Measurement of Higgs plus Associated Top Production in the Boosted Regime

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Following the discovery of the Higgs Boson at the LHC in 2012, there remain many measurements to be made to verify whether it is a Standard-Model Higgs boson, or something more exotic. Within the ATLAS collaboration, this project focuses on the channel where a Higgs boson is produced in association with a pair of top quarks, and then decays to b-quarks. In the SM, the coupling of fermions to the Higgs is proportional to the fermion mass and so accurate measurements of these couplings, called Yukawa couplings, is crucial. Any deviation from the predicted coupling strengths would be a powerful indication of new physics beyond the SM, with far-reaching consequences for our understanding of physics at large scales. The project will exploit the specific region of phase space where the top quarks and/or the Higgs boson are boosted to high transverse momenta, considerably larger than their rest mass. In this kinematic regime, the decay products of the top or Higgs are collimated into one "fat" jet. These events can be identified to select a high signal-purity sample to analyse. Specifically designed techniques looking at the substructure of these fat jets can also be used to identify them as originating from a specific heavy object. The project will build upon first measurements made in the semi-leptonic mode (where one top decay hadronically and one leptonically) with an initial aim to extend this to the all-hadronic mode (where both top quarks decay hadronically) using the full Run-2 dataset. In both modes there are large combinatorial problems which must be overcome using a refined set of variables. These variables will be used as inputs to a deep neural network which should improve the statistical significance of the results. The project will use robust statistical techniques to combine the results from different modes and channels.