Searches for new physics with top quarks in the final state and measurement of the differential cross-section of ttbar production in association with additional heavy flavor jets

Abstract

The Standard Model of particle physics is the most successful theory of elementary particles and their interactions. However, it does not describe some phenomena observed in the experiment, like the presence of dark matter in the Universe. To address this and other problems, the Standard Model has been extended with Supersymmetry. The Minimal Supersymmetric Standard Model with R-parity conservation predicts the existence of new particles, called superpartners of the Standard Model particles. The lightest supersymmetric particle is a good candidate for dark matter. This thesis presents a search for supersymmetry with R-parity conservation involving the pair-production of gluinos ~g decaying via supersymmetric top quarks into the top quark and the lightest neutralino X1^~0. It utilizes an LHC proton-proton collision dataset of the centre-of-mass energy Square root s =13 TeV with an integrated luminosity of 139 fb^-1 collected by the ATLAS detector in the 2015-2018 data taking period. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which are identified as originating from b-quarks. The analysis is done in two final states: one of which is required to have at least one lepton, and the second one with a veto on leptons. Since no significant excess of data with respect to the background was observed, the results are presented in terms of the combined 95% CL upper limit on the gluino and neutralino masses m~g and mX1^~0 using simplified signal models. The highest excluded m~g is approximately 2240 GeV, which corresponds to the massless neutralino, and the highest excluded mX1^~0 is 1300 GeV, which corresponds to approximately m~g =2100 GeV.

The production of top-antitop pairs in association with additional b-jets was found to be the most challenging process to discriminate from the hypothetical gluino production signal, which is not described well by the current Monte Carlo generators. This thesis presents differential cross-section measurements of ttbar+b and ttbar+bbar production using machine learning techniques. The studies will help to improve the ttbar production simulation with future Monte Carlo generators.

The Large Hadron Collider will be upgraded to high luminosity, corresponding to the average number of interactions per bunch crossing up to 200. To address the increased data rate, the entire ATLAS inner detector will be replaced with a new all-silicon tracker, ITk. The new tracker system will have a higher granularity, allowing to maintain a high reconstruction efficiency of tracks and vertices, which are vital for the ATLAS experiment. This thesis presents serial powering and data readout tests of demonstrators of the innermost part of the ITK, pixel detector.