Abstract
New Physics models in which the Standard Model particle content is enlarged via the addition of sterile fermions remain among the most minimal and yet most appealing constructions, particularly since these states are present as building blocks of numerous mechanisms of neutrino mass generation. Should the new sterile states have non-negligible mixings to the active (light) neutrinos, and if they are not excessively heavy, one expects important contributions to numerous high-intensity observables, among them charged lepton flavour violating muon decays and transitions, and lepton electric dipole moments. We briefly review the prospects of these minimal SM extensions to several of the latter observables, considering both simple extensions and complete models of neutrino mass generation. We emphasise the existing synergy between different observables at the Intensity Frontier, which will be crucial in unveiling the new model at work.
Highlights
Several observational problems fuel the need to extend the Standard Model (SM): among them, the baryon asymmetry of the Universe (BAU), the absence of a dark matter candidate, and neutrino oscillation phenomena
A common ingredient of many of the previously mentioned possibilities is the presence of additional neutral leptons, sterile states with a mass mνs, which only interact with the active neutrinos and possibly the Higgs
The phenomenological appeal of Heavy neutral leptons (HNL) is manifest: their non-negligible contributions might either lead to ease some existing tensions between SM predictions and experimental data, or render these SM extensions testable and even falsifiable. We focus on these New Physics (NP) candidates, and discuss the impact that they might have for numerous observables which can be probed at the high-intensity frontier
Summary
Several observational problems fuel the need to extend the Standard Model (SM): among them, the baryon asymmetry of the Universe (BAU), the absence of a dark matter candidate, and neutrino oscillation phenomena (i.e. neutrino masses and mixings). Depending on their mass regime and on their couplings to the “active” neutrinos, the new neutral fermions can lead to very distinctive phenomenological features, which in turn identify the possible means to explore their presence: additional neutral leptons can be searched for in cosmology and astrophysics, in high energy colliders, or in high-intensity experiments Concerning the latter, the fermionic singlets can be responsible for contributions to electric and magnetic leptonic dipole moments, and be responsible for numerous rare transitions and decays, including charged lepton flavour violating (cLFV), lepton number violating (LNV) and lepton flavour universality violating (LFUV) observables. We will consider two complementary approaches, firstly discussing the phenomenological impact of minimal SM extensions via a number nS of heavy sterile states (bottom-up approach, or “3 + nS toy-models”), and subsequently consider contributions of the HNL to several observables when the latter are naturally embedded in the framework of complete models of New Physics. Our final comments and discussion are collected in the Conclusions
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