Arts and crafts of particle phenomenology: a journey through analytical methods and applications

Thesis

In this thesis we set on a journey through modern techniques for the calculation of higher order corrections to key collider processes and their applications in the context of precision physics at the Large Hadron Collider (LHC). In particular, we will focus on the description of the highest energy stage of the particles collision, that goes under the name of "hard process", in the context of the Standard Model (SM) of fundamental interactions.

We identify two of the main ingredients that are needed for precision calculations of the hard process: scattering amplitudes and infrared subtraction. The two concepts are much intertwined, since infrared subtraction has the aim of organizing the cancellation of infrared divergences arising from loop amplitudes and phase space integrals in such a way that the two ingredients can be combined to give well-defined predictions for physical observables.

On the amplitude front, we offer an overview of contemporary techniques employed for multiloop amplitude calculations. Specifically, we undertake a computation of five-point two-loop scattering amplitudes in massless Quantum Chromodynamics (QCD) beyond the so-called large-Nc limit. These amplitudes, demanding cutting-edge computational methodologies, present avenues for exploring the non-planar sector of QCD and are ingredients e.g. for predictions that would facilitate the extraction of the strong coupling at very high energy scales.

In addressing infrared divergences, we present the challenges and frontiers of next-to-next-to-leading order (NNLO) subtraction schemes, with a focus on the Nested Soft-Collinear subtraction (NSC) scheme. By applying it to the case of mixed NNLO QCD-electroweak (EW) corrections to dilepton production at the LHC, we navigate through the intricacies of NNLO calculations within a simplified framework. Furthermore, we touch upon ongoing efforts towards extending the NSC scheme to address generic high-multiplicity NNLO processes.

Lastly, we apply these methodologies to phenomenological applications. We delve into the analysis of NNLO soft-virtual corrections to interference effects in the H → γγ decay channel, as well as NLO corrections to the rare H → Zγ decay, the former playing a role in constraining the Higgs boson decay width. Additionally, we present a comprehensive analysis of mixed QCD-EW corrections to dilepton production at the LHC in the high-invariant mass region, where the role of EW corrections is enhanced by the presence of Sudakov logarithms.

Authors: Devoto, F

• DOI: 10.5287/ora-wvrkmqmok
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: perturbative calculations; Higgs physics; quantum chromodynamics
• Subjects: theoretical particle physics