Searches for Supersymmetry and Dark Matter in final states with b-jets and ETmiss utilising the Higgs boson as a handle for new physics with the ATLAS detector



Teagle, Hamish
(2022) Searches for Supersymmetry and Dark Matter in final states with b-jets and ETmiss utilising the Higgs boson as a handle for new physics with the ATLAS detector. PhD thesis, University of Liverpool.

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Abstract

The Standard Model (SM) is the most powerful and successful description of nature we have. Its predictions have stood up to decades of scrutiny and, since the discovery of the Higgs boson by the ATLAS and CMS collaborations in 2012, all the fundamental particles of the theory have been observed. However, we also know the theory to be incomplete. This thesis describes several searches for evidence of physics beyond the SM in proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider (LHC). The searches are motivated in two theoretical bases: WIMP dark matter scenarios which propose additional massive weakly-interacting particles aiming to explain astrophysical observations, and Supersymmetry (SUSY) which is a complete theoretical framework based upon symmetry extensions to the SM and predicts many additional massive particles that partner those of the SM and, under certain constraints, also provides a WIMP dark matter candidate. In this work, the theoretical foundations of the SM and the relevant proposed extensions are briefly discussed. The methodology of the searches are then described along with their results. Firstly in chapter 8 two searches for Supersymmetric partners of the third generation quarks are outlined, targeting two decay chain scenarios. A dark matter model considering spin-0 mediators that expresses a similar experimental signature to the Supersymmetric signatures is also presented in this analysis. In chapter 9 a search for Supersymmetric partners to the electro-weak gauge bosons is presented taking two approaches: firstly a statistical analysis based on studying the shapes of key kinematic variables, and secondly a machine-learning (ML) based approach is targeting specific hard-to-reach areas of phase-space is presented. Lastly, in chapter 10 a search for pseudo- scalar mediated dark matter is presented motivated by the 2HDM+a model. The commonality between the searches lies in the experimental signatures they produce. They are all characterised by the presence of b-quarks and weakly interacting particles miss that are evidenced in the detector as missing transverse energy, ET . In addition, two of the proposed models contain the Higgs boson in their decays, this is utilised as a handle to identify the signatures. In the appendix an evaluation of the construction methods and the material content to be used in the construction of the support structure for the silicon detector systems in the ATLAS inner detector upgrade is outlined and the comparison with the design specification is discussed. The upgrade will be implemented for the high luminosity run of the LHC.

Item Type: Thesis (PhD)
Divisions: Faculty of Science and Engineering > School of Physical Sciences
Depositing User: Symplectic Admin
Date Deposited: 14 Apr 2022 15:08
Last Modified: 01 Sep 2022 08:11
DOI: 10.17638/03152369
Supervisors:
  • D'Onofrio, Monica
URI: https://livrepository.liverpool.ac.uk/id/eprint/3152369