== The interfaces ==
FeynRules has a couple of interfaces to other tools, allowing for an easy implementation a new model. Currently the following interfaces are supported by FeynRules:
* [:CalcHepInterface:CalcHep/CompHEP]
* [:FeynArtsInterface:FeynArts]
* [:MadGraphInterface:MadGraph]
* [:SherpaInterface:Sherpa]
For more details on how to run the interfaces in general please have a look at the [:UserManual:user manual].
==== The MC conventions ====
A couple of conventions concerning the SM gauge groups are shared by all MC interfaces. This it needed in order to treat in the correct way the running coupling of the coupling constants.
* The symbols referring to the SU(3) representation matrices, the structure constants and the symetric tensors are fixed to {{{T}}}, {{{f}}} and {{{dSU3}}}, and the SU(3) indices in the funcdamental and adjoint representations must be {{{Colour}}} and {{{Gluon}}} respectively. The declaration of the SU(3) gauge group is then {{{ SU3C == {
Abelian -> False,
GaugeBoson -> G,
StructureConstant -> f,
DTerm -> dSU3,
Representations -> {T, Colour},
CouplingConstant -> gs}
}
}}}
* The gauge couplings of the SM must be called {{{gs}}}, {{{gw}}} and {{{ee}}} respectively for the strong, the weak and the electromagnetic coupling.
* The electric charge and the hypercharge are named {{{Q}}} and {{{Y}}} respectively.
* The numerical values for the gauge couplings are assigned using the {{{SMINPUTS}}} block of the Les Houches accord, ''i.e.''
* The inverse of the electromagnetic coupling at the Zpole.
* The Fermi constant.
* The strong coupling constant.
* The Z pole mass

All of these for parameters '''must''' be declared as '''external''' parameters in the following way:
{{{ \[Alpha]EWM1== {ParameterType -> External,
BlockName -> SMINPUTS,
ParameterName -> aEWM1,
InteractionOrder -> {QED, 2},
Value -> 127.9,
Description -> "Inverse of the electroweak coupling constant"},
Gf == {ParameterType -> External,
BlockName -> SMINPUTS,
InteractionOrder -> {QED, 2},
Value -> 1.16639 * 10^(-5),
Description -> "Fermi constant"},
\[Alpha]S == {ParameterType -> External,
BlockName -> SMINPUTS,
ParameterName -> aS,
InteractionOrder -> {QCD, 2},
Value -> 0.118,
Description -> "Strong coupling constant at the Z pole."},
ZM == {ParameterType -> External,
BlockName -> SMINPUTS,
Value -> 91.188,
Description -> "Z mass"}}}}
* All the other gauge couplings, {{{ee}}}, {{{gw}}} and {{{gs}}} get their values through internal parameters. It turns out to be convenient to introduce the internal parameter αEW: {{{{{{ \[Alpha]EW == {ParameterType -> Internal,
Value -> 1/\[Alpha]EWM1,
ParameterName -> aEW,
Description -> "Electroweak coupling contant"},
ee == {ParameterType -> Internal,
Value -> Sqrt[4 Pi \[Alpha]EW],
InteractionOrder -> {QED, 1},
Description -> "Electric coupling constant"},
gw == {ParameterType -> Internal,
Value -> ee / sw,
InteractionOrder -> {QED, 1},
Description -> "Weak coupling constant"},
gs == {ParameterType -> Internal,
Value -> Sqrt[4 Pi \[Alpha]S],
InteractionOrder -> {QCD, 1},
ParameterName -> G,
Description -> "Strong coupling constant"}}}}}}}
where {{{sw}}} denotes the sin of the weak mixing angle.
Furthermore, in order to avoid a problem in numerical codes, all parameters (external and internal, as well as masses and decay rates) for which no numerical value was specified in the model file are automtically assigned the default value 1. Also, all particle where no PDG code was given, get an automatically assigned PDG code (starting from 6000001). However, it is '''strongly recommended''' to use the existing PDG codes whenever possible.
-- Main.ClaudeDuhr - 09 Feb 2008