Observation of the singly Cabibbo suppressed decay $D^0\to b_1(1235{)}^{-}{e}^{+}{\nu }_e$ and evidence for $D^{+}\to b_1(1235{)}^0{e}^{+}{\nu }_e$

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 $7.9{\mathrm{f}\mathrm{b}}^{-1}$ 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 $b_1(1235)\to \omega \pi$. The decay $D^0\to b_1(1235{)}^{-}{e}^{+}{\nu }_e$ is observed with a significance of 5.2$\sigma$ after considering systematic uncertainty, while evidence for the decay $D^{+}\to b_1(1235{)}^0{e}^{+}{\nu }_e$ is obtained with a 3.1$\sigma$ significance. The product branching fractions are determined to be $\mathcal{B}(D^0\to b_1(1235{)}^{-}{e}^{+}{\nu }_e)\times \mathcal{B}(b_1(1235{)}^{-}\to \omega {\pi }^{-})=(0.72\pm 0.18_{-0.08}^{+0.06})\times {10}^{-4}$ and $\mathcal{B}(D^{+}\to b_1(1235{)}^0{e}^{+}{\nu }_e)\times \mathcal{B}(b_1(1235{)}^0\to \omega {\pi }^0)=(1.16\pm 0.44\pm 0.16)\times {10}^{-4}$, where the first uncertainties are statistical and the second systematic. The ratio of their partial decay widths is determined to be $\frac{\mathrm{\Gamma }(D^0\to b_1(1235{)}^{-}{e}^{+}{\nu }_e)}{2\mathrm{\Gamma }(D^{+}\to b_1(1235{)}^0{e}^{+}{\nu }_e)}=0.78\pm 0.19_{-0.05}^{+0.04}$, which is consistent with unity, predicted by isospin invariance, within uncertainties.