Observation of the Decay B=> J/psi eta K and Search for X(3872)=> J/psi eta

Abstract

We report the observation of the $B$ meson decay $B^{\pm }\to J/\psi \eta K^{\pm }$ and evidence for the decay $B^0\to J/\psi \eta K_S^0$, using {90} million $BBbar$ events collected at the $\ensuremath{Υ{(4S)}}\xspace$ resonance with the $BaBar$ detector at the PEP-II $e^{+}{e}^{-}$ asymmetric-energy storage ring. We obtain branching fractions of $\mathcal{B}$$(B^{\pm }\to J/\psi \eta K^{\pm }$)=$(10.8\pm 2.3(stat.)\pm 2.4(syst.))\times 10^{-5}$ and $\mathcal{B}$$(B^0\to J/\psi \eta K_{\mathrm{S}}^0$)=$(8.4\pm 2.6(stat.)\pm 2.7(syst.))\times 10^{-5}$. We search for the new narrow mass state, the X(3872), recently reported by the Belle Collaboration, in the decay $B^\pm\to X(3872)K^\pm, X(3872)\to \jpsi η$ and determine an upper limit of $\mathcal{B}$$(B^\pm \to X(3872) K^\pm \to \jpsi ηK^\pm$) $<7.7\times 10^{-6}$ at 90% C.L.

7 pages and two figures, submitted to Phys. Rev. Letts