Speaker
Description
The transverse-momentum spectrum of Drell--Yan lepton pairs at low $p_T (ll)$ provides a sensitive probe of transverse-momentum-dependent (TMD) parton densities and of the interplay between perturbative initial-state radiation and non-perturbative QCD effects. The PDF2ISR method provides an initial-state parton shower whose backward evolution is consistent with the forward evolution of the collinear parton densities, and, within the Parton Branching (PB) framework, can be directly connected to the corresponding TMD distributions. We present recent developments and applications of PDF2ISR, including detailed studies of the primordial transverse-momentum treatment, recoil prescription, and the strong coupling at low scales. We determine the width and $\sqrt{s}$ of the dependence of the intrinsic-$k_T$ width and investigate its impact on the low-$p_T (ll)$ Drell--Yan spectrum.
The PDF2ISR framework is further extended to simulations with multileg matrix-element matching and multiparton interactions (MPI). We study the interplay between additional hard emissions, consistent initial-state evolution, and MPI, and compare the resulting predictions with those obtained using the standard PYTHIA 8 initial-state shower. These studies form an essential part towards demonstrating the applicability of PDF2ISR beyond the original proof of concept and provide a framework for consistently combining TMD and collinear parton densities with matrix-element matching, parton showers, and MPI.