QED manual (development version)

Load FeynCalc and the necessary add-ons or other packages

description = "El -> El, massless QED, 2-loops";
If[ $FrontEnd === Null, 
    $FeynCalcStartupMessages = False; 
    Print[description]; 
  ];
If[ $Notebooks === False, 
    $FeynCalcStartupMessages = False 
  ];
$LoadAddOns = {"FeynArts"};
<< FeynCalc`
$FAVerbose = 0; 
 
FCCheckVersion[10, 0, 0];

\text{FeynCalc }\;\text{10.0.0 (dev version, 2023-12-20 22:40:59 +01:00, dff3b835). For help, use the }\underline{\text{online} \;\text{documentation}}\;\text{, check out the }\underline{\text{wiki}}\;\text{ or visit the }\underline{\text{forum}.}

\text{Please check our }\underline{\text{FAQ}}\;\text{ for answers to some common FeynCalc questions and have a look at the supplied }\underline{\text{examples}.}

\text{If you use FeynCalc in your research, please evaluate FeynCalcHowToCite[] to learn how to cite this software.}

\text{Please keep in mind that the proper academic attribution of our work is crucial to ensure the future development of this package!}

\text{FeynArts }\;\text{3.11 (3 Aug 2020) patched for use with FeynCalc, for documentation see the }\underline{\text{manual}}\;\text{ or visit }\underline{\text{www}.\text{feynarts}.\text{de}.}

\text{If you use FeynArts in your research, please cite}

\text{ $\bullet $ T. Hahn, Comput. Phys. Commun., 140, 418-431, 2001, arXiv:hep-ph/0012260}

Generate Feynman diagrams

Nicer typesetting

diags = InsertFields[CreateTopologies[2, 1 -> 1, ExcludeTopologies -> {Tadpoles}], {F[2, {1}]} -> {F[2, {1}]}, 
    InsertionLevel -> {Particles}, ExcludeParticles -> {V[2 | 3], S[_], U[_], F[1 | 3 | 4]}];

Paint[DiagramExtract[diags, {1, 2, 5}], ColumnsXRows -> {3, 1}, SheetHeader -> False,   
   Numbering -> None, ImageSize -> {768, 256}];

Obtain the amplitude

ampRaw = FCFAConvert[CreateFeynAmp[DiagramExtract[diags, {1, 2, 5}], Truncated -> True, GaugeRules -> {}, 
        PreFactor -> 1], IncomingMomenta -> {p}, OutgoingMomenta -> {p},LoopMomenta -> {q1, q2}, 
        UndoChiralSplittings -> True, ChangeDimension -> D, List -> True, SMP -> True, 
        DropSumOver -> True] // SMPToSymbol;

ampRaw = {ampRaw[[1]], Nf ampRaw[[2]], ampRaw[[3]]};

Fix the kinematics

FCClearScalarProducts[];
ScalarProduct[p, p] = pp;

Calculate the amplitude

AbsoluteTiming[ampSimp = DiracSimplify[ampRaw /. me -> 0];]

\{0.880852,\text{Null}\}

Identify and minimize the topologies

{amp, topos} = FCLoopFindTopologies[ampSimp, {q1, q2}];

\text{FCLoopFindTopologies: Number of the initial candidate topologies: }2

\text{FCLoopFindTopologies: Number of the identified unique topologies: }2

\text{FCLoopFindTopologies: Number of the preferred topologies among the unique topologies: }0

\text{FCLoopFindTopologies: Number of the identified subtopologies: }0

subtopos = FCLoopFindSubtopologies[topos];

mappings = FCLoopFindTopologyMappings[topos, PreferredTopologies -> subtopos];

\text{FCLoopFindTopologyMappings: }\;\text{Found }1\text{ mapping relations }

\text{FCLoopFindTopologyMappings: }\;\text{Final number of independent topologies: }1

Rewrite the amplitude in terms of GLIs

AbsoluteTiming[ampReduced = FCLoopTensorReduce[amp, topos];]

\{1.84682,\text{Null}\}

AbsoluteTiming[ampPreFinal = FCLoopApplyTopologyMappings[ampReduced, mappings];]

\{1.45766,\text{Null}\}

AbsoluteTiming[ampFinal = ampPreFinal // DiracSimplify;]

\{2.81099,\text{Null}\}

reductionTable = Get[FileNameJoin[{FCGetNotebookDirectory[], "ReductionTable-El-El.m"}]];

resPreFinal = Collect2[Total[ampFinal /. reductionTable], GLI];

integralMappings = FCLoopFindIntegralMappings[Cases2[resPreFinal, GLI], mappings[[2]]]

\left\{\left\{G^{\text{fctopology1}}(1,0,1,1,0)\to G^{\text{fctopology1}}(0,1,1,0,1)\right\},\left\{G^{\text{fctopology1}}(0,1,1,0,1),G^{\text{fctopology1}}(1,1,0,1,1)\right\}\right\}

resFinal = Collect2[resPreFinal /. integralMappings[[1]], GLI]

\frac{i (D-2) e^4 G^{\text{fctopology1}}(0,1,1,0,1) \gamma \cdot p \left(3 D^3 \xi _A^2-35 D^2 \xi _A^2+126 D \xi _A^2-144 \xi _A^2-D^3+4 D^2 N_f+5 D^2-24 D N_f+18 D+32 N_f-72\right)}{2 (D-6) (D-4) \;\text{pp}}-\frac{1}{4} i (D-2) e^4 G^{\text{fctopology1}}(1,1,0,1,1) \left(D \xi _A^2-2 \xi _A^2+D-6\right) \gamma \cdot p

Check the final results

resGrozinSE = I FCI[e^4 (D - 2) ( 2 (D - 2)/(D - 6) (1/SPD[p, p]) GLI["fctopology1", {0, 1, 1, 0, 1}] Nf - 
        1/4 ((D - 2) (GaugeXi[A])^2 + D - 6) GLI["fctopology1", {1, 1, 0, 1, 1}] 
        + (1/2) (D - 3)/(D - 4) ((3 D - 8) (GaugeXi[A])^2 - D - 4) (1/SPD[p, p]) GLI["fctopology1", {0, 1, 1, 0, 1}]) GSD[p]];
FCCompareResults[resFinal, resGrozinSE, 
   Text -> {"\tCompare to Grozin's Lectures on QED and QCD, hep-ph/0508242, Eq. 5.51:", 
     "CORRECT.", "WRONG!"}, Interrupt -> {Hold[Quit[1]], Automatic}, Factoring -> Simplify];
Print["\tCPU Time used: ", Round[N[TimeUsed[], 4], 0.001], " s."]; 
 

\text{$\backslash $tCompare to Grozin's Lectures on QED and QCD, hep-ph/0508242, Eq. 5.51:} \;\text{CORRECT.}

\text{$\backslash $tCPU Time used: }24.135\text{ s.}