Heavy neutral lepton corrections to SM boson decays: lepton flavour universality violation in low-scale seesaw realisations

TL;DR

This paper investigates how low-scale seesaw models of neutrino mass can cause lepton flavour universality violation in SM boson decays, emphasizing the significance of electroweak corrections, especially in invisible Z decays, for constraining such models.

Contribution

It provides a detailed calculation of electroweak NLO corrections in low-scale seesaw models and highlights the importance of invisible Z decay measurements in testing these models.

Findings

Electroweak corrections can be as large as current experimental uncertainties.

Invisible Z decays offer stringent constraints on the Inverse Seesaw model.

Future experiments like FCC-ee can improve the probing of these models.

Abstract

We study lepton flavour universality violation in SM boson decays in low-scale seesaw models of neutrino mass generation, also addressing other electroweak precision observables. We compute the electroweak next-to-leading order corrections, which turn out to be important - notably in the case of the invisible decay width of the $Z$ boson, for which the corrections can be as large as the current experimental uncertainty. As a well-motivated illustrative study case, we choose a realisation of the Inverse Seesaw mechanism, and discuss the complementary role of lepton flavour conserving, lepton flavour violating and precision observables, both in constraining and in probing such models of neutrino mass generation. Our findings suggest that invisible $Z$ decays are especially important, potentially at the origin of the most stringent constraints for certain regimes of the Inverse Seesaw (while complying with charge lepton flavour violation and other electroweak precision tests). We also discuss the probing power of the considered observables in view of the expected improvement in experimental precision at FCC-ee.