Past exam of the particle physics course of the University of Oxford / 2018 / 3 / c / i

Direct CP violation means unequal magnitudes of CP-conjugate decay probability amplitudes:It changes decay probabilities even at $t=0$, before any mixing. For a common CP eigenstate final state, this reduces to $|A_f|\ne |{\bar{A}}_f|$. Usually it requires interfering decay contributions with different weak phases and different strong phases; it is not equivalent to a nonzero weak phase in one isolated probability amplitude.