Observation of the Semileptonic $D^+$ Transition into an Axial-Vector Meson $bar K_1(1270)^0$

By analyzing a 2.93~$rm fb^{-1}$ data sample of $e^+e^-$ collisions, recorded at a center-of-mass energy of 3.773 $rm ,GeV$ 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 $D^{+} rightarrow {bar{K}}_{1}(1270)^{0}e^{+} u_{e}$ decay is observed for the first time with a statistical significance greater than $10sigma$. Its decay branching fraction is determined to be ${mathcal B}[D^+to bar K_1(1270)^0 e^+ u_e]=(2.30pm0.26pm 0.18 pm 0.25)times10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${bar{K}}_{1}(1270)^{0} rightarrow K^{-} pi^{+} pi^{0}$.