The spin-orbit splitting of $\Lambda$ single-particle states in ${}_{\Lambda }^{13}\mathrm{C}$ was measured. The ${}^{13}\mathrm{C}(K^{-},{\pi }^{-}{)}_{\Lambda }^{13}\mathrm{C}$ reaction was used to excite both the $1/2^{-}$ and $3/2^{-}$ states simultaneously, which have predominantly ${}^{12}\mathrm{C}\left(0^{+}\right)\times p_{\Lambda }$ configuration. $\gamma$ rays from the states to the ground state were measured in coincidence with the ${\pi }^{-}$'s, by which $\ell s$ splitting was found to be $152\pm 54\left(\mathrm{stat}\right)\pm 36\left(\mathrm{syst}\right)\mathrm{keV}$. The value is 20–30 times smaller than exhibited by the $\ell s$ splitting in the nuclear shell model. This value gives us new insight into the YN interaction.

• Received 1 September 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.4255