First observation of the semileptonic decay Λc+→p K-e+νe

Using 4.5 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 Λ_c⁺→p K^-e⁺ν_e , Λ_c⁺→Λ (1520 )e⁺ν_e and Λ_c⁺→Λ (1405 )e⁺ν_e is performed. The Λ_c⁺→p K^-e⁺ν_e decay is observed with a significance of 8.2 σ and the branching fraction is measured to be B (Λ_c⁺→p K^-e⁺ν_e)=(0.88 ±0.1 7_stat±0.0 7_syst)×10^-3 . We also report evidence of Λ_c⁺→Λ (1520 )e⁺ν_e and Λ_c⁺→Λ (1405 )e⁺ν_e with significances of 3.3 σ and 3.2 σ , respectively, and measure B (Λ_c⁺→Λ (1520 )e⁺ν_e)=(1.02 ±0.5 2_stat±0.1 1_syst)×10^-3 and B (Λ_c⁺→Λ (1405 )[→p K^-]e⁺ν_e)=(0.42 ±0.1 9_stat±0.0 4_syst)×10^-3 . Combining these with the inclusive semileptonic Λ_c⁺ branching fraction measured by BESIII, the relative fraction is determined to be [B (Λ_c⁺→p K^-e⁺ν_e)/B (Λ_c⁺→X e⁺ν_e)]=(2.1 ±0. 4_stat±0. 2_syst)% , which provides a clear confirmation that semileptonic Λ_c⁺ decays are not saturated by the Λ ^ℓ+ν_ℓ final state.