Evidence for a neutral near-threshold structure in the $K_{\mathrm{S}}^0$ recoil-mass spectra in $e^{+}{e}^{-}\to K_{\mathrm{S}}^0{D}_s^{+}{D}^{\ast -}$ and $e^{+}{e}^{-}\to K_{\mathrm{S}}^0{D}_s^{\ast +}{D}^{-}$

Abstract

We study the processes $e^{+}{e}^{-}\to K_{\mathrm{S}}^0{D}_s^{+}{D}^{\ast -}$ and $e^{+}{e}^{-}\to K_{\mathrm{S}}^0{D}_s^{\ast +}{D}^{-}$, as well as their charge conjugated processes, at five center-of-mass energies between 4.628~GeV and 4.699~GeV, using data samples corresponding to an integrated luminosity of $3.8\mathrm{f}{\mathrm{b}}^{-1}$ 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 $D_s^{+}{D}^{\ast -}$ and $D_s^{\ast +}{D}^{-}$ production in the $K_{\mathrm{S}}^0$ 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 $(3992.2\pm 1.7\pm 1.6)\mathrm{MeV}/{\mathrm{c}}^2$ 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 $Z_{cs}(3985{)}^{+}$.

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