# FeynArts & FormCalc

## Main Question or Discussion Point

Hi all,

Is any one familiar with FeynArts and FormCalc programmes? When I calculate a process having $V_{CKM}$ in the couplings like $b \to s \gamma$, and use Insertion Level of Classes, after calculating the amplitude I get some parameters I don't understand, like

CKM[Gen4, 3], SumOver[Gen4, 3].

So what are their values ?

Bests.

## Answers and Replies

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Hepth
Gold Member
Its indicating a sum over Gen4 as your index. Ignore the "4", its just an internal name thats based on a counter. You probably have auto-generated indices like Col1 Col2 Gen3 Gen4 Gen5 Gen6 etc. Theyre just placeholders.

So what its saying is that you should sum this diagram over all Gen4==1..2..3.

SumOver[X,n] means a sum with X as the index, running from 1 to n.

This makes sense, as your diagram is probably the FFV triangle one where you have up charm and top contributions. So you sum the same diagram changing only the index Gen4. You may also notice things like Mass(F(Gen4),Loop), or else Mass(F(X),Loop), but then in your replacement rules (the rest of the amplitude) you'll have something like {Mass(F(X),Loop),....}->InsertionClasses{MQU(Gen4),....}

meaning that the mass in the loop needs to be replaced by the Gen4'th generation quark mass of the up type.

Is that clear? If not let me know I can help some more. I'm no expert but I've played with it quite a bit.

Hi, thanks for reply. I think FeynArts,if the program going well should not show these symbols at the final result. So I'm trying to find out what's missed or any bugs.

Hepth
Gold Member
How are you treating the result of CreateFeynAmp? These SHOULD be in the results of CreateFeynAmp. Especially at the Classes level. If you are looking at a specific diagram you can exclude some things so you get only the contribution you want, but otherwise the one diagram created from InsertFields is actually a sum over all diagrams with varying internal freedoms, like generations, spin, or color.

www.feynarts.de/FA3Guide.pdf

Again, I'm no expert in FA but I've been trying to use it a bit, though I'm mainly using FeynCalc these days. I never fully trusted FeynArts to give me what I want for some reason.

All I try to find is a program make calculations on the Loop level, can FeynCalc do this or its like CalcHep ..