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@…

A complete derivation and description of the model can be found in ​Eur.Phys.J.C 81 (2021) 1, 78 (​2007.03701v2). Please cite accordingly when making use of these model files.

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 couplings to the rest of the SM.

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), introducing low-energy effective operators which account for the interactions of on-shell mesons with 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 vertices which involve mesons and 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 into its mixings to electron, muon and tau neutrinos respectively. These three mixings 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.

Please refer to the instructions attached below for full technical details on the usage of the model files, as well as their applicability.

Model files and extensions

• ​https://feynrules.irmp.ucl.ac.be/attachment/wiki/HNLs/effective_HeavyN_Dirac.fr: Model file for Dirac neutrinos.
• ​https://feynrules.irmp.ucl.ac.be/attachment/wiki/HNLs/effective_HeavyN_Majorana.fr: Model file for Majorana neutrinos.
• ​https://feynrules.irmp.ucl.ac.be/attachment/wiki/HNLs/README.txt: Models usage instructions and applicability.
• ​https://feynrules.irmp.ucl.ac.be/attachment/wiki/HNLs/generate_form_Factors.py: Script to generate energy-dependent form factors.