We present a search for the decays of $B^0$ mesons into a final state containing a $\mathrm{\Lambda }$ baryon and missing energy. These results are obtained from a $711{\mathrm{fb}}^{-1}$ data sample that contains $772\times {10}^{6}B\overline{B}$ pairs and was collected near the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode and require the remainder of the event to consist of only a single $\mathrm{\Lambda }$. No evidence for these decays is found, and we set 90% confidence level upper limits on the branching fractions in the range $2.1–3.8\times {10}^{-5}$. This measurement provides the world’s most restrictive limits, with implications for baryogenesis and dark matter production.

• Received 29 October 2021
• Accepted 17 February 2022

DOI:https://doi.org/10.1103/PhysRevD.105.L051101

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Published by the American Physical Society