The vanishing analyzing powers in electron-proton elastic scattering; symmetries and dynamics

In high energy electron-proton elastic scattering the two parity-conserving analyzing powers vanish in the one-photon-exchange approximation. The electron analyzing-power vanishes from electron helicity-conservation, but the underlying reason(s) for the vanishing of the proton analyzing-power is (are) not clear. Therefore, a more detailed examination of this question has been made and is reported here. It is then noted that the proton analyzing-power is an optimum experimental observable for the determination of higher order contributions to this basic electron-nucleon electromagnetic interaction. (C) 1998 Elsevier Science B.V.