Spin asymmetry in proton-proton collisions as a probe of sea and gluon polarization in a proton.

Abstract

Quark and gluon spin densities in a proton are phenomenologically parametrized based on the European Muon Collaboration (EMC) data and on some plausible theoretical arguments. Four different characteristic values of gluon and sea polarizations suggested by various theoretical conjectures are considered. The sea polarization in a proton is probed by measuring the spin-spin asymmetry ${A}_{\mathrm{LL}}^{\mathrm{DY}}$ in the Drell-Yan process, while the helicity asymmetry ${A}_{\mathrm{LL}}^{\ensuremath{\gamma}}$ in direct photon production at high ${p}_{T}$ is employed to test the gluon spin content. Helicity asymmetries in both processes are quite sizable. ${A}_{\mathrm{LL}}^{\mathrm{DY}}$ is positive and of order ${10}^{\ensuremath{-}1}$ if the sea is polarized opposite to the proton spin, as suggested by the EMC data. However, even in the absence of the sea polarization at the EMC energies, we find ${A}_{\mathrm{LL}}^{\mathrm{DY}}$ to be large and negative. Experimental measurements of ${A}_{\mathrm{LL}}^{\mathrm{DY}}$ and ${A}_{\mathrm{LL}}^{\ensuremath{\gamma}}$ together will not only provide a clean probe of sea and gluon polarizations, but also test whether the combination $\ensuremath{\Delta}s\ensuremath{-}(\frac{{\ensuremath{\alpha}}_{s}}{4\ensuremath{\pi}})\ensuremath{\Delta}G$ inferred from the EMC data is valid, i.e., whether gluons contribute to the spin-dependent structure function ${g}_{1}^{p}(x,{Q}^{2})$ via the triangular anomaly.