Observation of charmed strange meson pair production in $\mathrm{{\rm Y}}(2S)$ decays and in $e^{+}{e}^{-}$ annihilation at $\sqrt{s}=10.52\mathrm{G}\mathrm{e}\mathrm{V}$

We observe the process $\mathrm{{\rm Y}}(2S)\to D_s^{(\ast )+}{D}_{sJ}^{-}$ and continuum production $e^{+}{e}^{-}\to D_s^{(\ast )+}{D}_{sJ}^{-}$ at $\sqrt{s}=10.52$ GeV (and their charge conjugates) using the data samples collected by the Belle detector at KEKB, where $D_{sJ}^{-}$ is $D_{s1}(2536{)}^{-}$ or $D_{s2}^{\ast }(2573{)}^{-}$. Both $D_{sJ}^{-}$ states are identified through their decay into $\bar{K}{\bar{D}}^{(\ast )}$. We measure the products of branching fractions $\mathcal{B}(\mathrm{{\rm Y}}(2S)\to D_s^{(\ast )+}{D}_{sJ}^{-})\mathcal{B}(D_{sJ}^{-}\to \bar{K}{\bar{D}}^{(\ast )})$ and the Born cross sections ${\sigma }^{\mathrm{B}\mathrm{o}\mathrm{r}\mathrm{n}}(e^{+}{e}^{-}\to D_s^{(\ast )+}{D}_{sJ}^{-})\mathcal{B}(D_{sJ}^{-}\to \bar{K}{\bar{D}}^{(\ast )})$, and then compare the ratios $R_1\equiv \mathcal{B}(\mathrm{{\rm Y}}(2S)\to D_s^{(\ast )+}{D}_{sJ}^{-})\mathrm{/}\mathcal{B}(\mathrm{{\rm Y}}(2S)\to {\mu }^{+}{\mu }^{-})$ for $\mathrm{{\rm Y}}(2S)$ decays and $R_2\equiv {\sigma }^{\mathrm{B}\mathrm{o}\mathrm{r}\mathrm{n}}(e^{+}{e}^{-}\to D_s^{(\ast )+}{D}_{sJ}^{-})\mathrm{/}{\sigma }^{\mathrm{B}\mathrm{o}\mathrm{r}\mathrm{n}}(e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-})$ for continuum production. We obtain $R_1\mathrm{/}{R}_2=9.7\pm 2.3\pm 1.1$, $6.8\pm 2.1\pm 0.8$, $10.2\pm 3.3\pm 2.5$, and $3.4\pm 2.1\pm 0.5$ for the $D_s^{+}{D}_{s1}(2536{)}^{-}$, $D_s^{\ast +}{D}_{s1}(2536{)}^{-}$, $D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}$, and $D_s^{\ast +}{D}_{s2}^{\ast }(2573{)}^{-}$ final states in the $D_{sJ}^{-}\to K^{-}{\bar{D}}^{(\ast )0}$ modes, respectively. Therefore, the strong decay is expected to dominate in the $\mathrm{{\rm Y}}(2S)\to D_s^{(\ast )+}{D}_{sJ}^{-}$ processes. We also measure the ratios of branching fractions $\mathcal{B}(D_{s1}(2536{)}^{-}\to K_S^0{D}^{\ast }(2010{)}^{-})\mathrm{/}\mathcal{B}(D_{s1}(2536{)}^{-}\to K^{-}{D}^{\ast }(2007{)}^0)=0.48\pm 0.07\pm 0.02$ and $\mathcal{B}(D_{s2}^{\ast }(2573{)}^{-}\to K_S^0{D}^{-})\mathrm{/}\mathcal{B}(D_{s2}^{\ast }(2573{)}^{-}\to K^{-}{D}^0)=0.49\pm 0.10\pm 0.02$, which are consistent with isospin symmetry. The second ratio is the first measurement of this quantity. Here, the first uncertainties are statistical and the second are systematic.