(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

2. Relevant equations

Hi, I'm studying Feynmann diagrams; electron-positron scattering.

If the 4-momentum of the incoming electron (positron) is p1 (p2) and of the outgoing electron (positron) is p3 (p4), momentum conservation gives

p1 + p2 = p3 + p4

If you now consider the scattering in the frame where thethreemomentum of the incoming positron = 0. Why does then defining threemomentum p4=p3 imply that they equal 0?

And why does defining p3*p4=0 (three vectors) imply that p3(threevector)=0 or p4=0?

3. The attempt at a solution

I tried: 4-vectors: p1+p2=p3+p4

square this: (p1)^2 + (p2)^2 + 2*p1*p2= (p3)^2 + (p4)^2 + 2*p3*p4

(p1)^2=(p3)^2=m (electron mass)

(p2)^2=(p4)^2=M (positron mass)

So: p1*p2=p3*p4

The lab frame condition gives:

p1(0)*p2(0)=p3(0)*p4(0)-p3*p4(three-vectors)

But what are the next steps????

(I hope I made clear the problem, it isn't very readable I'm afraid....)

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# Homework Help: Feynmann diagram

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