Search for top quark Flavour Changing Neutral Couplings with the CMS Experiment at the LHC

The top quark plays a key role to search for new physics phenomena beyond the standard Model of particle physics . Being the heaviest particle in the Standard Model (SM), physicists believe that it has an enhanced sensitivity to various new particles and interactions suggested by theories Beyond the Standard Model. One of interesting possibilities is the presence of flavour-changing neutral current (FCNC) interactions between the top quark and the Higgs boson. The observation of a SM-like Higgs boson by the ATLAS and CMS experiments in 2012 initiated the study of these FCNC interactions. In the SM, FCNC interactions of the top quark are suppressed. However, many theories beyond the SM predict the existence of these processes with higher branching ratios, where some are within the reach of the current experiments. This thesis concerns an experimental research at the Large Hadron Collider at CERN for FCNC interactions involving a top quark and a Higgs boson with a signature of two leptons in the final state with equal electric charge. The analysis is done using data of proton collisions collected in 2016 by the CMS experiment at a center-of-mass energy of 13 TeV and an integrated luminosity of 36fb-1. We probe for the FCNC interactions in top-quark pair decays as well as in the single top quark production through Hqt-couplings, where the quark q is either an up-quark or charm-quark. No significant deviation with respect to the predicted SM background is observed. The expected (observed) upper limits at 95% CL on the branching ratio are B(t cH) < 7.0 x 10-3 (6.8 x 10-3) and B(tuH) < 6.7 x 10-3 (6.1 x 10-3). These thesis results have been endorsed by the CMS collaboration.

Publication year:2018