The cos 2φ asymmetry of Drell-Yan and J/ψ production in unpolarized pp̄ scattering

We investigate the cos 2φ azimuthal asymmetry in Drell-Yan and J/ψ production from unpolarized pp̄ scattering at GSI-HESR energies. The contribution to this asymmetry arising from the leading-twist Boer-Mulders function h₁^⊥(x, k_T²), which describes a correlation between the transverse momentum and the transverse spin of quarks in an unpolarized hadron, is explicitly evaluated, and predictions for the GSI-HESR kinematic regime are presented. We show that the cos2φ asymmetry is quite sizable both on the J/ψ peak and in the Drell-Yan continuum region. Therefore these processes may offer experimentally viable access to the Boer-Mulders function in the early unpolarized stage of GSI experiments.