Observation of a Charmoniumlike State Produced in Association with a $J/\psi$ in $e^{+}{e}^{-}$ Annihilation at $\sqrt{s}\approx 10.6\mathrm{GeV}$

We report the first observation of a charmoniumlike state recoiling from the $J/\psi$ in the inclusive process $e^{+}{e}^{-}\to J/\psi +\mathrm{\text{anything}}$ at a mass of $(3.943\pm 0.006\pm 0.006)\mathrm{GeV}/c^2$. We also observe the decay of this state into $D^{*}\overline{D}$ and determine its intrinsic width to be less than $52\mathrm{MeV}/c^2$ at the 90% C.L. These results are obtained from a $357{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector near the ${\rm Y}(4S)$ resonance, at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.