MEASUREMENT OF LONGITUDINAL SINGLE-SPIN ASYMMETRY FOR W± BOSON PRODUCTION IN POLARIZED PROTON-PROTON COLLISIONS AT STAR AT FORWARD RAPIDITY

Abstract

Spin plays a key role in the determination of the properties of fundamental particles and their interactions. The spin structure of the proton is one of the most challenging open puzzles in Quantum Chromodynamics (QCD). It was believed that the proton spin was carried by the spin of its three valence quarks. However, The results of the EMC (European Muon Collaboration) experiments in 1987 suggested that the quark intrinsic spin contributes, ∆Σ = 0.12 ± 0.09 ± 0.14 of the proton spin setting off the proton spin crisis. ”Where is the rest of the proton spin is coming from?” remains a major challenge to our understanding of the structure of the proton. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) delivers the highest energy polarized proton-proton collisions at a center of mass energy up to 510 GeV and provides a unique opportunity to study quark and gluon spin structure of the proton and the QCD dynamics at high energy scale. The production of W −(+) bosons in polarized proton-proton collisions provides an ideal tool to study the spin-flavor structure of the proton sea quark distributions profiting from the parity-violating nature of the weak interactions. W −(+) bosons are produced in u ̄ + d (d ̄ + u) annihilation and can be detected through their leptonic decay mode. The STAR experiment can detect charged leptons e−(+) at mid and forward rapidity regions. In this analysis, the measurement of the longitudinal single-spin asymmetries at forward rapidity for W boson production will be presented based on the data sample collected in 2013 (RUN-13) corresponding to an integrated luminosity 220 pb−1 with an average beam polarization ∼ 56%.