=== Author ===
Richard Ruiz
* University of Durham (IPPP) 
* richard.ruiz AT durham.ac.uk
* https://www.ippp.dur.ac.uk/~rruiz/ 


=== 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 ''N''i (with i=1,2,3) is given by [ [#Atre 1] ]
{{{ 
#!latex 
$\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
{{{ 
#!latex 
$\mathcal{L}_{N} = \frac{1}{2}\overline{N_k} i\!\not\!\partial N_k - \frac{1}{2}m_{N_k} \overline{N_k}N_k, \quad k=1,\dots,3$;
}}}
and its interactions with the Weak gauge and Higgs bosons in the mass eigenbasis, i.e., after mixing with active neutrinos, are given by
{{{ 
#!latex 
\begin{eqnarray}
\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.}
\end{eqnarray}
}}} 
The mixing matrix elements V''lk'' parametrize the mixing between the (active) lepton flavor eigenstate ''l'' and the heavy mass eigenstate corresponding to the mass eigenvalue mNk.

=== Model Files === 

 * [https://feynrules.irmp.ucl.ac.be/attachment/wiki/typeIseesaw/heavyN.fr heavyN.fr]:  Main model file. Relies on sm.fr (default FR model file) being declared elsewhere. 
 * [https://feynrules.irmp.ucl.ac.be/attachment/wiki/typeIseesaw/heavyN.nb heavyN.nb]:  Mathematica notebook file that loads FR and model. Allows user to run quick sanity checks (optional) and outputs UFO file. 
 * [https://feynrules.irmp.ucl.ac.be/attachment/wiki/typeIseesaw/Seesaw_TypeI.tgz Seesaw_TypeI.tgz]: Standalone package containing heavyN.fr, heavyN.nb, massless.rst (default FR file), diagonalCKM.rst (default FR file), and sm.fr (default FR file). 

 * [https://feynrules.irmp.ucl.ac.be/attachment/wiki/typeIseesaw/Seesaw_TypeI_WithUFO.tgz 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).

 * [https://feynrules.irmp.ucl.ac.be/attachment/wiki/typeIseesaw/Seesaw_TypeI_UFO.tgz Seesaw_TypeI_UFO.tgz]: Standalone UFO folder as packaged in Seesaw_TypeI_withUFO.tgz.




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

~/Path $ wget http://feynrules.irmp.ucl.ac.be/raw-attachment/wiki/typeIseesaw/Seesaw_TypeI.tgz

~/Path $ tar -zxvf Seesaw_TypeI.tgz

~/Path $ wget http://feynrules.irmp.ucl.ac.be/raw-attachment/wiki/typeIseesaw/Seesaw_TypeI_WithUFO.tgz

~/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, V''lk'' for ''l'' = ''e, mu, tau'' and ''k'' = 1,2,3. Default values are V''lk'' = Identity(3x3), i.e., Ve1 = Vmu2 = Vta3 = 1 and all others zero.
  * '''Note''': VlN are restricted to be real in the model file.


=== Validation ===
* The model file was implemented in Ref. [ [#Alva 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.


=== References === 
[=#Atre] [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]

[=#Alva] [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]