First measurement of the t ̄ t production cross section in association with a Z boson decaying into a quark pair at the CMS Experiment

Abstract

In this thesis, data collected by the CMS Experiment at the CERN LHC is analysed. The data was collected during LHC Run 2 in the years 2016-2018 and corresponds to an integrated luminosity of 138 fb ̄ ¹. Using this data, the cross section of top quark pair production (tt ̄) in association with a Z boson is measured. The process is analysed for the first time in the decay channel, with the tt ̄ pair decaying dileptonically and the Z boson decaying hadronically. Only Z bosons with a transverse momentum greater than 200 GeV are considered. In this regime, the two jets from the Z boson form one single boosted jet that is used to identify the Z boson. The cross section is measured to be σ(ttZ) = 710 (+327 -299) fb. In addition, the calibration of the BCM1F detector, one of the luminosity measurement systems of the CMS Experiment, is shown. In order to calibrate the measurement, several corrections are developed and applied. For the luminosity measurement for the year 2018, the ratio with respect to nominally the best luminosity ranges between 1 and 1.02 after the corrections are applied.