Search for heavy fermions with LHC data

This thesis presents two searches for vector like top (VLT) and vector like bottom (VLB) using data collected by the ATLAS experiment during the second run of the Large Hadron Collider operations, at a center of mass energy of $\sqrt{s}$ = 13 TeV. These analyses target vector like quarks (VLQ) decaying to a $Z$ boson and a third generation quark, in events with the $Z$ boson decaying to electrons or muons. Since it is known from previous searches that, if VLQs exist in nature, they should have high masses, a selection based on multiplicity and transverse momentum of the final state objects was implemented. The first analysis presented uses a partial run-2 dataset, corresponding to data collected between 2015 and 2016, amounting to an integrated luminosity 36.1 fb$^{-1}$ of data. It features searches for pair and single production of vector like quarks. As no deviation from the Standard Model expectation was found, upper mass limits were derived. VLT (VLB) masses below 1.03 (1.01) TeV for the singlet hypothesis and 1.21 (1.13) TeV for the doublet hypothesis are excluded. The results, when assuming branching ratio exclusively to $Z$, are 1.34 and 1.22 TeV for the VLT and VLB, respectively. A second iteration of the analysis using the complete run-2 dataset, amounting to 139 fb$^{-1}$, is the main focus of the present thesis. It focuses exclusively on pair production of VLT and VLB, and features a new search strategy, which includes a signal region categorization based on multiplicities of tags of a deep neural network trained to classify jets as originated from top quarks, $W$ or $Z$ bosons, Higgs bosons or background jets. In this search no deviations from the Standard Model expectation were found so upper mass limits were derived. Masses below 1.27 and 1.20 TeV are excluded for singlet VLT and VLB, respectively. These values, for the doublet hypothesis, are 1.46 and 1.32 TeV for VLT and VLB, respectively. In the scenario where all VLQ decay exclusively to $Z$ bosons, the excluded masses are 1.42 and 1.60 TeV for VLB and VLT, respectively. These results represent a significant improvement from the previous iteration of the analysis and are currently the most stringent