CP ASYMMETRY IN TOP-QUARK RADIATIVE DECAYS

We analyze the CP asymmetry in the top-quark radiative decay t --> q-gamma, where q = c, u and gamma is an on-shell photon, due to the one-loop penguin diagram in the three-generation standard model. This penguin-dominated CP asymmetry is a new phenomenon of heavy-quark physics, and corresponds to the onset in the amplitude of the physical thresholds t --> real {Wl}, where l = b, s, d is a quark in the loop. A CP asymmetry in a radiative decay to an on-shell photon in the light-quark sector (e.g. B-meson decays) cannot be generated by the one-loop penguin. The origin of this asymmetry is thus formally of some interest, although the phenomenological application turns out to be rather limited, due to extremely small branching ratios for t --> q-gamma, at least to one-loop order (however, our calculations raise an interesting possibility with respect to the role of QCD corrections to this asymmetry). This calculation should also serve as a useful benchmark for the study of possible CP asymmetries in the top-quark sector, in the context of new physics.