TY - BOOK

T1 - Searching for Heavy Neutral Leptons at CERN

AU - Tastet, Jean-Loup

PY - 2021

Y1 - 2021

N2 - The Standard Model of particle physics (SM) is our best description of matterand interactions at subatomic scales. Despite its flawless record at describingthe results of high-energy experiments, it cannot be a fundamental theory, forit fails to describe a number of well-established observational phenomena: itcontains massless neutrinos (in contradiction to the observed neutrino flavoroscillations), cannot explain the observed matter-antimatter asymmetry of ourUniverse, and does not provide a candidate for the elusive dark matter.One of the simplest extensions of the Standard Model which could addressseveral — if not all — of these shortcomings consists in adding back the “missing” gauge singlet counterparts to neutrinos. These SM singlets can have aMajorana mass, whose scale is a priori unknown. If this Majorana mass is ator below the electroweak scale, the corresponding mass eigenstates — heavyneutral leptons (HNLs) — would interact solely through a small mixing withneutrinos. As a prime example of feebly interacting particles, they might haveevaded detection so far due to their tiny interactions. HNLs are currently beingactively searched by multiple experiments, and are among the main motivationsfor future “intensity-frontier” facilities, which will be uniquely sensitive to rareprocesses.This thesis, presented as a collection of three articles, investigates phenomenological aspects of heavy neutral leptons, in relation to their search atcurrent or proposed experiments. It concentrates on testing those propertiesof HNLs which are essential for resolving the aforementioned shortcomingsof the SM. The first article discusses whether one could test the HNL massdegeneracy — a core requirement for HNLs to generate the observed baryonasymmetry of the Universe — by observing their oscillations at the proposedSHiP experiment. The second investigates whether a new search channel atthe NA62 experiment could be used to close a currently unconstrained regionin parameter space. The last article reinterprets the results of an existing experimental search for HNLs by the ATLAS experiment within a minimal yetrealistic model of neutrino oscillations. By providing a scheme which allows toeasily recast their exclusion limits for arbitrary model parameters, this workcould greatly increase the scientific return of collider searches for HNLs.In summary, this thesis demonstrates how a minimal, realistic model ofheavy 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.

AB - The Standard Model of particle physics (SM) is our best description of matterand interactions at subatomic scales. Despite its flawless record at describingthe results of high-energy experiments, it cannot be a fundamental theory, forit fails to describe a number of well-established observational phenomena: itcontains massless neutrinos (in contradiction to the observed neutrino flavoroscillations), cannot explain the observed matter-antimatter asymmetry of ourUniverse, and does not provide a candidate for the elusive dark matter.One of the simplest extensions of the Standard Model which could addressseveral — if not all — of these shortcomings consists in adding back the “missing” gauge singlet counterparts to neutrinos. These SM singlets can have aMajorana mass, whose scale is a priori unknown. If this Majorana mass is ator below the electroweak scale, the corresponding mass eigenstates — heavyneutral leptons (HNLs) — would interact solely through a small mixing withneutrinos. As a prime example of feebly interacting particles, they might haveevaded detection so far due to their tiny interactions. HNLs are currently beingactively searched by multiple experiments, and are among the main motivationsfor future “intensity-frontier” facilities, which will be uniquely sensitive to rareprocesses.This thesis, presented as a collection of three articles, investigates phenomenological aspects of heavy neutral leptons, in relation to their search atcurrent or proposed experiments. It concentrates on testing those propertiesof HNLs which are essential for resolving the aforementioned shortcomingsof the SM. The first article discusses whether one could test the HNL massdegeneracy — a core requirement for HNLs to generate the observed baryonasymmetry of the Universe — by observing their oscillations at the proposedSHiP experiment. The second investigates whether a new search channel atthe NA62 experiment could be used to close a currently unconstrained regionin parameter space. The last article reinterprets the results of an existing experimental search for HNLs by the ATLAS experiment within a minimal yetrealistic model of neutrino oscillations. By providing a scheme which allows toeasily recast their exclusion limits for arbitrary model parameters, this workcould greatly increase the scientific return of collider searches for HNLs.In summary, this thesis demonstrates how a minimal, realistic model ofheavy 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.

M3 - Ph.D. thesis

BT - Searching for Heavy Neutral Leptons at CERN

PB - Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -