Search for a dark baryon in the Ξ−→π−+invisible decay

A search for a dark baryon is performed for the first time in the two-body decay Ξ−→π−+invisible using (10.087 ± 0.044) × 10 ⁹ J/ ψ events collected at a center-of-mass energy of 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(Ξ−→π−+invisible) are determined to be 4.2×10−5 ( 5.2×10−5), 6.9×10−5 ( 8.4×10−5), 6.5×10−4 ( 7.6×10−4), 1.1×10−4 ( 1.3×10−4) and 4.5×10−5 ( 5.5×10−5), under the dark baryon mass hypotheses of 1.07 GeV/ c ², 1.10 GeV/ c ², mΛ (1.116 GeV/ c ²), 1.13 GeV/ c ², and 1.16 GeV/ c ², respectively. The constraints obtained on the Wilson coefficients Cus,sL and Cus,sR are more stringent than the previous limits derived from the LHC searches for the colored mediators.