Search for the rare decays $W^{+}\to D_s^{+}\gamma$ and $Z\to D^0\gamma$ at LHCb

A search for the rare decays $W^{+}\to D_s^{+}\gamma$ and $Z\to D^0\gamma$ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 2.0 fb${}^{-1}$. No significant signal is observed for either decay mode and upper limits on their branching fractions are set using $W^{+}\to {\mu }^{+}\nu$ and $Z\to {\mu }^{+}{\mu }^{-}$ decays as normalization channels. The upper limits are $6.5\times {10}^{-4}$ and $2.1\times {10}^{-3}$ at 95% confidence level for the $W^{+}\to D_s^{+}\gamma$ and $Z\to D^0\gamma$ decay modes, respectively. This is the first reported search for the $Z\to D^0\gamma$ decay, while the upper limit on the $W^{+}\to D_s^{+}\gamma$ branching fraction improves upon the previous best limit.