Observation of the Singly Cabibbo Suppressed Decay D^{0}→b_{1}(1235)^{-}e^{+}ν_{e} and Evidence for...

By analyzing a data sample of e^{+}e^{-} collisions with center-of-mass energy sqrt[s]=3.773  GeV, corresponding to an integrated luminosity of 7.9  fb^{-1} collected with the BESIII detector operating at the BEPCII collider, we study semileptonic decays of the D^{0(+)} mesons into the axial-vector meson b_{1}(1235) via the decay b_{1}(1235)→ωπ. The decay D^{0}→b_{1}(1235)^{-}e^{+}ν_{e} is observed with a significance of 5.2σ after considering systematic uncertainty, while evidence for the decay D^{+}→b_{1}(1235)^{0}e^{+}ν_{e} is obtained with a 3.1σ significance. The product branching fractions are determined to be B[D^{0}→b_{1}(1235)^{-}e^{+}ν_{e}]×B[b_{1}(1235)^{-}→ωπ^{-}]=(0.72±0.18_{-0.08}^{+0.06})×10^{-4} and B[D^{+}→b_{1}(1235)^{0}e^{+}ν_{e}]×B[b_{1}(1235)^{0}→ωπ^{0}]=(1.16±0.44±0.16)×10^{-4}, where the first uncertainties are statistical and the second systematic. The ratio of their partial decay widths is determined to be {Γ[D^{0}→b_{1}(1235)^{-}e^{+}ν_{e}]/2Γ[D^{+}→b_{1}(1235)^{0}e^{+}ν_{e}]}=0.78±0.19_{-0.05}^{+0.04}, which is consistent with unity, predicted by isospin invariance, within uncertainties.