Searches for the baryon- and lepton-number violating decays $B^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\ell }}^{-}$, $B^{-}\to \mathrm{\Lambda }{\mathrm{\ell }}^{-}$, and $B^{-}\to \bar{\mathrm{\Lambda }}{\mathrm{\ell }}^{-}$

Searches for $B$ mesons decaying to final states containing a baryon and a lepton are performed, where the baryon is either ${\mathrm{\Lambda }}_c$ or $\mathrm{\Lambda }$ and the lepton is a muon or an electron. These decays violate both baryon and lepton number and would be a signature of physics beyond the standard model. No significant signal is observed in any of the decay modes, and upper limits in the range $(3.2-520)\times {10}^{-8}$ are set on the branching fractions at the 90% confidence level.