Observation of $D^0to K_1(1270)^- e^+ u_e$

Using 2.93 fb$^{-1}$ of $e^+e^-$ collision data taken with the BESIII detector at a center-of-mass energy of 3.773 $rm ,GeV$, the observation of the $D^0to K_1(1270)^- e^+ u_e$ semileptonic decay is presented. The statistical significance of the decay $D^0to K_1(1270)^- e^+ u_e$ is greater than $10sigma$. The branching fraction of $D^0to K_1(1270)^- e^+ u_e$ is measured to be $(1.09pm0.13^{+0.09}_{-0.16} pm 0.12)times10^{-3}$. Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of $K_1(1270)^- rightarrow K^-pi^+pi^-$. The fraction of longitudinal polarization in $D^0to K_1(1270)^- e^+ u_e$ is determined for the first time to be $0.50pm0.19_{rm stat}pm0.08_{rm syst}$.