# Feynrules + FeynArts - Calculating amplitudes

I am working on a model which is an extended model of the standard model (SM + Complex Scalar Field).

I am using FeynCalc (9.0.3) patched with FeynArts (3.11).

I run SetOptions[FourVector, FeynCalcInternal -> False]; such that I don't run into this.

I compute:

oneloop = CreateTopologies[1, 1 -> 2, ExcludeTopologies -> {Tadpoles, SelfEnergies}];
decay = InsertFields[oneloop, S[1] -> {S[5], S[5]}, InsertionLevel -> {Particles}, Model -> NotebookDirectory[] <> "SMH2DM_FA/SMH2DM_FA", GenericModel -> NotebookDirectory[] <>"SMH2DM_FA/SMH2DM_FA"];
amps = CreateFeynAmp[decay]


And everything runs smoothly. But I get some weird $$\xi$$ factors which I do not understand. I get something like this:

$$\int (...) \text{PropagatorDenominator}\left(\text{q}1,\text{MH} \sqrt{\xi _{S(\text{Gen}4)}}\right)\text{PropagatorDenominator}\left(\text{q}1-\text{k}1,\text{MA1} \sqrt{\xi _{S(\text{Gen}6)}}\right)\text{PropagatorDenominator}\left(\text{k}1+\text{k}2-\text{q}1,\text{MH} \sqrt{\xi _{S(\text{Gen}5)}}\right) dq1$$

If I run these commands which just FeynArts (no FeynCalc) I get:

$$\int (...) \text{FeynAmpDenominator}\left(\frac{1}{(\text{q}1)^2-\text{MH}^2},\frac{1}{(\text{q}1-\text{k}1)^2-\text{MA1}^2},\frac{1}{(-(\text{k}1)-\text{k}2+\text{q}1)^2-\text{MH}^2}\right) dq1$$

What is happening here?

You have to patch the model for it to be imported properly. If the model is in the /Feynarts/Models folder you do:

FAPatch[PatchModelsOnly -> True]


If the model is in the notebook's directory:

FAPatch[PatchModelsOnly -> True,
FAModelsDirectory ->
FileNameJoin[{NotebookDirectory[], "MyModelDir"}]]