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

A search for the rare decay of a $B^0$ or $B_s^0$ meson into the final state $J/\psi \gamma$ 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 $3{\mathrm{fb}}^{-1}$. The observed number of signal candidates is consistent with a background-only hypothesis. Branching fraction values larger than $1.5\times 10^{-6}$ for the $B^0\to J/\psi \gamma$ decay mode are excluded at 90% confidence level. For the $B_s^0\to J/\psi \gamma$ decay mode, branching fraction values larger than $7.3\times 10^{-6}$ are excluded at 90% confidence level; this is the first branching fraction limit for this decay.

Figure 1

Feynman diagram of the leading contribution to the decay $B_{(s)}\to J/\psi \gamma$ in the Standard Model. Radiation of the photon from the other quarks is suppressed by a factor of ${\mathrm{\Lambda }}_{\mathrm{QCD}}/m_b$ [1].

Figure 2

Mass distribution of signal candidates with the fit result overlaid, allowing for a $B_s^0\to J/\psi \gamma$ signal.

Figure 3

Observed ${\mathrm{CL}}_{\mathrm{S}}$ (black points), expected ${\mathrm{CL}}_{\mathrm{S}}$ (continuous line) and expected 1 and $2\sigma$ uncertainty (shaded bands) value as a function of the hypothesized branching fraction for the $B_s^0\to J/\psi \gamma$ decay (left) and $B^0\to J/\psi \gamma$ decay (right). The dashed line represents the branching fraction excluded at 90% C.L.