wiki:HNLs

Version 5 (modified by ManuelGonzalezLopez, 4 years ago) ( diff )

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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).

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. Of course, the measured neutrino masses and mixings cannot be reproduced with the inclusion of a single extra heavy state. That is not the purpose of this model; we rather work in a 3 + 1 scenario, assuming that only one heavy neutrino is light enough or exhibits sufficiently large mixings to be phenomenologically relevant.

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. Note that both quarks and mesons are not allowed to appear simultaneously in order to avoid double countings.

Specific instructions

  • The effective theory has been computed for on-shell mesons: mesons propagating in internal should be avoided by imposing computations at leading order in QED.
  • Semileptonic meson decays involve energy-dependent form factors. The model files include constant, averaged form factors, which provide good approximations. We provide a Python script that allows to modify the corresponding UFO to account for the correct energy dependance. Note that these form factors account for meson decays and would provide incorrect results for semileptonic lepton decays (although the vertices exist).
  • The model files are written in terms of neutrino mass eigenstates, which do not correspond to oscillation eigenstates (as we only include one HNL, only one active neutrino acquires a mass) or to the SM flavor eigenstates (except for the Dirac case in the limit of small mixings). Thus, we have given new particle IDs to the neutrinos.
  • The meson interactions implemented do not allow to generate HNL decays into 3 or more mesons, although this kind of processes can be estimated from the corresponding HNL decays into quarks. See the article referenced above for further details.

Please refer to instructions attached below for full technical details on the usage of the model files.

Model files and extensions

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