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On p.327 in my second edition of Peskin and Schroeder, I have an expression for the one loop correction to the 4-point amplitude of phi^4 theory:

[tex]i\mathcal{M}=-i\lambda - \frac{i \lambda^2}{32 \pi^2}\text{[Complicated integral]}[/tex]

Mathematica can do the integral for me, and all that I'm interested in for the moment- I think- is the imaginary part, which happens to be -I*Pi; I therefore find that to this order,

[tex]\text{Im}(\mathcal{M})=\frac{\lambda^2}{32 \pi}[/tex]

Examining the leading contribution to the optical theorem

[tex]\text{Im}(\mathcal{M}(t=0))=2E_{cm}p_{cm}\sigma_{TOTAL}[/tex]

we should compare this to the cross-section [itex]\sigma_{TOTAL}[/itex] we get from thetree-levelamplitude (eq. 4.100 in my Peskin):

[tex]\sigma_{TOTAL}=\frac{\lambda^2}{32 \pi s}[/tex]

So it looks like the optical theorem is only satisfied if [itex]2E_{cm}p_{cm}=s[/itex], which is only true for massless particles. This disturbs me. Not only have we not assumed that our particles are massless, but the counterterms are singular in this limit; the only way I can find to make sense of this result is that it somehow corresponds to the check on the optical theorem performed in bare perturbation theory, with a divergent bare coupling [itex]\lambda[/itex], but having unitarity restored in a physically meaningless limit, with an assumption about a physical quantity that is not only extraneous but incorrect, doesn't make me feel any better.

Can someone please explain to me what I'm doing wrong here? Thanks.

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# Optical theorem and renormalised perturbation theory (c.f. Peskin 10.2)

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