Search for Dark Matter in Association with Top Quarks with the CMS Detector

The evidence of the existence of dark matter is provided by astrophysical observationsat different scales. Nevertheless, information about its nature or non gravitationalinteractions is not yet available. Assuming interactions between dark matter and stan-dard model particles, dark matter can be produced in high energy collisions and theinteraction studied in detail.In this thesis, a search for dark matter produced in association with a top quark pairis performed using the data recorded by the CMS detector at two different center-of-mass energies of 8 and 13 TeV. The datasets correspond to integrated luminositiesof19.7fb−1and2.2fb−1, respectively. The analysis performed at8TeV considersonly the single-lepton decay of top quark pairs. It represented the first search of itskind in CMS and the results provide important insight on possible scalar interactionsbetween dark matter and standard model particles. The results are interpreted using adedicated effective field theory. Similar search is performed using the data collected in2015 at a center of mass energy of 13 TeV. This work contains important improvements,as for example the increase of signal acceptance by considering in addition the finalstate where both top quarks decay hadronically. The higher energies reached by theLHC in the run started in 2015 reduce the region of validity for the effective fieldtheory interpretation and simplified models are used instead to interpret the results.In both analyses, the observed data is compared with the predicted standard modelbackground, showing good agreement with the expectation.The same simplified model also predicts processes in which the dark matter particlesare produced in association with a single top quark. The prospects of an increasedsensitivity for dark matter scalar interactions due to this processes is firstly developedas part of this thesis.The associated production of dark matter particle with top quarks has been firstlyinvestigated with the data collected by the CMS detector in this thesis. The resultshave been interpreted employing the latest theoretical descriptions of dark matterinteractions. While, no evidence of the production of dark matter matter in proton-proton collisions have been found, important constraints have been set on the propertyof this mysterious component of the universe. New prospects to improve the darkmatter discovery potential in interactions with top quark couplings have also beenpresented