Version 11 (modified by richardphysics, 5 years ago) (diff)

Updates Model File text and notes. Readying for publication.


Richard Ruiz

Model Description

The Type I Seesaw Model extends the Standard Model (SM) field content by introducing three right-handed (RH) neutrinos, which are singlets under the SM gauge symmetry (no color, weak isospin, or weak hypercharge charges), as well as one RH Majorana mass for each RH neutrino. After electroweak symmetry breaking, the effective Type I Seesaw Lagrangian with three heavy Majorana neutrinos Ni (with i=1,2,3) is given by [ 1 ]

$\mathcal{L}_{\rm Type~I} = \mathcal{L}_{\rm SM} + \mathcal{L}_{N} + \mathcal{L}_{N~\text{Int.}} $,

where the first term is the Standard Model Lagrangian; in the mass basis, the heavy Majorana neutrinos' kinetic and mass terms are

$\mathcal{L}_{N} = \frac{1}{2}\overline{N_k} i\!\not\!\partial N_k - \frac{1}{2}m_{N_k} \overline{N_k}N_k, \quad k=1,\dots,3$;

and its interactions with the Weak gauge and Higgs bosons in the mass eigenbasis, i.e., after mixing with active neutrinos, are given by

\mathcal{L}_{N~\text{Int}} = 
&-&\frac{g}{\sqrt{2}} W_{\mu}^{+}\sum_{k=1}^{3}\sum_{\ell=e}^{\tau} \overline{N_k}V_{\ell k}^{*}\gamma^{\mu}P_{L}\ell^{-}
+{\rm H.c.}
&-&\frac{g}{2\cos\theta_W}Z_{\mu}\sum_{k=1}^{3}\sum_{\ell=e}^{\tau} \overline{N_k}V_{\ell k}^{*}\gamma^{\mu}P_{L}\nu_\ell
+{\rm H.c.}
&-&\frac{g m_N}{2 M_W}         h \sum_{k=1}^{3}\sum_{\ell=e}^{\tau} \overline{N_k}V_{\ell k}^{*}P_{L}\nu_\ell
+{\rm H.c.}

The mixing matrix elements Vlk parametrize the mixing between the (active) lepton flavor eigenstate l and the heavy mass eigenstate corresponding to the mass eigenvalue mNk.

Model Files

  • Main model file. Relies on (default FR model file) being declared elsewhere.
  • heavyN.nb: Mathematica notebook file that loads FR and model. Allows user to run quick sanity checks (optional) and outputs UFO file.
  • Seesaw_TypeI.tgz: Standalone package containing, heavyN.nb, massless.rst (default FR file), diagonalCKM.rst (default FR file), and (default FR file).
  • Seesaw_TypeI_withUFO.tgz: Same as Seesaw_TypeI.tgz but with UFO folder generated using restrictions massless.rst (only massive fermions are top quark and Majorana neutrino) and diagonalCKM.rst (CKM matrix set to unity).


  • To download any of the packages and unpack via the terminal, use the commands:

~/Path $ wget

~/Path $ tar -zxvf Seesaw_TypeI.tgz

~/Path $ wget

~/Path $ tar -zxvf Seesaw_TypeI_WithUFO.tgz

  • In addition to the SM parameters, this model contains 15 free parameters:
    • Three masses: mN1, mN2, mN3. Defaults are 300 GeV, 500 GeV, and 1 TeV, respectively.
    • Three widths: WN1, WN2, WN3. Defaults are 0.303 GeV, 1.50 GeV, and 12.3 GeV, respectively.
    • Nine mixing parameters, Vlk for l = e, mu, tau and k = 1,2,3. Default values are Vlk = Identity(3x3), i.e., Ve1 = Vmu2 = Vta3 = 1 and all others zero.
    • Note: VlN are restricted to be real in the model file.


  • The model file was implemented in Ref. [ 2 ] using FeynRules v2.0.6 and MG5_aMC@NLO (MG5) v2.1.0. See Tables 2 and 5 for further validation checks; note that mixing matrices have been factored out/normalized in tabulated results.

List of analyses that have used the model file

  • Please email me to update this space.


[1] A. Atre, T. Han, S. Pascoli and B. Zhang, The Search for Heavy Majorana Neutrinos, JHEP 0905, 030 (2009) arXiv:0901.3589 [hep-ph]

[2] D. Alva, T. Han, R. Ruiz, Heavy Majorana neutrinos from #!latex $W\gamma$ fusion at hadron colliders, JHEP 1502, 072 (2015), arXiv:1411.7305 [hep-ph]

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