Sudakov resummation in QCD

TL;DR

This thesis extends the renormalization group approach to resum large logarithms in QCD processes, including prompt photon production, and demonstrates its importance for matching theoretical predictions with experimental data.

Contribution

It generalizes the resummation method to complex two-scale kinematics and provides all-order formulas for various rapidity distributions without relying on factorization assumptions.

Findings

Resummation remains valid with complex two-scale kinematics.

NLL corrections are comparable to NLO fixed-order results in fixed-target experiments.

Resummation improves agreement between theory and experimental data.

Abstract

In this PhD thesis, we analyze and generalize the renormalization group approach to the resummation of large logarithms in the perturbative expansion due to soft and collinear multiparton emissions. In particular, we present a generalization of this approach to prompt photon production. It is interesting to see that also with the more intricate two-scale kinematics that characterizes prompt photon production in the soft limit, it remains true that resummation simply follows from general kinematic properties of the phase space. Also, this approach does not require a separate treatment of individual colour structures when more than one colour structure contributes to fixed order results. However, the resummation formulae obtained here turn out to be less predictive than previous results: this depends on the fact that here neither specific factorization properties of the cross section in the soft limit is assumed, nor that soft emission satisfies eikonal-like relations. We also derive resumation formulae to all logarithmic accuracy and valid for all values of rapidity for the prompt photon production and the Drell-Yan rapidity distributions. We show that for the fixed-target experiment E866/NuSea, the NLL resummation corrections are comparable to NLO fixed-order corrections and are crucial to obtain agreement with the data. Finally we outline also possible future applications of the renormalization group approach.