Observation of a Narrow Charmoniumlike State in ExclusiveB±→K±π+π−J/ψDecays

Abstract

We report the observation of a narrow charmoniumlike state produced in the exclusive decay process ${B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{K}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$. This state, which decays into ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$, has a mass of $3872.0\ifmmode\pm\else\textpm\fi{}0.6\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}\ifmmode\pm\else\textpm\fi{}0.5\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}$, a value that is very near the ${M}_{{D}^{0}}+{M}_{{D}^{*0}}$ mass threshold. The results are based on an analysis of 152M $B$-$\overline{B}$ events collected at the $\ensuremath{\Upsilon}(4S)$ resonance in the Belle detector at the KEKB collider. The signal has a statistical significance that is in excess of $10\ensuremath{\sigma}$.