New physics effects in doubly Cabibbo suppressed D decays

The most sensitive experimental searches for $D-\bar D$ mixing use $D^0 \toK^+ \pi^-$ decays. It is often assumed that effects of New Physics and, inparticular, CP violation, can appear through the mixing, while the $c \to d u\bar s$ decay amplitude cannot have significant contributions from New Physicsand is, therefore, CP conserving to a good approximation. We examine thisassumption in two ways. First, we calculate the contributions to the decay invarious 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 Physicscontributions are indeed small compared to the standard model doubly Cabibbosuppressed amplitude. Second, we show that many of our constraints holdmodel-independently. We find, however, one case where the model-independentbound is rather weak and a CP violating contribution of order 30% is notexcluded.Comment: 12 pages, no figures, Late