First Observation of the Decay $\mathit{B}\to \mathit{J}/\mathit{\psi }\mathit{\phi }\mathit{K}$

We present the first observation of the decay $B\to J/\psi \phi K$. Using $9.6\times {10}^6$ $B\overline{B}$ meson pairs collected with the CLEO detector, we have observed ten fully reconstructed $B\to J/\psi \phi K$ candidates, whereas the estimated background is $0.5\pm 0.2$ event. We obtain a branching fraction of $B(B\to J/\psi \phi K)=\left({8.8}_{-3.0\mathrm{}}^{+3.5\mathrm{}}\right[\mathrm{stat}]\pm 1.3[\mathrm{syst}\left]\right)\times {10}^{-5\mathrm{}}$. This is the first observed $B$ meson decay requiring the creation of an additional $s\overline{s}$ quark pair.