Global analysis of the Sivers functions at NLO+NNLL in QCD

TL;DR

This paper performs a comprehensive global fit of the quark Sivers function using TMD factorization at NLO+NNLL accuracy, analyzing data from various experiments to understand spin asymmetries in QCD.

Contribution

It introduces a simultaneous NLO+NNLL fit of the Sivers function across multiple processes, highlighting the impact of RHIC data and the sensitivity to QCD evolution effects.

Findings

Excellent agreement with SIDIS and Drell-Yan data

Tension with W/Z boson asymmetries at RHIC

Sensitivity of W/Z asymmetry description to Qiu-Sterman evolution

Abstract

We perform global fit to the quark Sivers function within the transverse momentum dependent (TMD) factorization formalism in QCD. We simultaneously fit Sivers asymmetry data from Semi-Inclusive Deep Inelastic Scattering (SIDIS) at COMPASS, HERMES, and JLab, from Drell-Yan lepton pair production at COMPASS, and from $W/Z$ boson at RHIC. This extraction is performed at next-to-leading order (NLO) and next-to-next-to leading logarithmic (NNLL) accuracy. We find excellent agreement between our extracted asymmetry and the experimental data for SIDIS and Drell-Yan lepton pair production, while tension arises when trying to describe the spin asymmetries of $W/Z$ bosons at RHIC. We carefully assess the situation, and we study in details the impact of the RHIC data and their implications through different ways of performing the fit. In addition, we find that the quality of the description of $W/Z$ vector boson asymmetry data could be strongly sensitive to the DGLAP evolution of Qiu-Sterman function, besides the usual TMD evolution. We present discussion on this and the implications for measurements of the transverse-spin asymmetries at the future Electron Ion Collider.