QCD Beyond Leading Power

TL;DR

This thesis advances the understanding of QCD's infrared structure beyond leading power by developing effective field theory techniques, introducing new factorization objects, and achieving the first resummation of subleading logarithms for collider observables.

Contribution

It introduces new subleading power gauge invariant objects, the $ heta$-jet and $ heta$-soft functions, and develops methods for resummation and multiloop calculations beyond leading power in QCD.

Findings

First resummation of collinear and soft logarithms beyond leading power.

Introduction of the $ heta$-jet and $ heta$-soft functions for subleading power factorization.

Development of a new regulator for rapidity divergences beyond leading power.

Abstract

In this thesis I study the infrared limits of QCD beyond leading power by developing effective quantum field theory techniques. I introduce the motivations for studying this subject both as a tool to deepen our understanding of the infrared structure of gauge theory amplitudes and cross sections in the soft, collinear and Regge limit as well as to improve predictions for collider observables. Using and extending the framework of Soft and Collinear Effective Theory (SCET), I explore the ingredients of factorization beyond leading power constructing subleading hard scattering operators and radiative jet and soft functions for processes such as Higgs production and Drell-Yan. I introduce new subleading power gauge invariant objects, the $\theta$-jet and $\theta$-soft functions, which arise in the renormalization of subleading power operators. I use them to achieve the first resummation of collinear and soft logarithms beyond leading power for a collider observable in QCD. I study the perturbative power corrections to differential distributions for color singlet production at the LHC retaining the full dependence on the kinematics of the color singlet. I highlight and solve the subtleties related to the regularization of rapidity divergences beyond leading power for which I propose a new regulator, the pure rapidity regulator. I present a new method to employ cutting edge multiloop techniques for the computation of the expansions of cross sections in the collinear limit. This allows the extraction of universal ingredients arising in the collinear limit of QCD to an unprecedented level of precision in perturbation theory. Finally, I study the Regge limit at the amplitude and cross section level beyond leading power, developing a Lagrangian formalism for the treatment of fermion mediated forward scattering processes in QCD and applying it to obtain the quark reggeization and BFKL resummation.