ONLINE COLLOQUIUM: "Illuminating dark matter with precision top-quark measurements" by Clara Nellist (Radboud University Nijmegen)



The Large Hadron Collider (LHC) at CERN, Switzerland, is the largest and most powerful particle accelerator ever built, colliding protons or ions in the centre of complex detectors located around the 27km ring. The LHC recently completed its seventh year of data acquisition and fourth year at 13 TeV collision energy. One of its greatest results so far was the discovery of the Higgs boson in 2012 by the ATLAS and CMS collaborations. Yet many important scientific questions, such as the nature of dark matter, still remain unanswered, and the search for physics beyond the Standard Model continues. The top quark is the most massive known fundamental particle, although the reason for its large mass is not fully understood. Due to its unique properties, the top quark is potentially sensitive to effects of new physics beyond the Standard Model (SM) and so it is crucial to perform precision measurements of top-quark properties and couplings in order to search for these hints. This is becoming possible with the unprecedented amount of data provided by the second operational run of the Large Hadron Collider (LHC). With this substantial dataset, we can now start to measure very rare cross-sections, such as the production of four top quarks in the final state. This is so rare that 70 000 top-quark pairs are produced at the LHC at 13 TeV for every four-top event. I will present the most recent results of the cross-section measurements with four top quarks in the final state by the ATLAS and CMS collaborations. Results for the coupling of top quarks with additional bosons (W, Z and H) will also be discussed for comparison. Possible enhancements of very rare SM cross-sections from new physics through the production of heavy objects in association with a top-quark pair are also possible. These enhancements, if discovered, would be potential dark matter candidates. To conclude the colloquium, I will discuss the exciting prospects for finding hints of dark matter through measurements with top quarks in the final state during Run 3 and at the High Luminosity LHC.