Charm Physics at B Factories

After the discovery by Tevatron of B s − Bs-oscillations D and D remains the last flavor neutral meson system with unobserved mixing. Neutral meson mixing is characterized by parameters x = ∆M/Γ and y = ∆Γ/2Γ, where ∆M and ∆Γ are the mass and width difference between the two CP eigenstates. The SM box diagram in D ↔ D transitions is strongly GIM and CKM suppressed. However, D mixing can be enhanced by long distance effects, involving onor off-shell D ↔ D transitions through intermediate states accessible to both mesons, that contribute both to x and y. New Physics contribution to loops can enhance x only, and observation of x y would be a signal of New Physics. Observation of CP violating effects in D would be another unambiguous signature of New Physics as the SM predicts tiny CP violation beyond the present experimental sensitivity. Experimentally one of the following techniques is exploited to search for D mixing: study of wrong-sign (WS) hadronic decays, search for WS D semileptonic decays, a timedependent Dalitz plot analysis or direct measurement of the lifetime difference between opposite CP eigenstates. The first method provides the restrictive mixing constraints, in spite of a complication due to presence of doubly-Cabibbo suppressed (DCS) contribution to the same WS final state. The interference of the mixing and DCS amplitudes results in rotation of measured x′ = x cos δ + y sin δ and y′ = −x sin δ + y cos δ by the strong phase difference δ between the mixing and DCS amplitudes. This year BaBar [2] has presented a strong evidence for D mixing in D → K+π− decays. The WS decay rate is a function of proper decay-time: