What is the reasoning for saying that the scattering matrix in quantum field theory is unitary? Take the initial state to be an electron and a positron. All sorts of crazy products can result in the final state, from photons to Z's to Higgs to an electron/positron with different momenta, to quark/gluon/hadron jets. Also, there is the greatest possibility, which is that the electron and positron completely miss each other and don't react. So all this sums up to 1? What if there's a particle yet to be discovered that can be produced from electron/positron collision? Then that would mean that any proof that the Standard Model obeys unitarity would be false, because the Standard Model predicted a probability of 1, but with the extra particle, the probability would be more than 1. Or was unitarity built in from the beginning, somewhere in the path integral (a normalization?)? The starting point for scattering calculations would be the calculation of [tex]W[J]=<0|0>_J [/tex] (the vacuum-to-vacuum expectation value in the presence of a source) and feeding W[J] into the LSZ-reduction formula? So somewhere at that juncture, unitarity was inserted by hand?