First observation of the semileptonic decay ${\Lambda }_{c}^{+}\to p{K}^{-}{e}^{+}{\nu }_{e}$

Using $4.5\text{}\text{}{\mathrm{fb}}^{-1}$ 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 ${\Lambda }_{c}^{+}\to p{K}^{-}{e}^{+}{\nu }_{e}$, ${\Lambda }_{c}^{+}\to \Lambda \left(1520\right){e}^{+}{\nu }_{e}$ and ${\Lambda }_{c}^{+}\to \Lambda \left(1405\right){e}^{+}{\nu }_{e}$ is performed. The ${\Lambda }_{c}^{+}\to p{K}^{-}{e}^{+}{\nu }_{e}$ decay is observed with a significance of $8.2\sigma $ and the branching fraction is measured to be $B\left({\Lambda }_{c}^{+}\to \phantom{\rule{0ex}{0ex}}p{K}^{-}{e}^{+}{\nu }_{e}\right)=\left(0.88±0.1{7}_{\mathrm{stat}}±0.0{7}_{\mathrm{syst}}\right)×{10}^{-3}$. We also report evidence of ${\Lambda }_{c}^{+}\to \Lambda \left(1520\right){e}^{+}{\nu }_{e}$ and ${\Lambda }_{c}^{+}\to \Lambda \left(1405\right){e}^{+}{\nu }_{e}$ with significances of $3.3\sigma $ and $3.2\sigma $, respectively, and measure $B\left({\Lambda }_{c}^{+}\to \Lambda \left(1520\right){e}^{+}{\nu }_{e}\right)=\left(1.02±0.5{2}_{\mathrm{stat}}±0.1{1}_{\mathrm{syst}}\right)×{10}^{-3}$ and $B\left({\Lambda }_{c}^{+}\to \Lambda \left(1405\right)\left[\to p{K}^{-}\right]{e}^{+}{\nu }_{e}\right)=\left(0.42±0.1{9}_{\mathrm{stat}}±0.0{4}_{\mathrm{syst}}\right)×{10}^{-3}$. Combining these with the inclusive semileptonic ${\Lambda }_{c}^{+}$ branching fraction measured by BESIII, the relative fraction is determined to be $\left[B\left({\Lambda }_{c}^{+}\to p{K}^{-}{e}^{+}{\nu }_{e}\right)/B\left({\Lambda }_{c}^{+}\to X{e}^{+}{\nu }_{e}\right)\right]=\left(2.1±0.{4}_{\mathrm{stat}}±0.{2}_{\mathrm{syst}}\right)%$, which provides a clear confirmation that semileptonic ${\Lambda }_{c}^{+}$ decays are not saturated by the $\Lambda {\ell }^{+}{\nu }_{\ell }$ final state.