Measuring luminosity and the $\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$ production cross section with the CMS experiment

Measurements of the luminosity and of top quark pair production in association with a Z boson ($\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$) in proton-proton collisions with the CMS experiment at the LHC are presented. The luminosity measurement of the CMS experiment is calibrated with the Van der Meer (VdM) method, based on data collected during beam separation scans. A calibration of the length scale is performed using data from constant-separation scans. The bias of the VdM method due to the assumption of factorisable proton densities of the beams is estimated with an improved method using the transverse densities as determined from beam-imaging scan data. The performance is evaluated with dedicated simulation studies. Both studies lead to a significant reduction of the systematic uncertainty for the measurement of the integrated luminosity. Measurements of $\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$ production in proton-proton collisions at a centre-of-mass energy of 13 TeV are performed using events with three charged leptons where the Z boson is identified in its decay to two oppositely charged leptons. The first differential $\mathrm{t}\bar{\mathrm{t}}\mathrm{Z}$ production cross section measurement as a function of kinematic properties of the Z boson is presented, based on a dataset corresponding to an integrated luminosity of 77.5/fb collected in 2016 & 2017. An extended differential measurement as a function of top quark observables is presented using the full Run 2 dataset, corresponding to an integrated luminosity of 137/fb. It includes a kinematic reconstruction of the $\mathrm{t}\bar{\mathrm{t}}$ system and allows to probe the QCD production mechanism. Within current experimental uncertainties, the measured differential cross sections are found to be in good agreement with standard model predictions.