A bstract We propose a novel method to study flavor-changing neutral currents in the e _(+) e _(?) → D _(?0) and e _(+) e _(?)?→? B ~( s ) _(?) transitions, tuning the energy of e _(+) e _(?)- collisions to the mass of the narrow vector resonance D _(?0) or B ~( s ) _(?) . We present a thorough study of both short-distance and long-distance contributions to e _(+) e _(?) → D _(?0) in the Standard Model and investigate possible contributions of new physics in the charm sector. This process, albeit very rare, has clear advantages with respect to the D _(0) → e _(+) e _(?) decay: the helicity suppression is absent, and a richer set of effective operators can be probed. Implications of the same proposal for B ~( s ) _(?) are also discussed.
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