Search for Dark Matter in the Monojet and Trackless Jets Final States with the CMS Detector at the LHC

All matter we know and see around us is made up of atoms, which consist of neg- atively charged electrons revolving around a positively charged nucleus. While the electrons are – as far as we currently know – fundamental particles, the nu- cleus contains protons and neutrons, which are in turn composed of up and down quarks. The theoretical framework that describes all these fundamental particles and their interactions is called the Standard Model of Particle Physics. While it is an extremely successful theory, multiple unresolved questions and observations cannot be explained by the Standard Model. Cosmological observations, for ex- ample, indicate that the known matter described by the Standard Model only con- tributes 15% of all the matter in the universe. The remaining matter is observed through gravitational interactions, but is not visible in observations of light at any wavelength, implying it is electrically neutral. Only very little is know about this so-called dark matter, and many theoretical models exist to explain its origin. Depending on their exact nature, dark matter particles might be produced in high-energy collisions at particle colliders. Many models assume that the dark matter particles interact weakly with ordinary matter, through a new force, making it possible to produce them in the collision of two Standard Model particles. This thesis covers two searches for dark matter performed at the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider. This par- ticle accelerator is currently the largest in the world, and provides proton-proton collisions with a record centre-of-mass energy of 13 TeV at a high collision rate. The CMS detector is a multi-purpose particle detector, used for various precision measurements of the Standard Model and many searches for new physics. In the first analysis, the dark matter particles are expected to leave the CMS detector undetected as they are neutral and weakly interacting. When they are produced in association with other particles, they can however be observed due to an imbalance of energies measured in the detector, called missing energy. This technique is used in the first dark matter search described in this thesis, called the monojet analysis, where the missing energy is balanced with one or more colli- mated sprays of particles emerging from the collision, so-called jets. The work in this thesis refined the background prediction and thus increased the sensitivity of the search. No significant excess above the predicted background was observed, setting new, stronger limits on several dark matter models, and excluding a larger part of the available parameter space. As no observation was made in this first analysis, a more unusual model is stud- ied as well. Instead of looking for weakly interacting massive particles, strongly interacting candidates were considered. These dark matter candidates would be produced in pairs through a new mediating particle, which has a probability to in- teract with matter that is similar to protons or neutrons. As a result, these particles will leave a signal in the detector that is similar to neutrons, which are electrically neutral as well. The investigated signature is therefore a pair of neutral or so-called trackless jets, which can efficiently be differentiated from the background consist- ing of charged jets. The result of this search is compatible with the predicted background, and again a part of parameter space was excluded. To conclude, the two searches covered in this thesis are very complementary, as the missing transverse energy signature used in the monojet search can trans- form into a trackless jets signature when the interaction probability becomes large enough. Although no sign of new physics was observed, these searches have led to the exclusion of more dark matter scenarios.