Observation of D⁰ b₁ (1235) ^- e^+ₑ and evidence for D^+ b₁ (1235) ⁰ e^+ₑ

By analyzing a data sample of e^+e^- collisions with center-of-mass energy 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 b₁ (1235) via the decay b₁ (1235). The decay D⁰ b₁ (1235) ^-e^+₄ is observed with a significance of 5. 2 after considering systematic uncertainty, while evidence for the decay D^+ b₁ (1235) ⁰ e^+ₑ is obtained with a 3. 1 significance. The product branching fractions are determined to be B (D⁰ b₁ (1235) ^-e^+₄) B (b₁ (1235) ^- ^-) = (0. 720. 18^+0. 06-₀. ₀₈) 10^-4 and B (D^+ b₁ (1235) ⁰e^+₄) B (b₁ (1235) ⁰~ ⁰) = (1. 160. 440. 16) 10^-4, where the first uncertainties are statistical and the second systematic. The ratio of their partial decay widths is determined to be (D⁰ b₁ (₁₂₃₅) ^-₄^+₄) 2 (D^+ b₁ (1235) ⁰e^{+₄) }=0. 780. 19^+0. 04-₀. ₀₅, which is consistent with unity, predicted by isospin invariance, within uncertainties.