Observation of D⁰→ b₁(1235)⁻ e⁺νₑ and evidence for $D^+→ b_1(1235)^0 e^+ν_e$

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 D⁰⁽⁺⁾ mesons into the axial-vector meson b₁(1235) via the decay b₁(1235)→ ωπ. The decay D⁰→ b₁(1235)⁻e⁺νₑ is observed with a significance of 5.2σ after considering systematic uncertainty, while evidence for the decay D⁺→ b₁(1235)⁰ e⁺νₑ is obtained with a 3.1σ significance. The product branching fractions are determined to be B(D⁰→ b₁(1235)⁻e⁺νₑ)× B (b₁(1235)⁻→ ω π⁻) = (0.72±0.18+0.06-0.08)×10⁻⁴ and B(D⁺→ b₁(1235)⁰e⁺νₑ)× B (b₁(1235)⁰~→ ω π⁰) = (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(D⁰→ b₁(1235)⁻e⁺νₑ)2Γ(D⁺→ b₁(1235)⁰e⁺νₑ)=0.78±0.19+0.04-0.05, which is consistent with unity, predicted by isospin invariance, within uncertainties.