New Physics Effects in Doubly Cabibbo Suppressed D Decays

The most sensitive experimental searches for $D-\bar D$ mixing use $D^0 \to K^+ \pi^-$ decays. It is often assumed that effects of New Physics and, in particular, CP violation, can appear through the mixing, while the $c \to d u \bar s$ decay amplitude cannot have significant contributions from New Physics and is, therefore, CP conserving to a good approximation. We examine this assumption in two ways. First, we calculate the contributions to the decay in various relevant models of New Physics: Supersymmetry without R-parity, multi-scalar models, left-right symmetric models, and models with extra quarks. We find that phenomenological constraints imply that the New Physics contributions are indeed small compared to the standard model doubly Cabibbo suppressed amplitude. Second, we show that many of our constraints hold model-independently. We find, however, one case where the model-independent bound is rather weak and a CP violating contribution of order 30% is not excluded.