The BESIII collaboration has reported “Observation of D⁺→ K_S⁰π⁰μ⁺ν_μ, Test of Lepton Flavor Universality and First Angular Analysis of D⁺→K^*(892^)0barl⁺ν_l”. These results have been published in Physical Review Letters on October 21, 2025. [Phys. Rev. Lett. 135, 171801 (2025)].
Semileptonic decays of charmed mesons provide important opportunities for rigorously testing the standard model (SM) and probing for potential new physics. Since the establishment of the SM, CP violation and its possible extensions have remained central topics in partial physics. To address the observed natter antimatter asymmetry in the universe, various models have been proposed to identify additional sources of CP violation. To explore if the anomalies observed in semileptonic B decays, deviating from SM predictions by 2 to 3 deviations, originate from real physics effects---it is crucial to investigate whether similar effects may appear in semileptonic charm decays which involve analogous quark level transitions. The high precision measurements of branching fraction ratios, hadronic transition form factors and the full set of angular coefficients, are therefore essential to constrain the relevant new physics parameters.

In this work, we report the first observation of the decay D⁺→ K_S⁰π⁰μ⁺ν_μ, and determine its branching fraction for the first time to be (0.896±0.017±0.008)%, while that of D⁺→ K_S⁰π⁰e⁺ν_e is determined with improved precision to be (0.943±0.012±0.010)%. These lead to their branching fraction ratio to be 0.94±0.02±0.01, which is consistent with the SM prediction. From the analysis of the decay dynamics of D⁺→ K_S⁰π⁰l⁺ν_l, the form factor ratios of D⁺→K^*(892^)0bar semileptonic transition are determined to be r_V=1.42±0.03±0.02 and r₂=0.75±0.03±0.01. We also present the first measurement of averaged angular observables and CP asymmetries in the full set of angular distributions and asymmetries in the decay rates of D⁺→K^*(892^)0barl⁺ν_l . The famous forward-backward asymmetries are measured to be A_FB=(-18.7±1.4±0.5)% for D⁺→K^*(892^)0bare⁺ν_e and A_FB=(-23.4±2.3±0.3)% for D⁺→K^*(892^)0barμ⁺ν_μ. The triple product asymmetries are found to be A_TP=(-0.4±1.3±0.0)% for D⁺→K^*(892^)0bare⁺ν_e and A_TP=(-1.9±2.0±0.3)% for D⁺→K^*(892^)0barμ⁺ν_μ. All results are consistent with SM.

Fig. 1. Projections of the data and amplitude analysis fits for the (top) μ⁺ and (bottom) e⁺ channels.

Fig. 2. Measured observables <A_i>, <S_i>, R_μ/e and the CP asymmetry for D⁺→K^*(892^)0barl⁺ν_l in different q² regions with 0.8<m_K*(892)0bar<1.0 GeV²/c⁴. The e⁺ (μ⁺) results are shown in blue (red). The horizontal lines and shaded bands represent the mean value and 1 standard deviation across full q² range.

Reference: Phys. Rev. Lett. 135, 171801 (2025)
Journal link: http://link.aps.org/doi/10.1103/w9vz-4fq9