Observation of the Decay $B^0\to J/\psi \eta$

We report the first observation of $B^0\to J/\psi \eta$ decay. These results are obtained from a data sample that contains $449\times {10}^6$ $B\overline{B}$ pairs accumulated at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We observe a signal with a significance of $8.1\sigma$ and obtain a branching fraction of $\mathbf{(}9.5\pm 1.7(\mathrm{stat})\pm 0.8(\mathrm{syst})\mathbf{)}\times {10}^{-6}$.

Figure 1

Feynman diagram for the leading contribution to the decay $B^0\to J/\psi \eta$.

Figure 2

$\Delta E$ distributions for the decay $B^0\to J/\psi \eta$. The plot on the left shows $B^0\to J/\psi \eta (\eta \to \gamma \gamma )$, while $B^0\to J/\psi \eta (\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0)$ is on the right. The curves show the signal (dash-dot lines) and background (dashed lines) contributions as well as the overall fit (solid lines).