Search for the rare decays $B^0\to J\mathrm{/}\psi \gamma$ and $B_s^0\to J\mathrm{/}\psi \gamma$

A search for the rare decay of a $B^0$ or $B_s^0$ meson into the final state $J\mathrm{/}\psi \gamma$ is performed, using data collected by the LHCb experiment in $pp$ collisions at $\sqrt{s}=7$ and $8$ TeV, corresponding to an integrated luminosity of 3 fb${}^{-1}$. The observed number of signal candidates is consistent with a background-only hypothesis. Branching fraction values larger than $1.7\times {10}^{-6}$ for the $B^0\to J\mathrm{/}\psi \gamma$ decay mode are excluded at 90% confidence level. For the $B_s^0\to J\mathrm{/}\psi \gamma$ decay mode, branching fraction values larger than $7.4\times {10}^{-6}$ are excluded at 90% confidence level, this is the first branching fraction limit for this decay.