质子自旋结构与RHIC上海夸克自旋分布的测量

Nucleon spin structure and measurements on sea quark polarization at RHIC

对核子内味道分离的夸克自旋分布的研究是核子自旋结构研究的重要前沿课题之一,也是RHIC自旋物理研究的重要核心课题.通过对极化质子-质子反应中W玻色子自旋不对称的测量, RHIC首次对■和■海夸克的自旋分布■和■给出了高精度限制,发现了■在0.05<x<0.25区间内是正向极化的,并显示核子内海夸克自旋结构存在同位旋不对称,即■> ■,首次清楚地揭示出自旋分布的同位旋不对称与非极化时符号相反.同时, RHIC还通过对极化质子-质子反应中■超子极化转移的测量,对反奇异海夸克■的螺旋度分布■以及横向极化度进行了研究.

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 AL=(■+-■-)/(■++■-).At RHIC,W’s can be detected via the leptonic channel W e with the kinematics of the decay lepton alone.Longitudinally polarized proton-proton collision events at■=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,■>■,in the proton for parton momentum fraction range of 0.05<x<0.25.This is opposite to the flavor asymmetry observed in the unpolarized quark distributions,where■>■over a wide x range has been observed.On the other hand,the phenomenological studies showed that the spin transfer to■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 DLL to■and■hyperons in longitudinally polarized proton-proton collisions at 200 GeV.STAR also completed the first measurement on transverse spin transfer DLL to■and■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.