I have yet another question... I was always thinking that the scattering amplitudes one computes in QFT are complex numbers of modulus between 0 and 1. And I was thinking that because it is supposed to be related to the probability of some transition between states happening. And then I tried to find out some support to this idea of mine but without success. Does anyone know if the scattering amplitudes are of modulus less than 1? And, most importantly, any references where I could find support for the claim, if it's true? Thanks!