# Peskin p. 160

Tags:
1. Apr 5, 2015

### physichu

In peskin p. 160 forth paragraph they say to verefy Ward identity in equation 5.74.
I don't succeed, they say some algebra is needed. I conjecture that this some algebra is what i miss.
Any help will be appreciated - thanks a lot.

2. Apr 5, 2015

### Hepth

Remember that $\not k \not k = 0$ and that from momentum conservation $k' - k = p - p'$

So you may end up with something like $\bar{u}(p') (\not p - \not p') u(p')$, which you can use equation of motion to get $m-m = 0$.

3. Apr 6, 2015

### physichu

Hurray... It got solved.
Many many thank's.