Searching for Heavy Neutral Leptons at CERN

The Standard Model of particle physics (SM) is our best description of matter and interactions at subatomic scales. Despite its flawless record at describing the results of high-energy experiments, it cannot be a fundamental theory, for it fails to describe a number of well-established observational phenomena: it contains massless neutrinos (in contradiction to the observed neutrino flavor oscillations), cannot explain the observed matter-antimatter asymmetry of our Universe, and does not provide a candidate for the elusive dark matter. One of the simplest extensions of the Standard Model which could address several — if not all — of these shortcomings consists in adding back the “missing” gauge singlet counterparts to neutrinos. These SM singlets can have a Majorana mass, whose scale is a priori unknown. If this Majorana mass is at or below the electroweak scale, the corresponding mass eigenstates — heavy neutral leptons (HNLs) — would interact solely through a small mixing with neutrinos. As a prime example of feebly interacting particles, they might have evaded detection so far due to their tiny interactions. HNLs are currently being actively searched by multiple experiments, and are among the main motivations for future “intensity-frontier” facilities, which will be uniquely sensitive to rare processes. This thesis, presented as a collection of three articles, investigates phenomenological aspects of heavy neutral leptons, in relation to their search at current or proposed experiments. It concentrates on testing those properties of HNLs which are essential for resolving the aforementioned shortcomings of the SM. The first article discusses whether one could test the HNL mass degeneracy — a core requirement for HNLs to generate the observed baryon asymmetry of the Universe — by observing their oscillations at the proposed SHiP experiment. The second investigates whether a