Extracting jet signals of New Higgs Physics: from traditional analysis to machine learning

Abstract

This thesis investigates possible parameter values of, and optimum jet reconstruction for the signals from, the two Higgs doublet model (2HDM). Possible parameter values are investigated by way of recasting a parameter scan published by ATLAS. The original analysis, performed with 36.1 fb−1 of run 2 data, investigated the possibility of observing cascade decays from the 2HDM. The study considered the process A → ZH → l +l −b ¯b (where l = e, µ) in the context of the standard four Yukawa types. A parameter space in the physical basis of the 2HDM is explored, seeking parameter combinations that are not forbidden by theoretical constraints or existing observations, and to which a detector would be sensitive. The existing study is recast in two directions, firstly the possibility of exchanging A and H is investigated. Secondly, the extrapolation to run 3 is calculated. Under exchange of H and A all detectable parameter combinations are forbidden. More promisingly, however, it is seen that run 3 will offer sensitivity to considerable areas of permissible parameter space. It is clear that these decay channels already offer potential for finding the 2HDM at the LHC. Another line of investigation that might compliment this, is the potential to improve sensitivity by better signal reconstruction techniques. In particular, jet reconstruction techniques that might expand sensitivity to cascade decays from the 2HDM ending in a four b-quark final state are sort. Firstly, the challenges of reconstructing these states with existing algorithms is evaluated, and the limitations posed by cuts in the trigger illustrated. A comparison is made between the prevalent anti-kT algorithm and a somewhat unusual algorithm termed variable-R. This finds that variable-R performs this task best, both in terms of mass peak reconstruction, and jet multiplicity. The second investigation into optimum jet construction aims to apply a novel method, spectral clustering, to the jet formation problem. Again, it is driven by an interest in reconstructing cascade decays from the 2HDM. This method proves to be insensitive to infra-red singularities in a practical sense. It is also shown to be very flexible, capable of clustering a range of signal types, without requiring alterations to its parameter settings.

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Identifiers

ORCID for Stefano Moretti: orcid.org/0000-0002-8601-7246

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