On null tests of time-reversal invariance in scattering and reactions

There have been suggestions in the literature, both recently and in the more distant past, that, in the lowest-order Born approximation, time-reversal (T)-odd experimental observables in certain reactions are required by T-symmetry to vanish. These observables are the final-state spin-correlation coefficient C{sub xy} in the reaction e{sup +}e{sup {minus}} {r_arrow} {tau}{sup +}{tau}{sup {minus}} and the target analysing power A{sub oy} in the inclusive process ep {r_arrow} eX with a polarized proton target. These assertions are in direct conflict with a theorem that states that there can be no null-test of T-symmetry in such processes; that is, T-symmetry does not require any single observable to vanish. This talk addresses the resolution of that conflict.