QCD calculation: Webber et al. Sec. 7 (with FeynRules)

Two-jet cross sections (Only QCD; mass ignored) again

Initialization

Exit[];

$Language="English";
<<FeynArts`
<<FormCalc`

FeynRules model files

model = FileNameJoin[{NotebookDirectory[],"models","SM","Standard_Model_FA","Standard_Model_FA"}]

You can check whether the files exist or not by:

FileExistsQ[model<>".mod"]
FileExistsQ[model<>".gen"]

Create Topology (and also ignore helicities)

topologies=CreateTopologies[0,2->2];
_Hel=0;

u c -> u c

diag=InsertFields[topologies,{F[3,{1}],F[3,{2}]}->{F[3,{1}],F[3,{2}]},InsertionLevel->{Classes},
ExcludeParticles->{V[1],V[2],V[3],V[4],S[1],S[2]},
GenericModel->model,
Model->model
];
Paint[diag]

This is wrong.
In a different model, particles have different labels, and parameters have different names.
We have to check the model file (.mod) to see which particle is which.

In this model, gluon is labelled as V[3]. So we should not “Exclude” V[3].

diag=InsertFields[topologies,{F[3,{1}],F[3,{2}]}->{F[3,{1}],F[3,{2}]},InsertionLevel->{Classes},
ExcludeParticles->{S[1],S[2],V[1],V[2]},
GenericModel->model,
Model->model
];
Paint[diag];

(* Remember the prefactor 4/9 *)
ClearProcess[];
amp = CalcFeynAmp[CreateFeynAmp[diag],FermionChains->VA];
sq=SquaredME[amp];
matrixelement = FullSimplify[(4/9)sq[[1]]//.sq[[2]]//.HelicityME[amp]//.ColourME[amp]]

matrixelement//.{MC2->0,MU2->0,Den[a_,b_]:>(1/(a-b))}

Here we see a unknown parameter.
The interpretation is stored in M$FACouplings.

M$FACouplings

matrixelement //. M$FACouplings //. {MC2->0,MU2->0,Den[a_,b_]:>(1/(a-b))}

This result is the same as we saw in Lecture1-4.nb.