Search for new physics in the dark sector with the CMS detector From invisible to low charge particles

The Standard Model of particle physics is the framework that describes all known phenomena and interactions between elementary particles. It has proven to give outstanding results over the years and was succesfully completed with the discovery of the Brout-Englert-Higgs boson in 2012 by the ATLAS and CMS collaborations at CERN. However, several observations escape its reach: the matter-antimatter asymmetry, the nature of Dark Matter or the quantization of the electric charge. These are all examples of measured facts not explained by the Standard Model formalism and that call for an extension to a Beyond the Standard Model (BSM) theory. In this thesis, we have looked for evidence of new physics using proton-proton collision data produced by CERN’s Large Hadron Collider (LHC) at a center-of-mass energy of 13 TeV. Collected from 2016 to 2018 by the CMS detector, it corresponds to an integrated luminosity of 136 fb$^{-1}$ . After an introduction to the theoretical context and the experimental tools, two analyses are presented. The first one is a search for Dark Matter particles recoiling against a jet and leaving the detector unnoticed. With this ”monojet” analysis, we are able to exclude mediator masses up to 1.8 TeV, and masses of Dark Matter particles up to 700 GeV. The second analysis is a search for fractionally charged particles. Using the fact that their stopping power is lower than Standard Model particles, we are able to exclude their existence up to masses of 765 GeV for a charge of 2/3 $e$.