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

## TL;DR

This paper reports the first observation of the semileptonic decay of the $D^+$ meson into an axial-vector meson ${ar{K}}_{1}(1270)^0$ with a measured branching fraction, using data from the BESIII detector.

## Contribution

First observation of the $D^+
ightarrow ar{K}_1(1270)^0 e^+
u_e$ decay with a precise measurement of its branching fraction.

## Key findings

- Decay observed with >10σ significance.
- Branching fraction measured as (2.30 ± 0.26 ± 0.18 ± 0.25)×10^{-3}.
- First experimental evidence of this semileptonic transition.

## Abstract

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 $D^{+} \rightarrow {\bar{K}}_{1}(1270)^{0}e^{+}\nu_{e}$ decay is observed for the first time with a statistical significance greater than $10\sigma$. Its decay branching fraction is determined to be ${\mathcal B}[D^+\to \bar K_1(1270)^0 e^+\nu_e]=(2.30\pm0.26\pm 0.18 \pm 0.25)\times10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${\bar{K}}_{1}(1270)^{0} \rightarrow K^{-} \pi^{+} \pi^{0}$.