Non-perturbative effects and soft-gluon dynamics in low-p_T Drell-Yan production

The transverse-momentum spectrum of Drell--Yan lepton pairs at small \ptll\ probes non-perturbative QCD effects, including intrinsic partonic transverse momentum and initial-state soft-gluon radiation. The novel \pythiaPB\ approach, which implements the Parton Branching (\PB ) framework in \pythia\ while employing the same evolution ingredients, is used to study this spectrum in the low-\ptll\ region. This framework is particularly well suited for such investigations, as it allows for a systematic treatment of the dominant non-perturbative effects and their interplay. We examine in detail the implementation of primordial transverse momentum in \pythiaPB . With an improved recoil treatment, we determine the width of the intrinsic-\kt\ distribution and find a \sqrts\ dependence that is steeper than in \PB , but much flatter than in \pythia . We identify the origin of these differences. Furthermore, different treatments of the strong coupling at low scales are investigated and confronted with available experimental data. We find that, at the highest centre-of-mass energy considered, the low-\ptll\ Drell--Yan spectrum becomes sensitive to the transition between the perturbative and non-perturbative regimes.