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Authors: S. Fazio, D. Smirnov

Title: Measurement of W^± boson single-spin asymmetry in transversely polarized pp collisions at STAR

ABSTRACT

The Sivers function f_1T^perp describes the correlation of parton transverse momentum with the transverse spin of the nucleon. There is evidence of a quark Sivers effect in semi-inclusive DIS (SIDIS) measurements. Important aspects of the Sivers effect include its process dependence and the color gauge invariance. As a consequence, the quark Sivers functions are of opposite sign in SIDIS and in Drell-Yan and this non-universality is a fundamental prediction from the gauge invariance of QCD. The experimental test of this sign change is one of the open questions in hadronic physics, and can provide a direct verification of QCD factorization. While luminosities required for a meaningful measurement of asymmetries in Drell-Yan production are challenging, W^+/- production with a comparable sensitivity to the predicted sign change can be measured at the STAR experiment. At these large Q² the result can also provide essential input for the evolution effects of the Sivers function.

We report on the first attempt to measure the transverse single spin asymmetry, A_N, of the W^+/- boson at RHIC. The W kinematics is reconstructed by employing MC-based corrections dictated by the acceptance of the STAR detector. The data sample of 25 pb^-1 was collected by STAR in transversely polarized proton collisions at sqrt(s)=500 GeV during the 2011 run.