Observation of the Decay $B\to J/\psi \eta K$ and Search for $X\left(3872\right)\to J/\psi \eta$

We report the observation of the $B$ meson decay $B^{\pm }\to J/\psi \eta K^{\pm }$ and evidence for the decay $B^0\to J/\psi \eta K_{\mathrm{S}}^0$, using $90\times {10}^6$ $B\overline{B}$ events collected at the ${\rm Y}\left(4S\right)$ resonance with the BABAR detector at the SLAC PEP-II $e^{+}{e}^{-}$ asymmetric-energy storage ring. We obtain branching fractions of $\mathcal{B}(B^{\pm }\to J/\psi \eta K^{\pm })=[10.8\pm 2.3(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 2.4(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\left)\right]\times {10}^{-5}$ and $\mathcal{B}(B^0\to J/\psi \eta K_{\mathrm{S}}^0)=[8.4\pm 2.6(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 2.7(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\left)\right]\times {10}^{-5}$. We search for the new narrow mass state, the $X\left(3872\right)$, recently reported by the Belle Collaboration, in the decay $B^{\pm }\to X\left(3872\right)K^{\pm },X\left(3872\right)\to J/\psi \eta$ and determine an upper limit of $\mathcal{B}[B^{\pm }\to X(3872)K^{\pm }\to J/\psi \eta K^{\pm }]<7.7\times {10}^{-6}$ at 90% confidence level.

Figure 1

For $B^{\pm }\to J/\psi \eta K^{\pm }$, the $\Delta E$ versus $m_{\mathrm{E}\mathrm{S}}$ event distribution (a) is shown with vertical and horizontal bands defined by limits, $|m_{\mathrm{E}\mathrm{S}}-m_{\mathrm{B}}|<7.5\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$ and $\left|\Delta E\right|<40\mathrm{M}\mathrm{e}\mathrm{V}$, respectively. The intersection of these bands corresponds to the signal-box region defined in the text. The $\Delta E$ projection (b) is shown for events in the vertical band that contains the $m_{\mathrm{E}\mathrm{S}}$ signal region. The $m_{\mathrm{E}\mathrm{S}}$ projection (c) is shown for events in the horizontal band that contains the $\Delta E$ signal region. The dashed histogram represents the estimated background and is described in the text.

Figure 2

The $\Delta E$ and $m_{\mathrm{E}\mathrm{S}}$ distributions for $B^0\to J/\psi \eta K_{\mathrm{S}}^0$. The descriptions of (a), (b), and (c) follow those of Figs. 1, 1, and 1, respectively.

Figure 3

The summed $J/\psi \eta$ mass distributions from $B^{\pm }\to J/\psi \eta K^{\pm }$ and $B^0\to J/\psi \eta K_{\mathrm{S}}^0$. The arrows indicate where the $\psi \left(2S\right)$ and $X\left(3872\right)$ signals would appear.