Search for the rare decays $W⁺ \to D⁺_s\gamma$ and $Z \to D⁰\gamma$ at LHCb

Abstract

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