A search for a dark baryon is performed for the first time in the two-body decay $\Xi^-\rightarrow\pi^-+{ \rm invisible}$ using $(10.087\pm0.044)\times10^{ 9}$ $J/\psi$ events collected at a center-of-mass energy of $\sqrt{ s}=3.097\,\mbox{ GeV}$ 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 $B(\Xi^-\rightarrow\pi^-+{ \rm invisible})$ are determined to be $4.2\times10^{ -5}$ ($5.2\times10^{ -5}$), $6.9\times10^{ -5}$ ($8.4\times10^{ -5}$), $6.5\times10^{ -4}$ ($7.6\times10^{ -4}$), $1.1\times10^{ -4}$ ($1.3\times10^{ -4}$) and $4.5\times10^{ -5}$ ($5.5\times10^{ -5}$), under the dark baryon mass hypotheses of 1.07$\,\mbox{ GeV}/c^2$, 1.10$\,\mbox{ GeV}/c^2$, $m_\Lambda$ (1.116$\,\mbox{ GeV}/c^2$), 1.13$\,\mbox{ GeV}/c^2$, and 1.16$\,\mbox{ GeV}/c^2$, respectively. The constraints obtained on the Wilson coefficients $C_{ u s, s}^L$ and $C_{ u s, s}^R$ are more stringent than the previous limits derived from the LHC searches for the colored mediators.
Categories: hep-ex