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

TL;DR

This paper reports the observation of specific B meson decays involving J/psi, eta, and K mesons, measures their branching fractions, and searches for the X(3872) state in these decays, setting an upper limit on its production.

Contribution

First observation of B to J/psi eta K decays and a search for X(3872) in these channels, providing new insights into B meson decay modes and exotic states.

Findings

Branching fraction for B+ to J/psi eta K+ measured as (10.8 ± 2.3 ± 2.4)×10^{-5}

Branching fraction for B0 to J/psi eta K0_S measured as (8.4 ± 2.6 ± 2.7)×10^{-5}

Upper limit set on B+ to X(3872) K+ to be less than 7.7×10^{-6} at 90% C.L.

Abstract

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