Measurement of the N-jettiness variables in the production of Z boson events with the CMS detector and performance of its electromagnetic calorimeter

This thesis presents the measurements of the differential cross section of the Z boson production in association with jets in proton-proton collisions at a center-of-mass energy of 13 TeV. The analyzed data have been collected by the CMS (Compact Muon Solenoid) experiment of LHC (Large Hadron Collider) during 2018 and corresponds to an integrated luminosity of 59 fb$^{−1}$. The cross section is measured as a function of track-based event shape variables: zerojettiness, one-jettiness and sum of the transverse momentum of particles, and jet-based event shape variables ($\tau$$_{sum}$ and $\tau$$_{max}$).
The measurement of event shape variables is performed using events where pairs of muons are produced in the decay of a real Z boson with an invariant mass between 76 and 106 GeV. Track-based variables are also measured for off-shell Z bosons with an invariant mass between 125 and 150 GeV, 150 and 350 GeV, and 350 and 1500 GeV, as wll as in four different Z boson transverse momentum regions. The measurements have been compared with three types of theoretical predictions with LO, NLO, and NNLO QCD accuracies obtained with two Monte Carlo generators, MADGRAPH5_aMC@NLO and GENEVA.

Part of the thesis is devoted to the studies of the performance and intercalibration of the Electromagnetic calorimeter (ECAL) during the Run 2 datataking period (2016, 2017, and 2018). This subtedector is crucial for the detection of photons and electrons; therefore, it is very important for many searches at CMS. Regular monitoring and calibration allowed an excellent performance to be achieved: the energy resolution was maintained within 1.7% in the central part of the detector. It is also shown that the performance from Run 2 is very close to the one from Run 1 despite ageing of the detector and much higher instantaneous luminosity.