Observation of $D^0\to {\rho }^0\gamma$ and Search for $CP$ Violation in Radiative Charm Decays

Here, we discuss the first observation of the radiative charm decay $D^0\to {\rho }^0\gamma$ and the first search for $CP$ violation in decays $D^0\to {\rho }^0\gamma$, $\phi \gamma$, and ${\overline{K}}^{*0}\left(892\right)\gamma$, using a data sample of $943{\mathrm{fb}}^{-1}$ collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The branching fraction is measured to be $B(D^0\to {\rho }^0\gamma )=(1.77\pm 0.30\pm 0.07)\times {10}^{-5}$, where the first uncertainty is statistical and the second is systematic. The obtained $CP$ asymmetries $A_{CP}(D^0\to {\rho }^0\gamma )=+0.056\pm 0.152\pm 0.006$, $A_{CP}(D^0\to \phi \gamma )=-0.094\pm 0.066\pm 0.001$, and $A_{CP}(D^0\to {\overline{K}}^{*0}\gamma )=-0.003\pm 0.020\pm 0.000$ are consistent with no $CP$ violation. Furthermore, we present an improved measurement of the branching fractions $B(D^0\to \phi \gamma )=(2.76\pm 0.19\pm 0.10)\times {10}^{-5}$ and $B(D^0\to {\overline{K}}^{*0}\gamma )=(4.66\pm 0.21\pm 0.21)\times {10}^{-4}$.