Measurement of the process ttZ(νν) in pp collision at √s = 13 TeV in the ATLAS experiment

This thesis presents a precision measurement of the associated production of a top-quark pair with a Z boson (ttZ), a key process that gives direct access to the top quark’s neutral electroweak couplings and constitutes a critical irreducible background in searches for new physics, including associated ttH production. The analysis focuses on the challenging di-leptonic final state where the Z boson decays to an invisible pair of neutrinos (Z → νν), leading to a signature characterized by two charged leptons, two b-jets, and significant missing transverse energy. The full Run 2 dataset of pp collisions at a center-of-mass energy of √s = 13.6 TeV collected by the ATLAS experiment at the LHC is utilized, corresponding to an integrated luminosity of L = 140 fb^{−1}. To enhance the statistical sensitivity to the ttZ signal, a dedicated phase space is defined to optimize the separation with the dominant backgrounds, primarily di-leptonic tt events and multiboson productions. The signal strength is extracted through a binned profile likelihood fit performed in a frequentist framework, testing the signal-plus-background hypothesis against the background-only null hypothesis. The statistical procedure is rigorously validated using an Asimov dataset before unblinding, which yields a statistical significance of Z = 3.16. This work constitutes one of the most sensitive measurements of the ttZ process in this channel to date. The results are presented as inclusive and differential cross sections, providing stringent tests of the SM. Finally, the results from the di-leptonic channel are combined with those from the zero- and one-lepton channels, yielding a comprehensive and statistically powerful measurement of t¯tZ production at the LHC, solidifying our understanding of the top quark’s electroweak interactions and narrowing the path for future discoveries. Furthermore, the dataset is interpreted within an Effective Field Theory (EFT) framework to constrain possible BSM effects.