Observation of a narrow<i>KK¯</i>state in<i>J/ψ</i>radiative decays

Evidence is presented for a narrow state, called \ensuremath{\xi}, in the decay modes J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\xi}, \ensuremath{\xi}\ensuremath{\rightarrow}${K}^{+}$${K}^{\mathrm{\ensuremath{-}}}$, and \ensuremath{\xi}\ensuremath{\rightarrow}${K}_{S}^{0}$${K}_{S}^{0}$. In the ${K}^{+}$${K}^{\mathrm{\ensuremath{-}}}$ mode, the \ensuremath{\xi} has a mass of 2.230\ifmmode\pm\else\textpm\fi{}0.006\ifmmode\pm\else\textpm\fi{}0.014 GeV/${c}^{2}$, a width of \ensuremath{\Gamma}${=0.026}_{\mathrm{\ensuremath{-}}0.016}^{+0.020}$\ifmmode\pm\else\textpm\fi{} 0.017 GeV/${c}^{2}$, a product branching ratio of ${(4.2}_{\mathrm{\ensuremath{-}}1.4}^{+1.7}$\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}10 $^{\mathrm{\ensuremath{-}}5}$, and a statistical significance of \ensuremath{\sim}4.5 standard deviations. In the ${K}_{S}^{0}$${K}_{S}^{0}$ mode, it has a mass of 2.232\ifmmode\pm\else\textpm\fi{}0.007\ifmmode\pm\else\textpm\fi{}0.007 GeV/${c}^{2}$, a width of \ensuremath{\Gamma}${=0.018}_{\mathrm{\ensuremath{-}}0.015}^{+0.023}$\ifmmode\pm\else\textpm\fi{} 0.010 GeV/${c}^{2}$, a product branching ratio of ${(3.1}_{\mathrm{\ensuremath{-}}1.3}^{+1.6}$\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}10 $^{\mathrm{\ensuremath{-}}5}$, and a statistical significance of \ensuremath{\sim}3.6 standard deviations. Limits on \ensuremath{\xi} decay to other final states are presented.