Evidence for a Neutral Near-Threshold Structure in the $K_{\mathit{S}}^0$ Recoil-Mass Spectra in $e^{+}{e}^{-}\to K_{\mathit{S}}^0{D}_s^{+}{D}^{*-}$ and $e^{+}{e}^{-}\to K_{\mathit{S}}^0{D}_s^{*+}{D}^{-}$

We study the processes $e^{+}{e}^{-}\to K_{\mathit{S}}^0{D}_s^{+}{D}^{*-}$ and $e^{+}{e}^{-}\to K_{\mathit{S}}^0{D}_s^{*+}{D}^{-}$, as well as their charge conjugated processes, at five center-of-mass energies between 4.628 and 4.699 GeV, using data samples corresponding to an integrated luminosity of $3.8{\mathrm{fb}}^{-1}$ collected by the BESIII detector at the BEPCII storage ring. Based on a partial reconstruction technique, we find evidence of a structure near the thresholds for $D_s^{+}{D}^{*-}$ and $D_s^{*+}{D}^{-}$ production in the $K_{\mathit{S}}^0$ recoil-mass spectrum, which we refer to as the $Z_{cs}(3985{)}^0$. Fitting with a Breit-Wigner line shape, we find the mass of the structure to be $(3992.2\pm 1.7\pm 1.6)\mathrm{MeV}/c^2$ and the width to be $(7.7_{-3.8}^{+4.1}\pm 4.3)\mathrm{MeV}$, where the first uncertainties are statistical and the second are systematic. The significance of the $Z_{cs}(3985{)}^0$ signal is found to be $4.6\sigma$ including both the statistical and systematic uncertainty. We report the Born cross section multiplied by the branching fraction at different energy points. The mass of the $Z_{cs}(3985{)}^0$ is close to that of the $Z_{cs}(3985{)}^{+}$. Assuming SU(3) symmetry, the cross section of the neutral channel is consistent with that of the charged one. Hence, we conclude that the $Z_{cs}(3985{)}^0$ is the isospin partner of the $Z_{cs}(3985{)}^{+}$.

Figure 1

Invariant-mass distributions of the singly tagged $D_s^{+}$ (a) and $D^{-}$ (b), together with the fits to the recoil-mass distributions $RQ(K_{\mathit{S}}^0{D}_s^{+})$ (c) and $RQ(K_{\mathit{S}}^0{D}^{-})$ (d) at 4.682 GeV. The points with error bars are data. The blue-dashed lines show the fit results of the sideband regions, which are denoted by the red arrows. The histograms show the distributions from the nonresonant signal MC samples, which are scaled according to the yields of $D^{*-}$ and $D_s^{*+}$. The orange arrows indicate the signal regions of the $D^{*-}$ in (c) and $D_s^{*+}$ in (d).

Figure 2

Simultaneous fit to the recoil-mass $RM(K_{\mathit{S}}^0)$ spectra in all datasets [(a)–(e)], and for all the data points combined (f). The green dashed curves show the $Z_{cs}^0$ signal contribution. The pink dash-dotted curves show the nonresonant process. The blue dotted curves show combinatorial backgrounds. The black long dash-dotted curves show contributions from the highly excited $D_{(s)}^{**}$ backgrounds.