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

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 ALLDY in the Drell-Yan process, while the helicity asymmetry ALL^γ in direct photon production at high pT is employed to test the gluon spin content. Helicity asymmetries in both processes are quite sizable. ALLDY is positive and of order 10^-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 ALLDY to be large and negative. Experimental measurements of ALLDY and ALL^γ together will not only provide a clean probe of sea and gluon polarizations, but also test whether the combination Δs-(αₛ4π)ΔG inferred from the EMC data is valid, i.e., whether gluons contribute to the spin-dependent structure function g₁ᵖ(x,Q²) via the triangular anomaly.