Large Hadron Collider phenomenology of vector-like quarks beyond the Narrow Width Approximation

Abstract

The topic of this thesis is the phenomenology of Vector-Like Quarks (VLQs) at the Large Hadron Collider (LHC), with a focus on understanding the difference between the commonly adopted experimental VLQ search strategies that use the Narrow Width Approximation (NWA) in comparison to search strategies aimed at exploring signals arising from the production and propagation of VLQs at large width (LW). The effects on the cross sections are found to be non-negligible, with particularly important contributions from the topologies where the VLQ is coupled to the light generation Standard Model (SM) quarks. These off-shell effects are studied in a model-independent way for final states compatible with processes of pair production of VLQs, compared to the on-shell topologies in the NWA with the same final state. An analogous analysis has been performed for VLQs interacting with bosonic Dark Matter (DM) candidates and SM quarks to explore at the same time the potential to characterise VLQs with LW and the discrimination of scalar or vector DM states. A study is also performed for processes of single VLQ production, where a model-independent interpretation of the results has been obtained by exploiting a suitable factorisation of the VLQ couplings. The topologies considered where those containing the production of a single VLQ decaying into a three particle final state This analysis found similar results compared to the two previously mentioned studies. The model-independent parametrisation for the interpretation of results of VLQ single production is currently being used in experimental searches by CMS, and as such I have been included as an author of a CMS analysis for single production of a Vector-Like (VL) top partner.

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Identifiers

ORCID for Dermot Oisin O'Brien: orcid.org/0000-0002-5589-641X

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

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