Observation of a narrowKK¯state inJ/ψradiative decays

Evidence is presented for a narrow state, called {ξ}, in the decay modes J/{ψ}{→}{γ}{ξ}, {ξ}{→}K⁺{K}^{{{-}}}, and ξ→{K}S⁰KS⁰. In the K⁺{K}^{{{-}}}mode, the ξ has a mass of 2.230±0.006±0.014 GeV/{c}², a width of Γ{=0.026}_{{{-}}0.016}+0.020± 0.017 GeV/{c}²$, a product branching ratio of ${(4.2}_{{{-}}1.4}+1.7±0.8)×10^{{{-}}5}, and a statistical significance of ~4.5 standard deviations. In the{K}S⁰KS⁰ mode, it has a mass of 2.232±{}0.007±{}0.007 GeV/c², a width of {Γ}=0.018_-0.015+0.023±{} 0.010 GeV/c², a product branching ratio of (3.1_-1.3+1.6±{}0.7)×{}10 ^-5, and a statistical significance of {~}3.6 standard deviations. Limits on {ξ} decay to other final states are presented.