Measurement Of Longitudinal Single-Spin Asymmetry Of W~± Bosons Production At RHIC-STAR

It has been a fundamental and longstanding challenge in particle and nuclear physics to understand the spin structure of nucleon. In polarized deep inelastic scattering (DIS) experiments, the nucleon spin structure can be accessed through the spin-dependent structure function from the polarized cross section. The total contribution of quark spin has been measured to be about ～25% of the proton spin. The first flavor separated contributions of quark and anti-quark spin have been obtained in semi-inclusive DIS experiments with involvement of the fragmentation functions. Subject to uncertainties of fragmentation functions, the polarized parton distribution functions (PDFs) of sea quark are still not well constrained compared o the valance quark.RHIC, the relativistic heavy ion collider, located at Brookhaven National Labo-ratory in United States, is the first polarized proton-proton collider in the world. Tak-ing the advantage of polarized proton-proton collision, RHIC can provide new insights into the nucleon spin structure, in particular the gluon and sea quark polarizations. At RHIC, the production of W± boson in longitudinally polarized proton-proton collisions is a powerful tool to study the flavor separation of nucleon spin. The coupling of W± bosons to left-handed quarks and right-handed anti-quarks naturally determines the he-licity of the incident quarks. This provides direct sensitivity to the polarized PDFs of sea quarks through the parity-violating longitudinal single-spin asymmetry, Al, which is defined as AL=(σ+-σ-)/(σ++σ-), where σ+(-) is the cross section when the polarized beam has positive (negative) helicity. At leading order, the AL of W± can be directly associated to â–³u and â–³d, the helicity dependent PDFs of u and d quarks.At STAR, one of the two main experiments at RHIC, the W production is measured via its leptonic decay based on the topological and kinematic features of W â†' ev signal with the Time Projection Chamber, Barrel and Endcap Electromagnetic Calorimeters. The W â†' ev event is characterized as an isolated high pT track pointing to an isolated large energy deposited in the electromagnetic calorimeter. It is expected that there is a large pT imbalance caused by the undetected neutrino. Based on these features, the W leptonic decay candidates whose ET spectrum is expected to peak at-40GeV (Jacobian peak) can be selected out from the large amount of QCD background.The longitudinal single spin asymmetry as function of the lepton pseudorapidity is extracted from the spin-sorted and charge-separated W± yields. From 2011 (9.4 pb-1) and 2012 (77.4 pb-1) data, STAR measured ALW± as a function of the decay lepton pse-duorapidity for the first time. While ALW+ is consistent with the theoretical predictions based on polarized PDFs mainly determined from DIS experiments, ALW- for the nega-tive lepton pseudorapidity is systematically larger than the theoretical predictions which indicates that these data provide new constraints for â–³u than previous data from semi-inclusive DIS experiments. The results have been included into global analysis, and the impact confirms the conclusion with a more positive â–³u in the range of 0.05< x< 0.2. The uncertainties for â–³u and â–³d are significantly reduced with the new constraints from STAR ALw± data. The 2013 data sample is much larger than previous years. The measured ALW± results from 2013 data are in general consistent with these of 2011 and 2012, and the statistical uncertainties is further reduced by 40%. The quantitative im-pact will be manifested after being included in global analysis. The ALW± measurements at STAR provide unique constraints on the polarized PDFs of the light sea quarks.