Comparison of transverse single-spin asymmetries for forward π0 production in polarized pp, pAl and pAu collisions at nucleon pair c.m. energy sNN =200 GeV

Abstract

The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN, for neutral pions produced in polarized proton collisions with protons (pp), with aluminum nuclei (pAl) and with gold nuclei (pAu) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7 <0.47, the asymmetries are found to rise with increasing transverse momentum. For larger xF, the asymmetry flattens or falls as pT increases. Parametrizing the ratio r(A)AN(pA)/AN(pp)=AP over the kinematic range, the ratio r(A) is found to depend only weakly on A, with P = -0.027±0.005. No significant difference in P is observed between the low-pT region, pT<2.5 GeV/c, where gluon saturation effects may play a role, and the high-pT region, pT>2.5 GeV/c. It is further observed that the value of AN is significantly larger for events with a large-pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r(A) is similar for isolated and nonisolated π0 events.