The Beijing Spectrometer Experiment (BESIII) has recently reported the world’s best measurement of the leptonic decay Ds+→μ+ν by analyzing the data sample taken at the center-of-mass energy of 4.178 GeV during 2016.

The leptonic charmed meson decays D(s)+→l+ν (l=e, μ, or τ) offer a unique window into both strong and weak effects in the charm quark sector. At leading order in standard model (SM), the partial width of the D(s)+→l+νdecay can be simply written as a function of the pseudo-scalar meson decay constant fD(s)+ and the c→d(s) Cabibbo-Kobayashi-Mashkawa (CKM) element |Vcd(s)|, which describe the strong and weak interactions between the c and d(s) quarks, respectively. Any significant deviations from the theoretically predicted value of fD(s)+ or the CKM matrix unitarity would imply new physics beyond the SM. On the other hand, in the SM, lepton flavor universality (LFU) requires equality of couplings between the three families of leptons and gauge bosons. The leptonic Ds+ decays, which are well understood in the SM, provide an ideal opportunity to test LFU.

With the world’s largest threshold Ds+ data sample and a high quality muon counter, BESIII precisely determined the branching fraction of Ds+→μ+νto be (0.549？0.016？0.015)%, where the first uncertainty is statistical and the second uncertainty is systematic. Combining our measured B[Ds+→μ+ν] with the world averages of B[Ds+→μ+ν] and B[Ds+→τ+ν], we determine the branching fraction ratio B[Ds+→τ+ν]:B[Ds+→μ+ν] = 9.98？？.52, which agrees with the SM predicted value of 9.72 within the uncertainty, thus indicating no LFU violation is found.

Based on our measured value of B[Ds+→μ+ν] and appropriate external inputs, we determined fDs+ = 246.2？3.6？3.5 MeV and |Vcs| = 0.985？0.014？0.014. These are all the most precise measurements to date, and provide important data to calibrate various theoretical calculations [e.g., quenched or unquenched lattice quantum chromodynamics (LQCD), QCD sum rules etc] on fDs+. The obtained value of |Vcs| is important to test the CKM matrix unitarity. Also, we determined the ratios fDs+:fD+=1.24？0.04？0.02 and |Vcd|2/|Vcs|2=0.048？0.003？0.001, which deviate from the LQCD predicted value by 2σand the SM expected value in CKMfitter by 2σ, respectively. More precise measurements are desirable to understand these tensions.

The results were published in the latest issue of Physical Review Letters (22 February, 2019, Volume 122, issue 7, 071802). The precision of fDs+ is expected to be further improved by the ongoing study of the leptonic decay Ds+→τ+νvia variousτdecay modes with the same data sample. Also, the potential to improve all these measurements by using more existing or future BESIII data samples taken around 4.178 GeV is also being studied.