Name: Vladyslav Shtabovenko (email_not_shown)
Date: 12/24/16-06:08:25 PM Z


The "ComplexInfinity" error was caused by the typesetting of PropagatorDenominator[0,0] and has nothing to do with the physics.
This is now fixed in the development version.

The diagram itself is zero in DR. One should of course better introduce some regulator for the massless photon propagator, although the result itself doesn't depend on it.

$LoadFeynArts = True;
<< FeynCalc`

$FAVerbose = 0;
tops = CreateTopologies[1, 1 -> 1];
ins = InsertFields[tops[[2]], F[1, {1}] -> F[1, {1}], Model -> QED,
   InsertionLevel -> {Particles}];
Paint[ins];

amp = FCFAConvert[CreateFeynAmp[ins], UndoChiralSplittings -> True,
   SMP -> True, LoopMomenta -> {l}, List -> False,
   ChangeDimension -> D, IncomingMomenta -> {p},
   OutgoingMomenta -> {p}] /. PD[0, 0] -> PD[0, mIR]
amp // Contract // TID[#, l] &

gives 0.

Cheers,
Vladyslav

> Dear All,
>
> Can anyone enlighten me as to how to handle complex infinities that arise in tadpole diagrams involving massless particles? For instance the following calculation in QED:
>
> tops = CreateTopologies[1, 1 -> 1];
> ins = InsertFields[tops[[2]], F[1, {1}] -> F[1, {1}], Model -> QED,
> InsertionLevel -> {Particles}];
> CreateFeynAmp[ins]
>
> Produces three diagrams whose amplitudes involve 'ComplexInfinity' due to the zero mass of the photon. How should these diagrams be handled by FeynArts/FeynCalc?
>
> Many thanks
> Jon



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