Shedding Light onCPViolation in the Charm System viaD→VγDecays

Abstract

Recent evidence for direct $CP$ violation in nonleptonic charm decays cannot be easily accommodated within the standard model. On the other hand, it fits well in new physics models generating $CP$ violating $\ensuremath{\Delta}C=1$ chromomagnetic dipole operators. We show that in these frameworks sizable direct $CP$ asymmetries in radiative $D\ensuremath{\rightarrow}{P}^{+}{P}^{\ensuremath{-}}\ensuremath{\gamma}$ decays ($P=\ensuremath{\pi}$, $K$), with ${M}_{PP}$ close to the $\ensuremath{\rho}$ or the $\ensuremath{\phi}$ peak, can be expected. Enhanced matrix elements of the electromagnetic dipole operators can partly compensate the long distance dominance in these decays, leading to $CP$ asymmetries of the order of several percent. If observed at this level, these would provide a clean signal of physics beyond the standard model and of new dynamics associated with dipole operators. We briefly comment on related $CP$ violating observables accessible via time dependent $D(\overline{D})\ensuremath{\rightarrow}{P}^{+}{P}^{\ensuremath{-}}\ensuremath{\gamma}$ studies and angular decay product distributions in rare semileptonic $D$ decays.