He get the result (in the lab frame):

[tex] \frac{\mbox d^2\sigma}{\mbox d E^\prime\mbox d\Omega^\prime} =\frac1{16\pi^2}\frac{E^\prime}{E}\bar{|\mathcal M|}^2[/tex].

I'm new to this kind of calculus, but couldn't get this.

Does Anyone have a suggestion?.

Thanks in advance