Developing an amplitude level approach to radiative processes in quantum chromodynamics

This thesis details studies into soft and collinear radiation dressing QCD hard process amplitudes. Primarily, we present a formalism which reduces the computation of arbitrary multiplicities of soft and collinear QCD radiation in the massless limit, dressing any QCD hard process, to a single Markovian evolution equation. We present the evolution equation at leading logarithmic order, with complete spin correlations, and notably without approximating the QCD colour charge; though our formalism can be systematically extended beyond this order. We investigate the formalism and find that it can be used to study the accuracy of modern day parton showers, motivate new parton showers with increased accuracy, and study the factorisation properties of QCD processes in hadron colliders. From our studies, we introduce a new form of dipole shower, constructed to inherit the accuracy of an angular-ordered shower without losing the benefits and generalised applicability of a traditional dipole shower. We also study observables which suffer from coherence violating logarithms in proton evolution at hadron colliders. We arrive at the conclusion that almost all observables at hadron colliders will violate coherence to some degree. Only observables entirely insensitive to wide-angle soft physics remain completely safe. This thesis consists of five individual publications and supplementary material providing context and greater detail. This thesis is also supplemented with an extended discussion of introductory material on quantum chromodynamics and quantum field theories more broadly.