Zeros in energy and angular distributions with real or virtual photons

We consider γq/sub i/→W+-q/sub j/ and its crossed channels to show how kinematics and quark-charge assignment together determine the occurrence of a zero in the amplitude. Then we calculate the amplitude with off-mass-shell photons which we use first to show how W+-→q/sub i/q-bar/sub j/e+e-, q/sub i/q-bar/sub j/→ W+-e+e-, and e+-q/sub i/→e+-W+-q/sub j/ can explore the region around the zeros, and second to point out that the photon reaction may be unique for observing these zeros