LHCTOPVLQ · Direct and indirect searches for new physics in events with top quarks using LHC proton-proton collisions at the CMS detector

Direct and indirect searches for new physics in events with top quarks using LHC proton-proton collisions at the CMS detector

The standard model (SM) of particle physics is currently the state of the art in describing the behaviour of particles at the most elementary (i.e. fundamental) level. It is a successful theory that can make very precise predictions in many areas of physics, but it has known limitations and is only an approximate and incomplete description of the physics of the Universe. Many theories exist for the fundamental nature of physics beyond the SM (BSM), but none are significantly favoured by existing experimental evidence. The discovery of a new particle, or significant deviations in the behaviour of particles from the predictions of the SM, would revolutionise our understanding of nature, and set the research agenda for the coming decades. The top quark is the heaviest elementary particle in the SM, a fact that may be connected to the theory of physics beyond the SM — particularly that of the Higgs mechanism of electroweak symmetry breaking, which is responsible for the existence of elementary particle mass. In this project, we look for signs of physics beyond the SM predictions in events with top quarks using LHC proton-proton collisions at the CMS detector. A general analysis framework to search for new particles in the CMS data, signalled by an excess of events with properties consistent with new particles and not the predicted background events from the well-known SM particle production processes, is developed. The search for a new particle related to the top quark (a top quark partner), a common prediction of new theories, is conducted as an example application. The strategy of directly searching for new particles is complemented by an indirect but model-independent search for new physics interacting with top quarks, by looking for deviations from SM predictions in precision measurements of the behaviour of the top quark. In order to enable these and similar searches to continue at the future High Luminosity LHC, we study the performance of the upgraded trigger for events with top quarks, including optimisation of reconstruction algorithms and working points.

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