Search for the semileptonic decays ${\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ at Belle

Using the full data sample of 980 ${\mathrm{f}\mathrm{b}}^{-1}$ collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays ${\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ ($\mathrm{\ell }=e$ or $\mu )$. No significant signals are observed in the ${\mathrm{\Xi }}^0{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ invariant-mass distributions. Taking the decay ${\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+}$ as the normalization mode, we report 90% credibility upper limits on the branching fraction ratios $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{e}^{+}{e}^{-})\mathrm{/}\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})<6.7\times {10}^{-3}$ and $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{\mu }^{+}{\mu }^{-})\mathrm{/}\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})<4.3\times {10}^{-3}$ based on the phase-space assumption for signal decays. The 90% credibility upper limits on the absolute branching fractions of $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{e}^{+}{e}^{-})$ and $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^0{\mu }^{+}{\mu }^{-})$ are found to be $9.9\times {10}^{-5}$ and $6.5\times {10}^{-5}$, respectively.