Measurement of the $t\bar{t}Z$ Production Cross Section in the Final State with Three Charged Leptons using 36.1 fb${}^{-1}$ of $pp$ Collisions at 13 TeV at the ATLAS Detector

A measurement of the production cross section for a top quark pair in association with a $Z$ boson ($t\bar{t}Z$) is presented in this PhD thesis. Final states with exactly three charged leptons (electrons or muons) are used, taking into account the decay of the top quark pair in the lepton+jets channel and the decay of the $Z$ boson into two charged leptons. The dataset used for this analysis corresponds to 36.1 fb${}^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV, recorded during 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The result of a profile likelihood fit to the event yields in four signal enriched regions and two background enriched regions is ${\sigma }_{t\bar{t}Z}=966_{-102}^{+114}(\text{stat.}{)}_{-114}^{+115}(\text{syst.})$ fb. The observed (expected) significance is $7.2$ ($6.4$) standard deviations from the background-only hypothesis. Within the experimental uncertainties, the result is in good agreement with the Standard Model prediction. This result is compared with two other $t\bar{t}Z$ analysis channels, using the same dataset but different lepton multiplicities. The analysis presented here is found to be the most sensitive one in terms of observed significance. The result of a combined fit of all three analysis channels is discussed. Two feasibility studies of possible future $t\bar{t}Z$ analysis techniques are demonstrated.