Amplitude zeros

We consider the general phenomenon of amplitude zeros, first discovered in the process $d\overline{u}\to W^{-}\gamma$. Using spin-0 particles, we investigate the circumstances for amplitude zeros to occur in the physical region and, as well, their location in phase space. A necessary condition is that all of the charges must be of the same sign or neutral. We analyze, in detail, both the massive and massless cases for radiative decays, photoproduction, and direct photon production.