Heavy Neutral Leptons and interactions with mesons

Contact Authors

Pilar Coloma

• Instituto de Física Teórica UAM-CSIC
• pilar.coloma@…

Enrique Fernández-Martínez

• Universidad Autónoma de Madrid & Instituto de Física Teórica UAM-CSIC
• enrique.fernandez-martinez@…

Manuel González-López

• Universidad Autónoma de Madrid & Instituto de Física Teórica UAM-CSIC
• manuel.gonzalezl@…

Josu Hernández-García

• Institute for Theoretical Physics, ELTE Eötvös Loránd University
• garcia.josu.hernandez@…

Model Description

We extend the SM neutrino sector by including a fourth, heavy neutrino (or Heavy Neutral Lepton, HNL) and its interactions with SM particles. Two versions of the model are available, accounting for Dirac and Majorana neutrinos respectively. As it is a gauge singlet, the HNL will only interact via mixing with the light neutrinos, inheriting their interactions. In particular, we focus on how the HNL interacts with mesons.

In order to do so, we remove the quarks and replace them with mesons (including pions, kaons, η, ρ's, ω, φ, D's and D_s's, with their corresponding masses and decay constants), developing a low-energy effective theory which accounts for the interactions of on-shell mesons which involve HNLs. This allows to compute HNL production via meson decay and HNL decay into lighter mesons, as well as any purely leptonic process. Note that we have only included meson interactions with HNLs and SM leptons, and not the complete meson Lagrangian. Purely hadronic processes are beyond the scope of this model.

The HNL interactions are controlled by its mass (set by default to 1 GeV) and its Yukawa couplings, which can be translated to its mixings to electron, muon and tau neutrinos respectively. These three parameters are set by default to 0.001. All these four parameters can be modified in order to explore wide regions of the parameter space.

For the sake of generality, the models include a switch which allows to remove the mesons and restore the SM quark content, keeping the HNL interactions to W and Z bosons. This allows to study processes involving heavy neutrinos at higher energies, such as a collider environment, where the low-energy effective treatment of mesons is not valid anymore. Note that both quarks and mesons are not allowed to appear simultaneously in order to avoid double countings.