Observation of the Semileptonic ${D}^{+}$ Decay into the ${\overline{K}}_{1}\left(1270{\right)}^{0}$ Axial-Vector Meson

By analyzing a $2.93\text{}\text{}{\mathrm{fb}}^{-1}$ data sample of ${e}^{+}{e}^{-}$ collisions, recorded at a center-of-mass energy of 3.773 GeV with the BESIII detector operated at the BEPCII collider, we report the first observation of the semileptonic ${D}^{+}$ transition into the axial-vector meson ${D}^{+}\to {\overline{K}}_{1}\left(1270{\right)}^{0}{e}^{+}{\nu }_{e}$ with a statistical significance greater than $10\sigma $. Its decay branching fraction is determined to be $B\left[{D}^{+}\to {\overline{K}}_{1}\left(1270{\right)}^{0}{e}^{+}{\nu }_{e}\right]=\phantom{\rule{0ex}{0ex}}\left(2.30±{0.26}_{-0.21}^{+0.18}±0.25\right)×{10}^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${\overline{K}}_{1}\left(1270{\right)}^{0}\to {K}^{-}{\pi }^{+}{\pi }^{0}$.