The BESIII Collaboration has reported the most precise Ds+ decay constant fDs+ in a newly published Physical Review Letters paper [Phys. Rev. Lett. 127 (2021) 171801].
In the Ds+ leptonic decays, the fDs+ describes the amplitude for the quark and antiquark within the Ds+ meson to have zero separation, a condition necessary for them to annihilate into the virtual W+ boson that produces the l+ν (l= e, μ, τ) pair. High precision charm data enables us to calibrate the lattice quantum chromodynamics (LQCD) calculations of fDs+ and helps to guide the development of LQCD calculation techniques for better understanding of non-perturbative QCD effects.
This measurement was performed with the data samples corresponding to an integrated luminosity of 6.32 fb-1 collected at center-of-mass energies between 4.178 and 4.226 GeV. The absolute branching fraction of Ds+→τ+ν has been determined to be BF(Ds+→τ+ν)= (5.27±0.10±0.12)×10-2, where τ+ →e+νν. Taking the CKM matrix element |Vcs|=0.97320±0.00011 from the CKMfitter group as an input, the fDs+ is extracted to be (251.1±2.4±3.0) MeV. Combining the recent measurements of BF(Ds+→τ+ν) based on τ+→ π+ν and τ+→ π+π0ν from the BESIII Collaboration and the world average value, the relative sensitivity of fDs+ achieves to be 1.1%, of which about 50% contribution is from our measurement. Subsequently, combining the above updated world average BF(Ds+→τ+ν) and the world average BF(Ds+→μ+ν), a ratio of 9.67±0.34 is obtained, which is consistent with the Standard Model expectation of lepton flavor universality, 9.75±0.01 within the experimental uncertainty.
The PRL publication: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.127.171801
The INSPIRE entry: https://inspirehep.net/literature/1867208