Probing gluon polarization in hadronic direct photon production

Inclusive direct photon production at large transverse momentum in proton-proton interactions with a longitudinally polarized beam and target is an incisive probe of the polarized gluon density in a proton. To help motivate future experiments and assist in their design, we provide predictions of the polarization asymmetry for a range of reasonable choices of the polarized gluon density. The cross sections are small at fixed-target energies, but the effort to measure them is necessary if the polarized gluon density ΔG(x, Q²) is to be determined. In order to obtain our predictions, we parametrize spin-dependent parton densities and compute their evolution with Q². Our densities provide good agreement with the spin-dependent structure function g₁ᵖ(x, Q²) measured in deep-inelastic lepton scattering.