- #1
physics_fun
- 28
- 0
Homework Statement
Homework Equations
Hi, I'm studying Feynman 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 the threemomentum 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?
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...)