Observation of D^{+}→K_{S}^{0}π^{0}μ^{+}ν_{μ}, Test of Lepton Flavor Universality, and First Angular Analysis of...

We report a study of the semileptonic decays D^{+}→K_{S}^{0}π^{0}ℓ^{+}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_{S}^{0}π^{0}μ^{+}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_{S}^{0}π^{0}e^{+}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 K[over ¯]^{*}(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)/A_{1}(0)=1.42±0.03_{stat}±0.02_{syst} and r_{2}=A_{2}(0)/A_{1}(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.