Parton intrinsic motion in inclusive particle production: unpolarized cross sections, single spin asymmetries and the Sivers effect
Umberto D'Alesio, Francesco Murgia
TL;DR
This work develops a perturbative QCD framework that incorporates intrinsic parton transverse momentum (k_perp) and spin effects to describe unpolarized inclusive cross sections for Drell-Yan, direct photon, and pion production. By using Gaussian k_perp distributions for PDFs and FFs, a regulator and LO partonic cross sections, the authors achieve reasonable agreement with a wide range of data and set up a consistent basis for studying transverse single spin asymmetries via the Sivers effect. They introduce a practical Sivers function model and fit it to E704 pion SSA data, obtaining parameter sets that also describe STAR RHIC results and make predictions for direct-photon SSA. The results support the viability of a unified k_perp dependent pQCD description for both unpolarized cross sections and SSA, while highlighting the need for further formal development and high-precision data to constrain the spin-dependent nonperturbative functions.
Abstract
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 transverse momentum 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.