Observation of $D^0\to K_1(1270{)}^{-}{e}^{+}\hat{I}{½}_e$

Abstract

Using 2.93 fb${}^{-1}$ of $e^{+}{e}^{-}$ collision data taken with the BESIII detector at a center-of-mass energy of 3.773 $\mathrm{GeV}$, the observation of the $D^0\to K_1(1270{)}^{-}{e}^{+}\hat{I}{½}_e$ semileptonic decay is presented. The statistical significance of the decay $D^0\to K_1(1270{)}^{-}{e}^{+}\hat{I}{½}_e$ is greater than $10\ddot{I}ƒ$. The branching fraction of $D^0\to K_1(1270{)}^{-}{e}^{+}\hat{I}{½}_e$ is measured to be $(1.09\pm 0.13_{-0.16}^{+0.09}\pm 0.12)\times 10^{-3}$. Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of $K_1(1270{)}^{-}\to K^{-}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$. The fraction of longitudinal polarization in $D^0\to K_1(1270{)}^{-}{e}^{+}\hat{I}{½}_e$ is determined for the first time to be $0.50\pm 0.19_{\mathrm{stat}}\pm 0.08_{\mathrm{syst}}$.

9 pages, 3 figures