Study of processes with a pair of top-antitop quarks and missing transverse energy in the final state in proton-proton collisions with the CMS detector at the Run 2 of the LHC

This thesis presents measurements of the top-antitop quark pair (t$\bar{t}$) production cross section and a search for new physics in the context of supersymmetry using data from proton-proton collisions at the CERN Large Hadron Collider (LHC), recorded by the CMS detector. Furthermore, a study of the effects of the radiation on the performance of the drift tubes (DT) chamber of the CMS detector is presented. The main goal of the DT chambers is identifying muons going through CMS with a high efficiency. In 2026 the LHC will be upgraded to run at high luminosity conditions. The CMS detectors will have to deal with extreme radiation conditions. The studies presented in this thesis characterize the efficiency loss under high radiation conditions during a long period of time. The hit efficiency is shown to decrease, but the global CMS muon reconstruction efficiency is shown to stay close to 100 %, even at the end of the data taking. Besides, the t$\bar{t}$t production cross section is measured using data taken during 2015 and 2016 with different conditions of the LHC. The measurements are performed using different datasets at √s = 13 TeV. The results are in agreement with the Standard Model predictions. The high precision of these measurements, with lower uncertainties than the best predictions, allows to test different aspects of the theory. The first and only measurement of the tt cross section at √s = 5.02 TeV is also presented. Finally, a search for the production of stop quarks is reported. The production of these particles with a mass close to that of the top quark cannot be probed by generic stop quark searches. The sensitivity to this process comes from a very precise estimate of the SM tt production and the use of a discriminant observable. No excess is found over the background prediction and exclusion limits are set up to a stop quark mass of 208 GeV.