Exploring top-quark and Z boson coupling in top pair production in association with a Z boson

In this thesis, a measurement of the production cross section of a top quark pair in association with a Z boson has been presented. The analysis used data from proton-proton collisions at a COM energy of 13 TeV collected by the CMS detector at the LHC in 2018, corresponding to an integrated luminosity of 59.7 fb$^{-1}$. In the analysis presented in this thesis, the two-lepton final state was studied, in which the leptons are of opposite sign and same flavour. As the event selection was not sufficient to yield a good distinction between signal and background, two multivariate techniques were used, namely a neural network and a boosted decision tree. The usage of a BDT and NN resulted in measurements of the production cross section of $\sigma_{BDT} = 1.283^{+0.107}_{-0.105}(\text{stat})^{+0.271}_{-0.242}(\text{syst}) \text{ pb}=1.283^{+0.291}_{-0.264} \text{ pb}$ and $\sigma_{NN} = 1.145^{+0.109}_{-0.110}(\text{stat})^{+0.370}_{-0.373}(\text{syst}) \text{ pb}=1.145^{+0.386}_{-0.389} \text{ pb}$ respectively. Both of these results are compatible with the most recent cross section measurement by the CMS collaboration, which was performed in the three- and four-lepton final states. However, the precision of the CMS result is significantly better than those achieved in this thesis. The cross section measurement obtained using the NN is in agreement with the standard model prediction $\sigma_{SM} = 0.84 \pm 0.10$ pb, while the measurement using the BDT deviates from the SM prediction by 1.68 standard deviations.