On the role of soft gluons in collinear parton densities and parton shower event generators

TL;DR

This paper investigates the significance of soft gluons in collinear parton densities and parton shower simulations, emphasizing their role in the non-perturbative Sudakov form factor and their impact on inclusive distributions.

Contribution

It introduces a method to analytically calculate the non-perturbative Sudakov form factor within the Parton Branching framework, highlighting the importance of soft gluons in parton density evolution.

Findings

Soft gluons are crucial for proper cancellation of divergent terms.

Non-perturbative Sudakov form factor is constrained by fits to collinear densities.

Soft gluons significantly affect inclusive distributions but not final state hadron spectra.

Abstract

The role of soft (non-perturbative) gluons in collinear parton densities and parton shower event generators is investigated with the Parton Branching method as a solution of the DGLAP evolution equations. It is found that soft gluons play a significant role. Within the Parton Branching frame, the Sudakov form factor can be split into a perturbative and non-perturbative part. The non-perturbative part can be calculated analytically under certain conditions. It is shown that the inclusion of soft (non-perturbative) gluons in the parton density evolution is essential for the proper cancellation of divergent terms. It is argued that the non-perturbative part of the Sudakov form factor has its correspondence in Transverse Momentum Dependent parton distributions. Within the Parton Branching approach, this non-perturbative Sudakov form factor is constrained by fits of inclusive, collinear parton densities. We show that the non-perturbative Sudakov form factor and soft gluon emissions are essential for inclusive distributions (collinear parton densities and Drell-Yan transverse momentum spectra). We also show by using Parton Branching TMD parton shower, that the effect of soft gluons plays essentially no role in final state hadron spectra and jets.