MSc thesis defense by Marie-Louise Riis

Title: Piggyback Riding the Higgs Searches: A Hunt for Heavy Neutral Leptons with Tau Triggers. A study of the ATLAS experiment’s sensitivity to Heavy Neutral Leptons coupled to τ-leptons


The Large Hadron Collider (LHC) at CERN is the most powerful particle accelerator today, performing collisions of protons at the highest achievable centre-of-mass energy, and a high intensity of collisions makes it possible to use its data to probe for rare events. LHC possesses sophisticated means, called triggers, in to single out events of interest from among the huge amount of background processes. A number these triggers have been specifically created to probe the nature of the Higgs boson.

In this thesis we demonstrate for the first time how the same technology could be used to search for hypothetical particles – Heavy Neutral Leptons (HNLs). HNLs can mediate neutrino oscillations and at the same time be responsible for generation of baryon asymmetry of the Universe. This makes them interesting candidates for ‘beyond the Standard Model particles’. HNLs have interactions with electron, muon and tau flavours, each governed by its own coupling constant. Up to now only couplings to the electron and muon flavors have been explored at the LHC and no probes of the interaction with the tau channel have been performed. We develop a search strategy that utilises existing LHC triggers tuned towards the detection of two tau-leptons.

We demonstrate that if production of heavy neutral lepton is accompanied by one or two jets triggers select a sizeable amount of the HNL-mediated events. The resulting signature process is a tri-lepton signal with two tau-leptons and one electron or muon and with extra jet(s). We demonstrate that the searches by the ATLAS experiment for such events hold a discovery potential or can improve the existing limits on the coupling strength between HNL and tau flavour by about an order of magnitude (during Run 3) or even more (during the high-luminosity phase). The improvements are possible within the mass range from about 5 GeV/c2 to about 50 GeV/c2. Our estimates are done at the signal-only level and may be over-optimistic. They demonstrate nevertheless the feasibility of piggy-back riding on the ‘shoulders’ of Higgs searches.

Censor: Rasmus Mackeprang