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Phenomenology of extra quarks at the LHC

Phenomenology of extra quarks at the LHC
Phenomenology of extra quarks at the LHC
In this thesis, we study in a model independent way models of new Physics featuring extra quarks (XQs). These quarks are predicted by several extensions of the Standard Model (SM) but have never been observed yet even though many searches have been designed to find them at the Large Hadron Collider (LHC).

After an introduction about the SM and the LHC, we present the main properties of these XQs and a model independent parametrisation that can be used to describe their phenomenology with generic hypotheses about their mixing with SM quarks, both in the case of XQ coupling with SM bosons and with Dark Matter (DM) candidates.

In these two cases we study the offshellness effects in pair-production and decay and show that if the Narrow-Width Approximation (NWA), that we describe in detail, is a good approximation of the full result in the small width over mass ratio limit, sizeable differences occur when the XQ width becomes larger. The conclusion of our analysis is that even though the small width assumption is always conservative it is not possible to trivially rescale the mass bounds obtained considering processes of pair production and decay in the NWA to determine constraints for XQs with large widths.

We also study the role of interference in the process of pair production of new heavy XQs decaying to SM particles and show that in the NWA the interference contribution can be described by considering a parameter which contains only the relevant couplings and the scalar part of the propagators of the new quarks, both at the cross section level and at the distribution level.

Finally, we study how various Supersymmetry (SUSY) searches perform for our simplified model with XQs decaying to DM. We show that cross section upper limit maps and efficiency maps obtained for stop simplified models in stop searches can also be applied to analogous XQ models, provided the NWA applies: the bound for XQs can therefore be obtained from the SUSY ones just by rescaling the exclusion with the XQ cross section.
University of Southampton
Prager, Hugo
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Prager, Hugo
49e874e6-4343-4cc7-a85c-d6e78b0a8ab0
Moretti, Stefano
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Panizzi, Luca
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Shepherd-Themistocleous, C.H.
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Barducci, Daniele
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Belyaev, Alexander
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Prager, Hugo (2017) Phenomenology of extra quarks at the LHC. University of Southampton, Doctoral Thesis, 153pp.

Record type: Thesis (Doctoral)

Abstract

In this thesis, we study in a model independent way models of new Physics featuring extra quarks (XQs). These quarks are predicted by several extensions of the Standard Model (SM) but have never been observed yet even though many searches have been designed to find them at the Large Hadron Collider (LHC).

After an introduction about the SM and the LHC, we present the main properties of these XQs and a model independent parametrisation that can be used to describe their phenomenology with generic hypotheses about their mixing with SM quarks, both in the case of XQ coupling with SM bosons and with Dark Matter (DM) candidates.

In these two cases we study the offshellness effects in pair-production and decay and show that if the Narrow-Width Approximation (NWA), that we describe in detail, is a good approximation of the full result in the small width over mass ratio limit, sizeable differences occur when the XQ width becomes larger. The conclusion of our analysis is that even though the small width assumption is always conservative it is not possible to trivially rescale the mass bounds obtained considering processes of pair production and decay in the NWA to determine constraints for XQs with large widths.

We also study the role of interference in the process of pair production of new heavy XQs decaying to SM particles and show that in the NWA the interference contribution can be described by considering a parameter which contains only the relevant couplings and the scalar part of the propagators of the new quarks, both at the cross section level and at the distribution level.

Finally, we study how various Supersymmetry (SUSY) searches perform for our simplified model with XQs decaying to DM. We show that cross section upper limit maps and efficiency maps obtained for stop simplified models in stop searches can also be applied to analogous XQ models, provided the NWA applies: the bound for XQs can therefore be obtained from the SUSY ones just by rescaling the exclusion with the XQ cross section.

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Final thesis - Version of Record
Available under License University of Southampton Thesis Licence.
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Published date: November 2017

Identifiers

Local EPrints ID: 424495
URI: http://eprints.soton.ac.uk/id/eprint/424495
PURE UUID: b7df1daa-729f-458a-939c-a965c5d99a02
ORCID for Alexander Belyaev: ORCID iD orcid.org/0000-0002-1733-4408

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Date deposited: 05 Oct 2018 11:37
Last modified: 14 Mar 2019 01:40

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