Collins function for pion-in-jet production in polarized $pp$ collisions: a test of universality and factorization

TL;DR

This paper tests the universality and factorization of the Collins function in pion-in-jet production during polarized proton-proton collisions, using recent TMD extractions and comparing theoretical predictions with STAR data, supporting TMD factorization.

Contribution

It provides the first test of Collins function universality and TMD factorization in pion-in-jet production in polarized $pp$ collisions using a simplified TMD approach.

Findings

Good agreement with STAR data on jet and pion distributions.

Supports the universality of the Collins function.

Suggests limited impact of TMD evolution on asymmetries.

Abstract

We present an updated study of the Collins azimuthal asymmetries for pion-in-jet production in polarized $pp$ collisions. To this aim, we employ a recent extraction of the transversity and Collins fragmentation functions from semi-inclusive deep inelastic scattering and $e^+ e^-$ annihilation into hadron pairs processes, obtained within a simplified transverse momentum dependent (TMD) approach at leading order in the strong coupling constant $\alpha_s$. In the present case we adopt a collinear configuration for the initial state, keeping transverse momentum effects only in the fragmentation mechanism. Our theoretical estimates, when compared against 200~GeV and 510~GeV data from the STAR Collaboration, show a generally good agreement for the distributions in the transverse momentum of the jet, the pion longitudinal momentum fraction and its transverse momentum with respect to the jet direction. While not being a proof, due the assumptions and limitations behind the entire approach, these findings corroborate the hypothesis of TMD factorization for such processes as well as of the universality of the Collins function and, once again, of a reduced impact of the proper TMD evolution on azimuthal asymmetries. We will also present predictions based on an extraction of the Collins and transversity distributions where information from data on single spin asymmetry for inclusive pion production in $p^\uparrow p$ collisions is included through a Bayesian reweighting procedure.