Search for the baryon number and lepton number violating decays ${\tau }^{-}\to \mathrm{\Lambda }{\pi }^{-}$ and ${\tau }^{-}\to \bar{\mathrm{\Lambda }}{\pi }^{-}$ at Belle II

We present a search for the baryon number $B$ and lepton number $L$ violating decays ${\tau }^{-}\to \mathrm{\Lambda }{\pi }^{-}$ and ${\tau }^{-}\to \bar{\mathrm{\Lambda }}{\pi }^{-}$ produced from the $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ process, using a 364 fb${}^{-1}$ data sample collected by the Belle~II experiment at the SuperKEKB collider. No evidence of signal is found in either decay mode, which have $\mathrm{\mid }\mathrm{\Delta }(B-L)\mathrm{\mid }$ equal to $2$ and $0$, respectively. Upper limits at 90% credibility level on the branching fractions of ${\tau }^{-}\to \mathrm{\Lambda }{\pi }^{-}$ and ${\tau }^{-}\to \bar{\mathrm{\Lambda }}{\pi }^{-}$ are determined to be $4.7\times {10}^{-8}$ and $4.3\times {10}^{-8}$, respectively.