Observation of Ds+ →η′μ+νμ, Precision Test of Lepton Flavor Universality with Ds+ →η (′)l+νl, and First Measurements of Ds+ →η (′)μ+νμ Decay Dynamics

By analyzing 7.33 fb(-1) of e(+)e(-) annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector, we report the observation of the semileptonic decay D-s(+) -> eta'mu(+) nu(mu), with a statistical significance larger than 10 sigma, and the measurements of the D-s(+) -> eta mu(+) nu(mu) and D-s(+) -> eta mu(+) nu(mu) and D-s(+) -> eta'mu(+) nu(mu) decay dynamics for the first time. The branching fractions of D-s(+) -> eta'mu(+) nu(mu) and D-s(+) -> eta'mu(+) nu(mu) are determined to be (2.235 +/- 0.051(stat) +/- 0.052(syst))% and (0.801 +/- 0.055(stat) +/- 0.028(syst))%, respectively, with precision improved by factors of 6.0 and 6.6 compared to the previous best measurements. Combined with the results for the decays D-s(+) ->eta e(+)nu(e)e and D-s(+) ->eta'e(+)nu(e), the ratios of the decay widths are examined both inclusively and in several l(+) nu(e) four-momentum transfer ranges. No evidence for lepton flavor universality violation is found within the current statistics. The products of the hadronic form factors f(+,0)(eta()'()) (0) c -> s Cabibbo-Kobayashi-Maskawa matrix element |V-cs | are determined. The results based on the two-parameter series expansion are f(+,0)(eta) (0)|V-cs | = 0.452 +/- 0.010(stat) +/- 0.007(syst) and f(+,0)(eta)' (0)|V-cs | = 0.504 +/- 0.037(stat) +/- 0.012(syst), which help to constrain present models on f(+,0)(eta()'()) (0). The forward-backward asymmetries are determined to be < A(FB) (eta)> = -0.059 +/- 0.031(stat) +/- 0.005(syst) and < A(FB) (eta')> = -0.064 +/- 0.079(stat) +/- 0.006(syst) for the first time, which are consistent with the theoretical calculation.