Using 20.3~fb$^{ -1}$ of $e^+e^-$ annihilation data collected at a center-of-mass energy of 3.773~GeV with the BESIII detector, we report the first observation of the semileptonic decays $D^0\to K^-\omega e^+\nu_e$ and $D^+\to K_S^0\omega e^+\nu_e$ with significances of $8.0\sigma$ and $5.8\sigma$, respectively, including systematic uncertainties. Their decay branching fractions are measured to be ${ \cal B}(D^0\to K^-\omega e^+\nu_e)=(9.3^{ +2.1}_{ -1.9}\pm 0.7)\times10^{ -5}$ and ${ \cal B}(D^+\to K_S^0\omega e^+\nu_e)=(6.6^{ +2.0}_{ -1.8}\pm 0.6)\times10^{ -5}$. Combining with the latest measurements of $D^{ 0(+)}\to K^-\pi^+\pi^{ -(0)} e^+\nu_e$ and assuming $\bar{ K}_1(1270)$ to be the sole mediating resonance in all processes, the branching ratios are determined to be $\frac{ \Gamma(K_1(1270)^-\to K^-\pi^+\pi^-)}{ \Gamma(K_1(1270)^-\to K^-\omega)} = 3.4^{ +0.8}_{ -0.7} \pm 0.3$ and $\frac{ \Gamma(\bar{ K}_1(1270)^0\to K^-\pi^+\pi^0)}{ \Gamma(\bar{ K}_1(1270)^0\to \bar{ K}^0\omega)} = 9.6^{ +3.0}_{ -2.7} \pm 0.8$. The combined branching fraction is determined to be $\mathcal B(\bar{ K}_1(1270)\to \bar{ K}\omega) = (7.5\pm 1.3 \pm 0.5)\%$, which is the most precise measurement from a collider experiment. The first uncertainties are statistical, and the second are systematic.
Categories: hep-ex