The large amount of new high energy data being collected by the LHC experiments has the potential to provide new information about the nature of the fundamental forces through precision comparisons with the Standard Model. These precision measurements require intensive perturbative scattering amplitude computations with large multiplicity final states. In this thesis we develop new on-shell methods for the analytic computation of scattering amplitudes in QCD which offer improved evaluation speed and numerical stability over currently available techniques and also allow us to explore the structure of amplitudes in gauge theories. We apply these techniques to extract compact analytic expression for the triple collinear splitting functions at one-loop in QCD and supersymmetric gauge theories which contribute to the universal factorisation at N${}^3$LO. We also investigate improvements to dimensionally regulated one-loop amplitude computations by combining the six-dimensional spinor helicity formalism and a momentum twistor parameterisation with the integrand reduction and generalised unitarity methods. This allowed the development of a completely algebraic approach to the computation of dimensionally regulated amplitudes in QCD including massive fermions. We present applications to Higgs plus five-gluon scattering in the large top mass limit and top pair production with up to three partons. In the case of massive one-loop amplitudes we present a new approach to the problem of wave-function renormalisation which only requires gauge invariant, on-shell building blocks. Massive one-loop amplitudes contain information that cannot be extracted from unregulated cuts, the new approach instead constrains the amplitudes using the universal poles in $6-2\eps$ dimensions which can be computed from an effective Lagrangian on dimension six operators.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:753773 |
| Date | January 2018 |
| Creators | Buciuni, Francesco |
| Publisher | Durham University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.dur.ac.uk/12732/ |
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