Arena, Eloisa 
ORCID: 0000-0001-5970-8677
(2023)
Search for associated production of a Z boson with an invisibly decaying Higgs boson or dark matter candidates with the ATLAS detector using full Run-II data at LHC.
PhD thesis, University of Liverpool.
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Abstract
This thesis presents a search for invisible decays of the Higgs boson produced together with a leptonically decaying Z boson. In the Standard Model, the Higgs boson can decay into invisible particles only through the H→ZZ → 4ν process, with a predicted branching ratio Β(H→inv) of 1.06 10^{-3} for m_H =125 GeV. However, some extensions of the Standard Model allow the Higgs boson to decay into a pair of stable or long-lived particles, which can be classified as invisible because their final states cannot be detected by the ATLAS detector. Observation of a Β(H→inv) significantly above the Standard Model value would give a strong indication for physics beyond Standard Model, in particular in the context of models which predicts a direct coupling between the Higgs boson and dark matter particles. The analysis presented in this thesis makes use of the proton-proton collision dataset collected by the ATLAS experiment during the years 2015-2018, produced by the Large Hadron Collider at a centre-of-mass energy of √s= 13 TeV, corresponding to an integrated luminosity of 139 fb^{-1}. Particular emphasis is given to the estimation of the ZZ→2l2ν background through the ZZ→4l process, which represents one of the main contributions of the author to the analysis. Assuming a Standard Model cross-section for ZH production, the observed (expected) upper limit on the branching ratio of the Higgs boson to invisible particles is found to be 0.187 (0.186) at the 95% confidence level. A reinterpretation of the ZH→inv analysis in terms of dark matter models is also presented. This thesis also details studies on the ATLAS electron identification method, focusing on both testing the robustness of the current implementation, as well as possible improvements in preparation for the Run-III data-taking.
| Item Type: | Thesis (PhD) |
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| Divisions: | Faculty of Science and Engineering > School of Physical Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 04 Jul 2023 11:40 |
| Last Modified: | 04 Jul 2023 11:41 |
| DOI: | 10.17638/03171037 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3171037 |
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