I am encountering a similar problem with much more complex terms than the ones shown in the figures below. Basically, I wrote the chain for a process(in this case, a simple 2 particle scattering in QED).

We add 2 same order processes which vary only in the interchange of final fermions (as shown in expressions $M_1$ and $M_2$ respectively). On performing the $\textit{FermionSpinSum}$ on $M = M_1+M_2$, we can see that there are spinors in the answers, which leads me to conclude that somehow the re-ordering of the spinor indices have not taken place.

M1=(-e^3 /SP[q2])*(Spinor[p3, m2].GA[μ].Spinor[-p4,m2]).MT[μ, δ].(Spinor[p2, m2].GA[δ].Spinor[p1,m2]);

M2= (-e^3 /SP[q2]) (Spinor[p3, m2].GA[μ].Spinor[-p4,m2]).MT[μ, δ].(Spinor[-p1, m2].GA[δ].Spinor[p2,

M = M1 + M2;

FermionSpinSum[ComplexConjugate[M] M]

The output is, enter image description here

Is this correct? Can someone point out any corrections please? I am facing a similar problem with a much more complicated second term, but I believe this is the crux of the issue.

  • $\begingroup$ At a minimum provide your code as text, not pictures. $\endgroup$
    – MarcoB
    May 22, 2020 at 18:22
  • $\begingroup$ Sorry @MarcoB. I've added the code as best as I could format it. I don't know why the Greek notations are not rendering here. There is no problem in running the same in a notebook. $\endgroup$
    – Lelouch
    May 22, 2020 at 18:37
  • 1
    $\begingroup$ I fixed them. I use Halirutan's plugin for the M.SE editor to change the pasted encoded characters into actual Greek letters, as well as to add links to the documentation. It's great! I also use this handy online converter at http://steampiano.net/msc/ when I am on mobile. $\endgroup$
    – MarcoB
    May 22, 2020 at 19:35
  • $\begingroup$ What process exactly are you looking at? $\endgroup$ May 22, 2020 at 20:13
  • 1
    $\begingroup$ This looks like Bhabha scattering, cf. e.g. (feyncalc.github.io/FeynCalcExamplesMD/QED/Tree/ElAel-ElAel) but the spinor chains look weird, and it is not really clear what reaction you are trying to calculate. Can you write the amplitude in LaTeX in your question? BTW, there is an example gallery that contains most common simple tree-level QED processes: feyncalc.github.io/examples $\endgroup$
    – vsht
    May 22, 2020 at 22:13


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