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

Evidence is presented for a narrow state, called ξ, in the decay modes J/ψ→γξ, ξ→$K^{+}$$K^{\mathrm{-}}$, and ξ→$K_S^0$$K_S^0$. In the $K^{+}$$K^{\mathrm{-}}$ mode, the ξ has a mass of 2.230±0.006±0.014 GeV/$c^2$, a width of Γ${=0.026\mathrm{}}_{\mathrm{-}0.016}^{+0.020\mathrm{}}$± 0.017 GeV/$c^2$, a product branching ratio of ${(4.2\mathrm{}}_{\mathrm{-}1.4}^{+1.7\mathrm{}}$±0.8)×10 ${}_{}{}^{\mathrm{-}5}{}_{}{}^{}$, and a statistical significance of ∼4.5 standard deviations. In the $K_S^0$$K_S^0$ mode, it has a mass of 2.232±0.007±0.007 GeV/$c^2$, a width of Γ${=0.018\mathrm{}}_{\mathrm{-}0.015}^{+0.023\mathrm{}}$± 0.010 GeV/$c^2$, a product branching ratio of ${(3.1\mathrm{}}_{\mathrm{-}1.3}^{+1.6\mathrm{}}$±0.7)×10 ${}_{}{}^{\mathrm{-}5}{}_{}{}^{}$, and a statistical significance of ∼3.6 standard deviations. Limits on ξ decay to other final states are presented.