Rare radiative charm decays within the standard model and beyond

Abstract

We present standard model (SM) estimates for exclusive $c \to u \gamma$ processes in heavy quark and hybrid frameworks. Measured branching ratios ${\cal{B}}(D^0 \to (\phi, \bar K^{*0}) \gamma)$ are at or somewhat exceeding the upper range of the SM and suggest slow convergence of the $1/m_D, \alpha_s$-expansion. Model-independent constraints on $|\Delta C|=|\Delta U|=1$ dipole operators from ${\cal{B}}(D^0 \to \rho^0 \gamma)$ data are obtained. Predictions and implications for leptoquark models are worked out. While branching ratios are SM-like CP asymmetries $\lesssim 10 \%$ can be induced. In SUSY deviations from the SM can be even larger with CP asymmetries of $O(0.1)$. If $\Lambda_c$-baryons are produced polarized, such as at the $Z$-pole, an angular asymmetry in $\Lambda_c \to p \gamma$ decays can be studied that is sensitive to chirality-flipped contributions.