# Unpolarised TMD PDFs and FFs and the role of transverse momentum dependence in azimuthal spin asymmetries

## TL;DR

This paper examines how the correlation between intrinsic transverse momentum in unpolarised TMD PDFs and FFs affects predictions of azimuthal spin asymmetries in SIDIS, $e^+e^-$, and Drell-Yan processes, emphasizing the importance of these effects in TMD phenomenology.

## Contribution

It highlights the significance of correlation effects between transverse momenta in unpolarised TMDs and their impact on asymmetry predictions, which has been underexplored in previous studies.

## Key findings

- Correlation effects can significantly alter asymmetry sizes.
- These effects influence the interpretation of TMD evolution and process dependence.
- Accounting for these correlations is crucial for accurate TMD phenomenology.

## Abstract

In the TMD approach, the average transverse momentum of the unpolarised TMD PDFs and FFs is crucial not only to reproduce unpolarised cross sections and hadron multiplicities, but also for the understanding of azimuthal and spin asymmetries. Information on these transverse momenta is nowadays obtained mainly by fitting multiplicities data for SIDIS, where the intrinsic motion in the initial parton distributions and in the hadronisation process are strongly correlated and difficult to estimate separately without ambiguities. In this contribution we discuss the consequences of this correlation effects on the predictions for the Sivers and Collins asymmetries measured in SIDIS and $e^+e^-$ annihilations, and under active investigation for Drell-Yan processes at RHIC and at CERN by the COMPASS experiment. We show that these effects may be relevant and can sensibly modify the size of the predicted asymmetries. Therefore, they must be taken into careful account when investigating other aspects of TMDs, like the evolution properties of the Sivers and Collins functions and the expected process dependence of the Sivers function.