We investigated the internal motion of partons through its effects on the very low transverse momentum spectra of Drell-Yan pairs produced in hadron-hadron collisions at next-to-leading order (NLO) using the Parton Branching (PB) Method. This method describes the evolution of transverse momentum-dependent parton distributions.
Unlike standard Monte Carlo event generators, which require parton intrinsic transverse momentum (intrinsic \(k_T\)) distributions to strongly depend on the collision energy, the PB Method does not exhibit such dependence.
In order to understand the different behaviour, we try to mimic the parton-shower event generators by imposing a minimal transverse momentum of the radiated parton at a branching at the level of 1-2 GeV, resulting in removal of certain fraction of the soft gluon contribution. It will be shown that the origin of the intrinsic-\(k_T\) energy dependence is in the interplay between the two non-perturbative processes: the internal motion and the soft gluon emission.