Name: V. Shtabovenko Date: 11/15/17-03:57:09 AM Z

Hi Luca,

OneLoop is essentially broken since many FeynCalc versions. The function
does a lot of weird things (like mixing D- and 4-dimensional tensors)
which makes the final output rather ambiguous and may easily lead to
inconsistent result. Especially when the distinction between D and 4
dimensions is important like in the t’Hooft-Veltman scheme.

The Dirac algebra related functions (like DiracTrick) impose strict
checks on those things, so the “sloppiness” of OneLoop leads to a fatal
error.

This is why in the provided examples we use TID which does not suffer
from these issues.

I actually even started to rewrite OneLoop, but this is a slow process
which does not have a very high priority. Eventually it will be finished
until the next FeynCalc release.

Cheers,
Vladyslav

Am 14.11.2017 um 22:38 schrieb Luca Marzola:
> Hello guys,
>
> I have been trying to use FeynArts and FeynCalc to compute the electron g-2 in QED, similarly to the example you provide. When I invoke OneLoop I obtain the following error
>
> DiracTrick::failmsg: Error! DiracTrick has encountered a fatal problem and must abort the computation. The problem reads: Incorrect combination of dimensions and g^5 scheme!
>
> I read the question already asked by Maksym in the mailing list on the 07/24/17, but still I cannot understand what is going on. Essentially I am feeding OneLoop with the amplitude straight from FCFAConvert[]:
>
> $LoadFeynArts = True;
> «FeynCalc`;
>
> SetOptions[{Plot, ParametricPlot, LogPlot, LogLogPlot},
> BaseStyle -> FontSize -> 12, LabelStyle -> Black, Frame -> True,
> Axes -> None, FrameTicksStyle -> Black, AspectRatio -> 2/3,
> ImageSize -> 400, PlotStyle -> Thick];
> SetDirectory[NotebookDirectory[]];
>
> (*FaynArts verbosity *)
> $FAVerbose = 0;
>
> proc0 = {-F[1, {1}], F[1, {1}]} -> {V[1]};
> (*F[1,{1}] is the electron*)
>
> top1 = CreateTopologies[1, 2 -> 1,
> ExcludeTopologies -> {Tadpoles, WFCorrections}];
>
> ins1 = InsertFields[top1, proc0, ExcludeParticles -> {},
> InsertionLevel -> {Classes}, GenericModel -> “Lorentz”,
> Model -> “QED”];
>
> Paint[ins1, ColumnsXRows -> {1, 1}, Numbering -> None,
> SheetHeader -> None];
>
> amp1 = FCFAConvert[
> CreateFeynAmp[ins1, Truncated -> True,
> PreFactor -> -(2 \[Pi])^-4], ChangeDimension -> D,
> UndoChiralSplittings -> True, IncomingMomenta -> {-p2, p1},
> OutgoingMomenta -> {-p2 + p1}, LoopMomenta -> {-k}, List -> False,
> SMP -> True,
> FinalSubstitutions -> {SMP[“m_e”] -> m, SMP[“e”] -> e }];
>
> OneLoop[k, amp1, OneLoopSimplify -> True]
>
>>> DiracTrick::failmsg: Error! DiracTrick has encountered a fatal problem and must abort the computation. The problem reads: Incorrect combination of dimensions and g^5 scheme!
>
> What am I doing wrong? I�m on Mathematica 11.2.0.0, macOS high sierra, and latest installation of FeynCalc and FeynArts (installed and patched by FeynCalc with your automatic installation method).
>
> Thank you for your help and kind regards,
> L.
>