Rare radiative charm decays within the standard model and beyond

A bstract We present standard model (SM) estimates for exclusive c → u γ processes in heavy quark and hybrid frameworks. Measured branching ratios ℬ D 0 → ϕ K ¯ ∗ 0 γ are at or somewhat exceeding the upper range of the SM and suggest slow convergence of the 1 /m D , α s -expansion. Model-independent constraints on |Δ C | = | Δ U | = 1 dipole operators from ℬ( D 0 → ρ 0 γ ) data are obtained. Predictions and implications for leptoquark models are worked out. While branching ratios are SM-like CP asymmetries ≲ 10% can be induced. In SUSY deviations from the SM can be even larger with CP asymmetries of O (0 . 1). If Λ c -baryons are produced polarized, such as at the Z -pole, an angular asymmetry in Λ c → p γ decays can be studied that is sensitive to chirality-flipped contributions.