Measurement of the production cross section for the single top quark in association with a Z boson at the ATLAS detector

This thesis focuses on one of the rarest top quark processes, the electroweak single top quark production in association with a Z boson in the t-channel (tZq). The process, observed in 2019, is interesting because it can give insights on the coupling between the top quark and the Z boson. Moreover, this process is an important background to the tHq process, which allows to probe the tH Yukawa coupling, and in particular it is sensitive to its sign. This thesis aims at improving the inclusive tZq cross section measurement performed by the ATLAS experiment, using the full Run2 dataset, in which a 15% uncertainty on the cross section was achieved, with $σ^{\text{Observed}}_{tZq} = 97\ ^{+14}_{−13}\ \text{fb} = 97^{+13}_{−12}\ (\text{stat}) \pm 7\ (\text{syst})\ \text{fb}$. In Chapter 1 a brief introduction on the Standard Model is presented, whereas Chapter 2 focuses on the Top quark. The LHC and the ATLAS detector are described in Chapter 3, and the analysis setup, in which samples, object reconstruction and event selection are reported, is presented in Chapter 4. Chapter 5 describes the data-driven estimation of some of the dominant backgrounds, whereas Chapter 6 presents an AI-based analysis aimed at improving the separation between signal and the background sources. In Chapter 7 the binned-profile Likelihood fits to measure the cross section are presented, and finally, in Chapter 8, ATLAS' first measurement of the ratio between the cross sections of top and antitop quark production in association with a Z is reported.