First Observation of D^{0(+)}→K[over ¯]ωe^{+}ν_{e} and Determination of the Branching Fraction of K[over...

Using 20.3  fb^{-1} of e^{+}e^{-} annihilation data collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, we report the first observation of the semileptonic decays D^{0}→K^{-}ωe^{+}ν_{e} and D^{+}→K_{S}^{0}ωe^{+}ν_{e} with significances of 8.0σ and 5.8σ, respectively. Their decay branching fractions are measured to be B(D^{0}→K^{-}ωe^{+}ν_{e})=(9.4_{-1.8}^{+2.0}±0.6)×10^{-5} and B(D^{+}→K_{S}^{0}ωe^{+}ν_{e})=(8.0_{-2.1}^{+2.4}±0.7)×10^{-5}. Combining these with the latest measurements of D^{0(+)}→K^{-}π^{+}π^{-(0)}e^{+}ν_{e} and assuming K[over ¯]_{1}(1270) to be the sole mediating resonance in all processes, the branching ratios are determined to be {[Γ(K_{1}(1270)^{-}→K^{-}π^{+}π^{-})]/[Γ(K_{1}(1270)^{-}→K^{-}ω)]}=3.4_{-0.7}^{+0.8}±0.3 and {[Γ(K[over ¯]_{1}(1270)^{0}→K^{-}π^{+}π^{0})]/[Γ(K[over ¯]_{1}(1270)^{0}→K[over ¯]^{0}ω)]}=7.9_{-2.1}^{+2.4}±0.7. The combined branching fraction is determined to be B(K[over ¯]_{1}(1270)→K[over ¯]ω)=(8.4±1.4±0.5)%, which is the most precise measurement from a collider experiment. Furthermore, for the first time based on a single vector-pseudoscalar decay channel, the K[over ¯]_{1}(1270) mass is measured to be (1336±9±2)  MeV/c^{2}, providing unique experimental support for the two-pole structure of the K[over ¯]_{1}(1270) resonance. The first uncertainties are statistical, and the second are systematic.