The BESIII collaboration has reported the observation of Ds⁺→η^'μ⁺ν_μ, precision test of lepton flavor universality with Ds⁺→η^(') l⁺ ν_l, and first measurements of Ds⁺→η^(') μ⁺ ν_μ decay dynamics. These results have been published in Physical Review Letters on 28 February 2024. [Phys. Rev. Lett. 132, 091802 (2024)].

Leptons are the fundamental particles in nature. The couplings between the three generations of leptons (electron, muon and tau) and the gauge bosons are equal in the standard model, known as lepton flavor universality. The discovery of any violation of lepton flavor universality would imply the existence of new physics. Therefore, precision test of lepton flavor universality is a major frontier in particle physics. In recent years, evidences of lepton flavor universality violation have been implied in several B decays, however, it is still unknown whether these evidences are real physical effects or experimental bias. Semileptonic charm decays involve electron or muon, which are ideal window to test μ-e lepton flavor universality. Based on 7.33 fb^-1 of e⁺ e^- annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV, the observation of Ds⁺→η^'μ⁺ν_μ and the studies of Ds⁺→η^(')μ⁺ν_μ dynamics are presented for the first time. The branching fractions of Ds⁺→ημ⁺ν_μ and Ds⁺→η^'μ⁺ν_μ are measured to be (2.235±0.051±0.052)% and (0.801±0.055±0.028)%, respectively, with precision improved by about sixfold over the previous best measurements. Combining our branching fractions or partial decay rates of Ds⁺→η^(')μ⁺ν_μ with that of Ds⁺→η(^') e⁺ ν_e reported in an early BESIII work, we test the μ-e lepton flavor universality with Ds⁺→η^(') l⁺ ν_l. Up to the precision of 3.3%, no evidence for lepton flavor universality violation is found, which is the most precise test of lepton flavor universality in the semileptonic D_s^+ decays to date.