# Optical theorem

1. Jul 8, 2009

### parton

I'm confused. I read a line in a book where there was a qualitative explanation about the optical theorem, namely that it is connected with conservation of particle number. But I think that this is only true if we just consider elastic scattering. Inelastic processes would of course allow a change in the particle number so there is no conservation. The fundamental connection of the optical theorem is that with conservation of probability (because it follows from the unitarity of the S-matrix).

Is my "explanation" correct, or am I missing something?

2. Jul 9, 2009

### clem

The theorem also applies for inelastic scattering.
It shows that the number of particles continuing forward is decreased by the number either scattered elastically or absorbed (or changed into something else).
That is why the total cross section (elasstic + inelastic) appears in the theorem.

3. Jul 9, 2009

### parton

my question is whether theres is particle number conservation in general? I would say no, because of the fact that the optical theorem also applies for inelastic scattering and there we could imagine processes where particles are created or annihilated. So there is no partcile number conservation.

Right?

4. Jul 9, 2009

### clem

Right.

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