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

Pinna, Deborah. Search for Dark Matter in Association with Top Quarks with the CMS Detector. 2017, University of Zurich, Faculty of Science.

## Abstract

The evidence of the existence of dark matter is provided by astrophysical observations at different scales. Nevertheless, information about its nature or non gravitational interactions is not yet available. Assuming interactions between dark matter and standard model particles, dark matter particles can be produced in high energy proton collisions. In this thesis, a search for dark matter particle produced in association with a top quark pair is performed using the data recorded by the CMS detector at the LHC at two different centre-of-mass energies of 8 and 13 TeV. The datasets correspond to integrated luminosities of 19.7 fb$^{−1}$ and 2.2 fb−1, respectively. The analysis performed at 8 TeV considers only the single-lepton decay of top quark pairs. This is the first search of its kind in CMS and the results provide important insight on possible scalar interactions between dark matter and standard model particles. The results are interpreted using a dedicated effective field theory. Similar search is performed using the data collected in 2015 at a centre of mass energy of 13 TeV. This second search contains important improvements, as for example the increase of signal acceptance by considering in addition the final state where both top quarks decay hadronically. The higher energies reached by the LHC during 2015 reduced the region of validity of the effective field theory interpretation and simplified models are used instead to interpret the results. In both analyses, the observed data is compared with the predicted standard model background and good agreement with the expectation is found. Simplified models also predicts processes in which the dark matter particles are produced in association with single top quarks. The prospects of an increased sensitivity for dark matter scalar interactions rising from those processes is developed in this dissertation. The associated production of dark matter particles with top quarks has been investigated for the first time with the data collected by the CMS detector in this work. While, no evidence of the production of dark matter matter in proton-proton collisions is found, important constraints are set on the existence of dark matter particle. New prospects to improve the dark matter particles discovery potential in interactions with top quark couplings are also presented.

## Abstract

The evidence of the existence of dark matter is provided by astrophysical observations at different scales. Nevertheless, information about its nature or non gravitational interactions is not yet available. Assuming interactions between dark matter and standard model particles, dark matter particles can be produced in high energy proton collisions. In this thesis, a search for dark matter particle produced in association with a top quark pair is performed using the data recorded by the CMS detector at the LHC at two different centre-of-mass energies of 8 and 13 TeV. The datasets correspond to integrated luminosities of 19.7 fb$^{−1}$ and 2.2 fb−1, respectively. The analysis performed at 8 TeV considers only the single-lepton decay of top quark pairs. This is the first search of its kind in CMS and the results provide important insight on possible scalar interactions between dark matter and standard model particles. The results are interpreted using a dedicated effective field theory. Similar search is performed using the data collected in 2015 at a centre of mass energy of 13 TeV. This second search contains important improvements, as for example the increase of signal acceptance by considering in addition the final state where both top quarks decay hadronically. The higher energies reached by the LHC during 2015 reduced the region of validity of the effective field theory interpretation and simplified models are used instead to interpret the results. In both analyses, the observed data is compared with the predicted standard model background and good agreement with the expectation is found. Simplified models also predicts processes in which the dark matter particles are produced in association with single top quarks. The prospects of an increased sensitivity for dark matter scalar interactions rising from those processes is developed in this dissertation. The associated production of dark matter particles with top quarks has been investigated for the first time with the data collected by the CMS detector in this work. While, no evidence of the production of dark matter matter in proton-proton collisions is found, important constraints are set on the existence of dark matter particle. New prospects to improve the dark matter particles discovery potential in interactions with top quark couplings are also presented.