Observation of D⁺→K_S⁰π⁰μ⁺ν_μ, Test of Lepton Flavor Universality, and First Angular Analysis of D+ → K ¯^∗(892)⁰ℓ⁺ν_ℓ

We report a study of the semileptonic decays D⁺→K_S⁰π⁰ℓ⁺ν_ℓ (ℓ=e, μ) based on 20.3 fb^-1 of e⁺e^- collision data collected at the center-of-mass energy of 3.773 GeV with the BESIII detector. The D⁺→K_S⁰π⁰μ⁺ν_μ decay is observed for the first time, with a branching fraction of (0.896±0.017_stat±0.008_syst)%, and the branching fraction of D⁺→K_S⁰π⁰e⁺ν_e is determined with the improved precision as (0.943±0.012_stat±0.010_syst)%. From the analysis of the dynamics, we observe that the dominant K̄^∗(892)⁰ component is accompanied by an S-wave contribution, which accounts for (7.10±0.68_stat±0.41_syst)% of the total decay rate of the μ⁺ channel and (6.39±0.17_stat±0.14_syst)% of the e⁺ channel. Assuming a single-pole dominance parametrization, the hadronic form factor ratios are extracted to be r_V=V(0)/A₁(0)=1.42±0.03_stat±0.02_syst and r₂=A₂(0)/A₁(0)=0.75±0.03_stat±0.01_syst. Based on the first comprehensive angular and the decay-rate CP asymmetry analysis, the full set of averaged angular and CP asymmetry observables are measured as a function of the momentum-transfer squared; they are consistent with expectations from the standard model. No evidence for violation of μ-e lepton-flavor universality is observed in either the full range or the five chosen bins of momentum-transfer squared.