Weak and electromagnetic tests of T symmetry

It has often been stated that a T-odd reaction observable, which is the expectation value of a T-odd operator (e.g., σ· (ki - kf)), vanishes from T symmetry. Since this would provide a single-observable null test of T symmetry, this statement is in direct conflict with a theorem that proves the nonexistence of such a null test in exclusive reactions; and the theorem has been extended to inclusive reactions also. Since the number of amplitudes for a given spin structure is considerably reduced in electroweak processes, as compared with those of the strong interaction, many electroweak experimental (spin) observables vanish. There are some misconceptions in the literature concerning the correspondence between T-odd observables and electroweak tests of T symmetry, and these will be discussed