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

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 and adopting a collinear configuration for the initial state. Our theoretical estimates, when compared against 200 GeV and 510 GeV (preliminary) data from the STAR Collaboration, show a generally good agreement for the $p_{{\rm j}T}$ (the transverse momentum of the jet), $z$ and $j_T$ (the pion longitudinal momentum fraction and its transverse momentum with respect to the jet direction) distributions. This corroborates the hypothesis of the universality of the Collins function as well as of the TMD factorization for such processes 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.