Observation of $ {D}^{+}\to {K}_{S}^{0}{\pi }^{0}{\mu }^{+}{\nu }_{\mu }$, Test of Lepton Flavor Universality, and First Angular Analysis of $ {D}^{+}\to {\overline{K}}^{*}(892{)}^{0}{\ell }^{+}{\nu }_{\ell }$

We report a study of the semileptonic decays $ {D}^{+}\to {K}_{S}^{0}{\pi }^{0}{\ell }^{+}{\nu }_{\ell }$ ($ \ell =e$, $ \mu $) based on $ 20.3\text{}\text{}{\mathrm{fb}}^{-1}$ of $ {e}^{+}{e}^{-}$ collision data collected at the center-of-mass energy of 3.773 GeV with the BESIII detector. The $ {D}^{+}\to {K}_{S}^{0}{\pi }^{0}{\mu }^{+}{\nu }_{\mu }$ decay is observed for the first time, with a branching fraction of $ (0.896\pm 0.01{7}_{\mathrm{stat}}\pm 0.00{8}_{\mathrm{syst}})\%$, and the branching fraction of $ {D}^{+}\to {K}_{S}^{0}{\pi }^{0}{e}^{+}{\nu }_{e}$ is determined with the improved precision as $ (0.943\pm 0.01{2}_{\mathrm{stat}}\pm 0.01{0}_{\mathrm{syst}})\%$. From the analysis of the dynamics, we observe that the dominant $ {\overline{K}}^{*}(892{)}^{0}$ component is accompanied by an $ S$-wave contribution, which accounts for $ (7.10\pm 0.6{8}_{\mathrm{stat}}\pm 0.4{1}_{\mathrm{syst}})\%$ of the total decay rate of the $ {\mu }^{+}$ channel and $ (6.39\pm 0.1{7}_{\mathrm{stat}}\pm 0.1{4}_{\mathrm{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\pm 0.0{3}_{\mathrm{stat}}\pm 0.0{2}_{\mathrm{syst}}$ and $ {r}_{2}={A}_{2}(0)/{A}_{1}(0)=0.75\pm 0.0{3}_{\mathrm{stat}}\pm 0.0{1}_{\mathrm{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 $ \mu -e$ lepton-flavor universality is observed in either the full range or the five chosen bins of momentum-transfer squared.