NLO Analysis of Small-$k_T$ Region in Drell-Yan Production with Parton Branching

Abstract

The Parton-Branching Method (PB) facilitates the determination of Transverse Momentum Dependent (TMD) parton densities across a wide \kt\ range, spanning small to large transverse momentum scales. In the small $k_T$ region, both intrinsic parton motion and resummed ultra-soft gluons are significant contributors. Our analysis highlights their crucial role in shaping integrated and TMD parton densities. Using PB-derived TMD parton densities and a NLO calculation in MC@NLO style, we compute the transverse momentum spectrum of Drell-Yan pairs across a broad mass range. The spectrum's sensitivity to the intrinsic $k_T$ distribution allows us to fine-tune parametric parameters. Starting from the PB-NLO-HERAI+II-2018 set2 TMD parton distributions, we determine the intrinsic $k_T$ distribution width, resulting in a slightly wider profile than the default set. Importantly, this width remains independent of Drell-Yan pair mass and center-of-mass energy ($\sqrt{s}$), distinguishing our approach.