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 new search channel at
the NA62 experiment could be used to close a currently unconstrained region
in parameter space. The last article reinterprets the results of an existing experimental search for HNLs by the ATLAS experiment within a minimal yet
realistic model of neutrino oscillations. By providing a scheme which allows to
easily recast their exclusion limits for arbitrary model parameters, this work
could greatly increase the scientific return of collider searches for HNLs.
In summary, this thesis demonstrates how a minimal, realistic model of
heavy neutral leptons can nonetheless have a rich phenomenology, and discusses some important implications for experiments. In particular, it highlights the impact of model assumptions on experimental limits, and the need to interpret results within realistic models.