Observation of a near-threshold structure in the $K^{+}$ recoil-mass spectra in $e^{+}{e}^{-}\to K^{+}(D_s^{-}{D}^{\ast 0}+D_s^{\ast -}{D}^0)$

We report a study of the processes of $e^+e^-\to K^+ (D_s^- D^{0} + D^{-}_s D^0)$basedon$e^+e^-$ annihilation samples collected with the BESIII detector operating at BEPCII at five center-of-mass energies ranging from 4.628 to 4.698 GeV with a total integrated luminosity of 3.7 fb${}^{-1}$. An excess over the known contributions of the conventional charmed mesons is observed near the $D_s^{-}{D}^{\ast 0}$ and $D_s^{\ast -}{D}^0$ mass thresholds in the $K^{+}$ recoil-mass spectrum for events collected at $\sqrt{s}=4.681$ GeV. The structure matches a mass-dependent-width Breit-Wigner line shape, whose pole mass and width are determined as $(3982.5_{-2.6}^{+1.8}\pm 2.1)$ MeV/$c^2$ and $(12.8_{-4.4}^{+5.3}\pm 3.0)$ MeV, respectively. The first uncertainties are statistical and the second are systematic. The significance of the resonance hypothesis is estimated to be 5.3 $\sigma$ over the contributions only from the conventional charmed mesons. This is the first candidate of the charged hidden-charm tetraquark with strangeness, decaying into $D_s^{-}{D}^{\ast 0}$ and $D_s^{\ast -}{D}^0$. However, the properties of the excess need further exploration with more statistics.