Observation of the semileptonic decays D⁰ KS⁰^-⁰ e^+ ₑ and D^+ KS⁰^+^- e^+ ₑ

By analyzing e^+e^- annihilation data corresponding to an integrated luminosity of 2. 93 fb^-1 collected at a center-of-mass energy of 3. 773 GeV with the BESIII detector, the first observation of the semileptonic decays D⁰ KS⁰^-⁰ e^+ ₑ and D^+ KS⁰^+^- e^+ ₑ is reported. With a dominant hadronic contribution from K₁ (1270), the branching fractions are measured to be B (D⁰ K₁ (1270) ^- (K⁰S^-⁰) e^+ₑ) = (1. 69^+0. 53-₀. ₄₆0. 15) 10^-4 and B (D^+ K₁ (1270) ⁰ (K⁰S^+^-) e^+ₑ) = (1. 47^+0. 45-₀. ₄₀0. 20) 10^-4 with statistical significance of 5. 4 and 5. 6, respectively. When combined with measurements of the K₁ (1270) K^+^- decays, the absolute branching fractions are determined to be B (D⁰ K₁ (1270) ^-e^+ₑ) = (1. 05^+0. 33-₀. ₂₈0. 120. 12) 10^-3 and B (D^+ K₁ (1270) ⁰e^+ₑ) = (1. 29^+0. 40-₀. ₃₅0. 180. 15) 10^-3. The first and second uncertainties are statistical and systematic, respectively, and the third uncertainties originate from the assumed branching fractions of the K₁ (1270) K decays.