The relevance of intrinsic (or primordial) transverse momentum of partons in the inclusive production of particles at high energy and moderately large $p_T$ has been known for a long time, beginning with Drell-Yan and diphoton processes, and continuing with photon and meson production in hadronic collisions. In view of its renewed interest in the context of polarized processes and single spin asymmetries we perform, in the framework of perturbative QCD with the inclusion of spin and ${\mathit{k}}_{\perp }$ effects, a detailed analysis of several such processes in different kinematical situations. We show that the inclusion of these effects leads, at the level of accuracy reachable in this approach, to an overall satisfactory agreement between theoretical predictions and experimental unpolarized data, thus giving support to the study of spin effects and single spin asymmetries within the same scheme. We present results for transverse single spin asymmetries, generated by the so-called Sivers effect, in inclusive pion and photon production in proton-proton collisions. We compare our results with the available experimental data and with previous results obtained using simplified versions of this approach.

• Received 11 August 2004

DOI:https://doi.org/10.1103/PhysRevD.70.074009