W± production and single-spin asymmetries in polarized p⃗ + p collisions at = 500 GeV at STAR

Abstract

W boson production in longitudinally polarized p+p collisions provides a clean and novel probe of the flavor dependence of (sea) light anti-quark polarization distributions in the nucleon. The W+(−) are produced in leading order via u + d̄(d + ū) fusion, and can be studied through detection of their decay charged leptons. The STAR Time Projection Chamber (TPC) provides excellent charged particle tracking at mid-rapidity, and allowed for robust e+/e− separation for pT up to ∼50 GeV/c in this measurement. Electromagnetic calorimeters (EMCs) determine the precise lepton energy. The large acceptances of the STAR TPC and EMC systems cover most of the decay lepton (e±) phase space, and allow for strict isolation conditions to be imposed on the lepton, while also enabling a veto on substantial away-side energy. These two requirements reduce the 'QCD' background by several orders of magnitude, resulting in clean yield extraction. Preliminary results for the W+ and W− production cross sections and parity-violating single-spin asymmetries AL, obtained from the 2009 data set at = 500 GeV, are presented.