Observation of a Near-Threshold Structure in the $K^{+}$ Recoil-Mass Spectra in $e^{+}{e}^{-}\to K^{+}(D_s^{-}{D}^{*0}+D_s^{*-}{D}^0)$

We report a study of the processes of $e^{+}{e}^{-}\to K^{+}{D}_s^{-}{D}^{*0}$ and $K^{+}{D}_s^{*-}{D}^0$ based on $e^{+}{e}^{-}$ 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 $3.7{\mathrm{fb}}^{-1}$. An excess of events over the known contributions of the conventional charmed mesons is observed near the $D_s^{-}{D}^{*0}$ and $D_s^{*-}{D}^0$ mass thresholds in the $K^{+}$ recoil-mass spectrum for events collected at $\sqrt{s}=4.681\mathrm{GeV}$. The structure matches a mass-dependent-width Breit-Wigner line shape, whose pole mass and width are determined as $({3982.5}_{-2.6}^{+1.8}\pm 2.1)\mathrm{MeV}/c^2$ and $({12.8}_{-4.4}^{+5.3}\pm 3.0)\mathrm{MeV}$, 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 for a charged hidden-charm tetraquark with strangeness, decaying into $D_s^{-}{D}^{*0}$ and $D_s^{*-}{D}^0$. However, the properties of the excess need further exploration with more statistics.

Figure 1

Distributions of the invariant mass $M(K^{+}{K}^{-}{\pi }^{-})$ (a) and $M(K_S^0{K}^{-})$ (b) in data and MC simulations at $\sqrt{s}=4.681\mathrm{GeV}$. The $Z_{cs}(3985{)}^{-}$ signal MC component is normalized to the observed $D_s^{-}$ yield in data. Arrows indicate the mass region requirements.

Figure 2

Distribution of the $K^{+}{D}_s^{-}$ recoil mass in data and signal MC samples at $\sqrt{s}=4.681\mathrm{GeV}$. Horizontal arrows indicate the sidebands and vertical arrows indicate the signal region. The magnitudes of the three-body nonresonant processes and $Z_{cs}(3985{)}^{-}$ signal processes are scaled arbitrarily. The histogram of wrong-sign (WS) events is scaled by a factor of 1.18 to match the sideband data.

Figure 3

Simultaneous unbinned maximum likelihood fit to the $K^{+}$ recoil-mass spectra in data at $\sqrt{s}=4.628$, 4.641, 4.661, 4.681, and 4.698 GeV. Note that the size of the $D^{*0}{\overline{D}}_1^{*}(2600{)}^0(\to D_s^{-}{K}^{+})$ component is consistent with zero.

Figure 4

The $K^{+}$ recoil-mass spectrum in data at $\sqrt{s}=4.681\mathrm{GeV}$ after subtraction of the combinatorial backgrounds.