Last modified 5 years ago Last modified on 01/10/13 15:36:41

Type III Seesaw


  • Carla Biggio
    • Institut de Fisica d'Altes Energies (IFAE), Universitat Autonoma de Barcelona (UAB), Spain
    • email: biggio _at_
  • Florian Bonnet
    • Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Padova, Italy
    • email: florian.bonnet _at_

Model Description

This model describes the minimal realisation of the Type III seesaw. It is an extension of the Standard Model where a fermionic SU(2) triplet is added to the matter content in order to give mass to one neutrino via a seesaw mechanism. The fermionic triplet is introduced under the form of a heavy Dirac charged lepton tr+/- and a heavy Majorana neutral lepton tr0. For an exhaustive description of the model, see references below.


  • "mu->e gamma and tau->l gamma decays in the fermion triplet seesaw", A. Abada, C. Biggio, F. Bonnet, B. Gavela, T. Hambye, Phys. Rev. D78 (2008) 033007 arXiv:0803.0481
  • "Implementation of the Type III Seesaw model in FeynRules/MadGraph and prospects for discovery with early LHC data", C. Biggio, F. Bonnet, accepted for publication on Eur. Phys. J. C arxiv:1107.3463

Model Files

  • For MadGraph/MadEvent and BRIDGE users: this is a modified version of the file, that can be useful when dealing with small mixing angles (the original file sets to zero couplings smaller than 10-5)
  • typeIIIseesaw.nb: this is an example of the Mathematica notebook that loads the model, performs few checks, and export it to the MadGraph and CalcHep format.


In order to validate the implementation of the model we have checked several processes:

  • Comparison with the FeynRules generated Standard Model in MadGraph (sm_FR) of SM 2-to-2 processes :
    • l+l- -> ll
    • l+l- -> VV
    • VV -> VV
    • HH -> VV
    • GG -> GG
    • qq~ -> qq~
    • qq~ -> VV
    • qq~ -> GG

  • Comparison with the FeynRules generated Standard Model in MadGraph (sm_FR) of SM particles decay widths and branching ratios calculated with BRIDGE

  • Comparison with the theoretical predictions of the branching ratios calculated with BRIDGE
  • Comparison with previous studies (see Phys.Rev D78 (2008) 033002) of the production cross section of a pair of triplets at the LHC with 14 TeV as center-of-mass energy:
    • pp -> tr+tr-
    • pp -> tr+/-tr0

Details of the validation procedure can be found on arxiv:1107.3463.


The model has been tested on the following machines configurations:

  • iMac Intel Core 2 Duo, Mac OS X 10.6.6, FeynRules v1.4.3/FeynRulesv1.6beta, Mathematica 8
  • Mac Book Intel Core Duo, Mac OS X 10.4.11, FeynRules v1.4.3, Mathematica 6

We also checked the compatibility of our model with Madgraph version 4 and version 5. (N.B.: when running this model with mg4 some channels are not generated. Please, use mg5 to have the correct result.)


  • We implemented the couplings to Goldstone bosons so that the implementation allows for the use of the Feynman gauge. We have checked that the exportation to the CalcHep format is successful, however we did not check the CalcHep output.
  • N.B.: when using this model with CalcHep the variables (such as input parameters) longer than 6 characters must be redifined to shorter words (thanks to Jayash Panigrahi).
    • 1.1: "IndexRange[ Index[SU2W] ] = Range[3]" has been substituted by "IndexRange[ Index[SU2W] ] Unfold[Range[3]]" , in order to get UFO output. If you do not need them, you can use the 1.0 version.