Effective Field Theories for Top Quarks at the LHC and Physics Beyond the Higgs Boson

Abstract

In this thesis we have considered applications of two different types of effective field theory. The first of these, Soft Collinear Effective Theory (SCET) has been applied to the calculation of top quark pair production at hadron colliders. Building on factorisation theorems developed using SCET we present results for resummed differential cross sections of the top pair invariant mass. The highest accuracy of resummation achieved and matched to fixed order is at NNLO+NNLL'. Resummed predictions are compared to fixed order ones and we find that while the (N)NLO results can be sensitive to the choice of factorisation scale, the resummed results exhibit more stable behaviour. We perform a number of additional analyses to further investigate the choice of scale on the invariant mass distribution. In addition, we also present results for the p_T distribution of the top quark.

The second piece of work presented in this thesis concerns the use of the Standard Model Effective Field Theory (SMEFT). The SMEFT augments the Standard Model with higher dimensional operators which can be viewed as the low energy realisation of some as yet undiscovered physics at high scale. We use the dimension-6 SMEFT to calculate the QCD corrections to Higgs decay to bottom quarks at NLO. The main result of this section is the NLO partial decay rate for the Higgs including the dimension-6 Wilson coefficients. Finally we attempt to remove the presence of large mass logarithms through the conversion of the b-quark mass to the scheme. We assess the reliability of this prediction in the limit of massless bottom quarks and find excellent agreement with the result with full mass dependence.