| Summary: | <p>In this thesis we consider the phenomenology of Quantum Chromodynamics (QCD) in the context of precision predictions for the Large Hadron Collider (LHC). Our emphasis will be given to jets, the collimated sprays of hadrons present at high-energy colliders, their substructure, prediction and simulation.</p>
<p>To this end, we first introduce the fundamentals of QCD, resummation and jet substructure. We then introduce QCD parton shower event generators and the notion of their formal accuracy. We proceed to introduce two new algorithms for incorporating full-colour effects into parton showers and give particular emphasis to their testing and validation. Next, we introduce a novel algorithm for evaluating the (subjet) multiplicity at high-energy colliders called the Lund multiplicity. It is based on the Lund plane jet substructure technique and provides a simple procedure for evaluating the subjet multiplicity with an infra-red cutoff at a general collider. We first apply it to the case of $e^+e^-$ collisions at the Large Electron Positron (LEP) collider. In this context we, for the first time, resum the average Lund and Cambridge multiplicities to next-to-next-to-double logarithmic accuracy and perform a phenomenological theory-to-data comparison with published LEP results. Finally, we extend the study to the context of high-energy jets at the LHC. Particularly, we resum their average Lund multiplicities to next-to-next-to-double logarithmic accuracy in $Z$+jet and inclusive jet events at the LHC. We then perform a phenomenological analysis for a realistic collider scenario and provide predictions ready for theory-to-data comparison. Alongside the phenomenological applications of this observable, the resummed average Lund multiplicities provide a necessary benchmark for testing the next generation of accurate parton showers.</p>
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