Search for a dark baryon in the ${\mathrm{\Xi }}^{-}\to {\pi }^{-}+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e}$ decay

A search for a dark baryon is performed for the first time in the two-body decay ${\mathrm{\Xi }}^{-}\to {\pi }^{-}+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e}$ using $(10.087\pm 0.044)\times {10}^9$ $J\mathrm{/}\psi$ events collected at a center-of-mass energy of $\sqrt{s}=3.097GeV$ with the BESIII detector at the BEPCII collider. No significant signal is observed, and the 90% (95%) confidence level upper limits on the branching fraction $B({\mathrm{\Xi }}^{-}\to {\pi }^{-}+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e})$ are determined to be $4.2\times {10}^{-5}$ ($5.2\times {10}^{-5}$), $6.9\times {10}^{-5}$ ($8.4\times {10}^{-5}$), $6.5\times {10}^{-4}$ ($7.6\times {10}^{-4}$), $1.1\times {10}^{-4}$ ($1.3\times {10}^{-4}$) and $4.5\times {10}^{-5}$ ($5.5\times {10}^{-5}$), under the dark baryon mass hypotheses of 1.07$GeV\mathrm{/}{c}^2$, 1.10$GeV\mathrm{/}{c}^2$, $m_{\mathrm{\Lambda }}$ (1.116$GeV\mathrm{/}{c}^2$), 1.13$GeV\mathrm{/}{c}^2$, and 1.16$GeV\mathrm{/}{c}^2$, respectively. The constraints obtained on the Wilson coefficients $C_{us,s}^L$ and $C_{us,s}^R$ are more stringent than the previous limits derived from the LHC searches for the colored mediators.