First Measurement of the D s + → K 0 μ + ν μ Decay

We report the first measurement of the semileptonic decay D_{s}^{+}→K^{0}μ^{+}ν_{μ}, using a sample of e^{+}e^{-} annihilation data corresponding to an integrated luminosity of 7.33 fb^{-1} collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector at the BEPCII collider. The branching fraction of the decay is measured to be B(D_{s}^{+}→K^{0}μ^{+}ν_{μ})=(2.89±0.27_{stat}±0.12_{syst})×10^{-3}, where the first uncertainty is statistical and the second is systematic. Based on a simultaneous fit to the partial decay rates in q^{2} intervals measured in D_{s}^{+}→K^{0}μ^{+}ν_{μ} and D_{s}^{+}→K^{0}e^{+}ν_{e} decays, the product value of the form factor f_{+}^{K^{0}}(0) and the Cabibbo-Kobayashi-Maskawa matrix element |V_{cd}| is measured to be f_{+}^{K^{0}}(0)|V_{cd}|=0.140±0.008_{stat}±0.003_{syst}, which is the most precise determination to date. In addition, lepton flavor universality is tested for the first time with D_{s}^{+}→K^{0}ℓ^{+}ν_{ℓ} decays in full and separate q^{2} intervals. No violation is found.