The BESIII collaboration has reported “First Measurement of the Decay Dynamics in the Semileptonic Transition of the D⁺⁽⁰⁾ into the Axial-vector Meson K₁(1270)”. These results have been published in Physical Review Letters on 25 August 2025. [Phys. Rev. Lett. 135, 091801 (2025)].

Semileptonic decays of charmed mesons provide an unique platform to investigate the internal structure of involved hadrons. Over the past few decades, the form factors of D mesons semileptonic transitions into pseduo-scalar and vector mesons have been extensively studied in both theory and experimental aspects. However, for the axial-vector mesons, there are only some theoretical predictions, without any experimental measurements yet. As a Cabbibo-favored transition into the lowest lying axial-vector meson octet, the D→K₁(1270) transition is expected to be the most promising channel with significantly higher statistics. The measurement of corresponding decay dynamcis will not only test these theoretical calculations, but also shed a light on the long-pending mixing angle θ_K1 between K₁(1270) and K₁(1400). In additional, the combination of up-down asymmetry in D→K₁(1270) e⁺ν_e and B→K₁(1270) γ offers a novel approach to extract the photon polarization in b→sγ, which is sensitive to right-handed new physics contributions.
In this work, we report the first measurement of the decay dynamic of D⁺⁽⁰⁾ → K^-π⁺π^0(-)e⁺ν_e.The amplitude analysis gives the form factors of D→K₁(1270) semileptonic transition to be r_A=(-11.2±1.0±0.9) ×10^-2 and r_V=(-4.3±1.0±2.5) ×10^-2 for the first time. The branching fractions of D⁺→K₁(1270)⁰ e⁺ν_e and D⁰→K₁(1270)^- e⁺ν_e  are determined with improved precision to be (2.27±0.11±0.07±0.07) ×10^-3 and (1.02±0.06±0.06±0.03) ×10^-3, respectively. For D⁺→K₁(1400)⁰ e⁺ν_e and D⁰→K₁(1400)^- e⁺ν_e, no significant signal is seen and corresponding branching fraction upper limits are set to be 1.4×10^-4 and 0.7×10^-4 at 90% C.L., respectively. The measured form factors and branching fractions allow stringent test on various theoretical works, and none of them is in agreement with our results. Meanwhile, based on the form factors from theoretical inputs, the mixing angle θ_K1 can be determined. Moreover, the angular analysis yields an up-down asymmetry A^'_ud=0.01±0.11, which is consistent with Standard Model predicted values 0.092±0.022.

Reference: Phys. Rev. Lett. 135, 091801 (2025)
Journal link: https://doi.org/10.1103/xj42-xgzf     

Fig 1. Comparison of (a) r_A, (b) r_V, (c) B(D⁰→K₁(1270)^- e⁺ν_e) and (d) B(D⁰→K₁(1400)^- e⁺ν_e) measured in this work and predicted by various theoretical approaches as a function of θ_K1.