Nucleon spin structure and measurements on sea quark polarization at RHIC

Abstract

The nucleon spin structure in terms of its parton constituents is a fundamental question in QCD. The sea quark contribution to the nucleon spin is an important piece for a complete understanding of the nucleon spin structure. The production of W bosons in longitudinally polarized proton-proton collisions at RHIC provides a unique probe for the sea quark polarization, through the parity-violating single-spin asymmetry A L = ( σ + − σ − ) / ( σ + + σ − ) . At RHIC, W ’s can be detected via the leptonic channel W → e ν with the kinematics of the decay lepton alone. Longitudinally polarized proton-proton collision events at s =500 GeV were recorded at RHIC through the years 2009, 2011, 2012 and 2013. Both STAR and PHENIX experiments completed their measurements on longitudinal single spin asymmetries for W bosons as a function of decay lepton rapidity. These results provide new constraints on sea quark polarizations through the QCD global analyses, which clearly indicate the existence of a flavor asymmetry in the polarization of the light-quark sea, Δ u ¯ > Δ d ¯ , in the proton for parton momentum fraction range of 0.05 x d ¯ > u ¯ over a wide x range has been observed. On the other hand, the phenomenological studies showed that the spin transfer to Λ ¯ anti-hyperons provides a natural connection to helicity or transversity distributions of strange and anti-strange quarks in the proton. STAR experiment made the first and an improved measurement on the longitudinal spin transfer D LL to Λ and Λ ¯ hyperons in longitudinally polarized proton-proton collisions at 200 GeV. STAR also completed the first measurement on transverse spin transfer D LL to Λ and Λ ¯ hyperons in transversely polarized proton-proton collisions at 200 GeV. These data and future measurements with higher precision and larger momentum fraction in the forward region at RHIC will shed new lights on the polarized parton distributions for strange and anti-strange quark in the nucleon.