Observation of the Semileptonic $D^{+}$ Decay into the ${\overline{K}}_1(1270{)}^0$ Axial-Vector Meson

By analyzing a $2.93{\mathrm{fb}}^{-1}$ 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 $D^{+}\to {\overline{K}}_1(1270{)}^0{e}^{+}{\nu }_e$ with a statistical significance greater than $10\sigma$. Its decay branching fraction is determined to be $\mathcal{B}[D^{+}\to {\overline{K}}_1(1270{)}^0{e}^{+}{\nu }_e]=(2.30\pm {0.26}_{-0.21}^{+0.18}\pm 0.25)\times {10}^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${\overline{K}}_1(1270{)}^0\to K^{-}{\pi }^{+}{\pi }^0$.

Figure 1

The $M_{\mathrm{BC}}$ distributions of the ST candidates in the data sample (dots with error bars). Blue solid curves are the fit results and red dashed curves represent the background contributions of the fit. The pair of red arrows in each subfigure indicate the $M_{\mathrm{BC}}$ window.

Figure 2

(a) The $M_{K^{-}{\pi }^{+}{\pi }^0}$ vs $U_{\text{miss}}$ distribution of the SL candidate events and (b), (c) the projections to $M_{K^{-}{\pi }^{+}{\pi }^0}$ and $U_{\text{miss}}$, respectively, with the residual $\chi$ distributions of the 2D fit. Dots with error bars are data. Blue solid, red, and black dashed curves are the fit result, the fitted signal, and the fitted background, respectively.