Chiral Perturbation Theory and Final-State Theorem

Timelike scalar and vector form factors are recalculated using chiral perturbation theory and dispersion theory. It is shown that chiral perturbation theory at the one-loop level violates the final-state theorem (i.e., unitarity). In order to satisfy this theorem, chiral perturbation theory should be applied to the inverse of the form factor whose result is shown to be equivalent to the Pad\'e-approximant method.