vsht
commented
7 years ago This has nothing to do with massive or massless.
Spinor[U/V](Bar) are just placeholders for Spinor
and make little sense when not inside a closed spinor chain.
Inside Spinor the sign in front of Momentum is used to distinguish
between U and V spinors, c.f.
https://github.com/FeynCalc/feyncalc/wiki/FAQ#spinors
or
http://www.feynarts.de/FA3Guide.pdf p. 73
Compare the output of
SpinorUBar[p1].GS[q].SpinorU[p2] // FCI // StandardForm
SpinorUBar[p1].GS[q].SpinorV[p2] // FCI // StandardForm
SpinorVBar[p1].GS[q].SpinorU[p2] // FCI // StandardForm
SpinorVBar[p1].GS[q].SpinorV[p2] // FCI // StandardForm
In[51]:= SpinorVBar[k2,m]//FCI //StandardForm Out[51]//StandardForm= Spinor[-Momentum[k2],m,1]
Whereas
In[52]:= SpinorVBar[k2]//FCI //StandardForm Out[52]//StandardForm= Spinor[Momentum[k2],0,1]
Why is there a difference in sign of the momenta in these two cases ?
Similarly
In[67]:= Spinor[-k2].GA[[Mu]].Spinor[k1] //FCI //StandardForm Out[67] Spinor[-Momentum[k2],0,1].DiracGamma[LorentzIndex[[Mu]]].Spinor[Momentum[k1],0,1]
Whereas
In[66]:= SpinorVBar[k2]. GA[[Mu]].SpinorU[k1] //FCI //StandardForm Out[66]=Spinor[Momentum[k2],0,1].DiracGamma[LorentzIndex[[Mu]]].Spinor[Momentum[k1],0,1]