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Title: Supersymmetry searches involving multiple b-jets and missing energy at the ATLAS experiment
Author: Kourlitis, Evangelos
ISNI:       0000 0004 9352 1250
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Supersymmetry is an extension of space-time symmetries that predicts new bosonic parters for the Standard Model (SM) fermions and vice versa. The main subject of this dissertation is the search for these partners in final states involving multiple jets originating from the hadronisation of bottom quarks (b-jets) and missing energy, at the LHC √s = 13 TeV proton-proton collision data collected by the ATLAS detector during the Run 2 period (2015 to 2018). Moreover, in light of the Phase-II upgrade of the ATLAS detector, radiation tolerance studies of future silicon microstrip sensors are also presented. In particular, a sensor performance characterisation facility was developed at the University of Sheffield. A description of the facility along with initial benchmark measurements of microstrip sensors, before and after irradiation are presented. The results support the successful commissioning of the facility and are found to be in agreement with what is expected from the literature and similar measurements conducted at other facilities. Regarding the Supersummetry searches, first, a search concerning the pair production of gluinos decaying via third generation squarks into the lightest neutralino is presented. The data used constitute a partial Run 2 dataset collected during 2015 and 2016, corresponding to an integrated luminosity of 36.1/fb. The final state studied comprise of multiple energetic jets, at least three of which must be identified as b-jets, and large missing energy. The dataset is split into samples containing charged leptons or not, to further increase the search sensitivity. No excess above the predicted SM background is found, hence upper limits (at 95% confidence level) have been placed on the models under investigation. For neutralino masses below 300 GeV, gluino masses up to 1.97 (1.92) TeV have been excluded in models involving decays via top (bottom) squarks. An interpretation of the limits as function of the branching ratio of the gluinos into the various third generation squark configurations is provided as well. A search for the pair production of bottom squarks (sbottoms) is also presented. This work use the full LHC Run 2 dataset that amounts to a total of 139/fb. Specifically, the the sbottoms are searched for in scenarios in which they decay to the second-lightest neutralino and a b-quark. Subsequently, each second-lightest neutralino is considered to decay to a Higgs and the lightest neutralino. Two mass configurations are targeted: either the lightest neutralino has a constant mass at 60 GeV or the mass difference between the two neutralino states is constant at 130 GeV. The final state explored consist of zero leptons, at least three b-jets and large missing energy. No significant excess above the predicted SM background is observed in data thus sbottom masses up to 1.5 TeV have been excluded. Finally, few concluding remarks and future prospects are discussed.
Supervisor: Costanzo, Davide Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available