Study of associated production of the top-quark pair and Z boson

The thesis presents the measurement of the total cross section of the top-quark pair and Z boson associated production (t t-barZ) at pp collisions data collected by the ATLAS detector at √p = 13 TeV. The first part presents the first observation of the (t t-barZ process by the ATLAS detector at 2015 and 2016 data corresponding to the luminosity of 36.1 fb^-1. This part is focused on the dilepton channel, with the leptonically decaying Z boson and hadronically decaying top-quark pair. Because of a high background rate in the 2l channel, a multivariate analysis needs to be employed. Optimization of the MVA and the set of its input variables are described in details in the thesis. Fitting the MVA output in 3 signal regions of the 2 channel, the following value of the ttZ total cross section has been obtained: σ^2l,measured_ttbarZ = 0.64^+0.15(stat.)^+0.20_0,1₉(syst.) pb = 0.64^+0.25_-0_.24 pb: (4) with 3.0 σ significance (exclusion of background only hypothesis). Combining the 2l channel with 3l and 4l t t-barZ channels and adding two t t-barW channels to the fit 8.9 σ signal significance and the following value of cross section have been obtained: σ^2l,measured_ttbarZ = 0:95 ± 0.08(stat.) ± 0.10(syst.) pb = 0.95 ± 0.13 pb. (5) The both results are compatible with the Standard Model prediction. The second part of the thesis deals with analysis of the full Run II dataset collected by the ATLAS detector at pp collisions from 2015 to 2018. This part is focused on the 4 channel, with both Z boson and top-quark pair decaying dileptonically. Only expected results obtained from a Monte Carlo simulation are presented. The expected cross section obtained from the 4l channel t at dataset corresponding to the full Run II dataset is μ^4l,expected_ttbarZ = 1.000^+0.142_-0.13 2 (stat.) ^+0.074_-0.0 6 8 (syst.) = 1:000^+0.160_-0_.149; (6) where the theory prediction has been taken as the unit of μ^4l,expected_ttbarZ. (Author)