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

Abstract

By analyzing a 2.93~$\mathrm{f}{\mathrm{b}}^{-1}$ data sample of $e^{+}{e}^{-}$ collisions, recorded at a center-of-mass energy of 3.773 $\mathrm{GeV}$ with the BESIII detector operated at the BEPCII collider, we have searched for the semileptonic $D^{+}$ transition into the axial-vector meson ${\bar{K}}_1(1270{)}^0$. The $D^{+}\to {\bar{K}}_1(1270{)}^0{e}^{+}\hat{I}{½}_e$ decay is observed for the first time with a statistical significance greater than $10\ddot{I}ƒ$. Its decay branching fraction is determined to be $\mathcal{B}[D^{+}\to {\bar{K}}_1(1270{)}^0{e}^{+}\hat{I}{½}_e]=(2.30\pm 0.26\pm 0.18\pm 0.25)\times 10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${\bar{K}}_1(1270{)}^0\to K^{-}\ddot{I}{€}^{+}\ddot{I}{€}^0$.