collisions at RHIC provides a direct probe for the spin-flavor structure of the proton through the parity-violating single-spin asymmetry, $A_L$. At STAR, the leptonic decay channel $W \to e\nu$ can be measured with the electromagnetic calorimeters and time projection chamber. STAR has previously measured $A_L$ as a function of the decay electron and decay positron pseudorapidities from datasets taken in 2011 and 2012. This has provided significant constraints on the $\bar{u}$ and $\bar{d}$ quark helicity distributions. In 2013 the STAR experiment collected an integrated luminosity of $\sim$300 pb$^{-1}$ at $\sqrt{s}=510$ GeV with an average beam polarization of $\sim$56\%, which is more than three times larger than the total integrated luminosity of previous years. The new preliminary results of the $W$ $A_L$ analysis for the dataset collected in 2013 are reported.