First Observation of the Semileptonic Decay ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{e}^{+}{\nu }_e$

Using $4.5{\mathrm{f}\mathrm{b}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data samples collected at the center-of-mass energies ranging from 4.600~GeV to 4.699~GeV with the BESIII detector at the BEPCII collider, a first study of the semileptonic decays ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{e}^{+}{\nu }_e$, ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1520)e^{+}{\nu }_e$ and ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1405)e^{+}{\nu }_e$ is performed. The ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{e}^{+}{\nu }_e$ decay is observed with a significance of $8.2\sigma$ and the branching fraction is measured to be $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{e}^{+}{\nu }_e)=(0.88\pm {0.17}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.07}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$. We also report evidence of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1520)e^{+}{\nu }_e$ and ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1405)e^{+}{\nu }_e$ with significances of $3.3\sigma$ and $3.2\sigma$, respectively, and measure $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1520)e^{+}{\nu }_e)=(1.02\pm {0.52}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.11}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$ and $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }(1405)[\to p{K}^{-}]e^{+}{\nu }_e)=(0.42\pm {0.19}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.04}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$. Combining these with the inclusive semileptonic ${\mathrm{\Lambda }}_c^{+}$ branching fraction measured by BESIII, the relative fraction is determined to be $[\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{e}^{+}{\nu }_e)\mathrm{/}\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to X{e}^{+}{\nu }_e)]=(2.1\pm {0.4}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.2}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\mathrm{\%}$, which provides a clear confirmation that semileptonic ${\mathrm{\Lambda }}_c^{+}$ decays are not saturated by the $\mathrm{\Lambda }{\mathrm{\ell }}^{+}{\nu }_{\mathrm{\ell }}$ final state.