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Feasibility Study of $D_s^+ \to τ^+ ν_τ$ Decay and Test of Lepton Flavor Universality with Leptonic $D_s^+$ Decays at STCF

We report a sensitive study of $D_s^+ \to τ^+ ν_τ$ decay via $τ^+ \to e^+ ν_e \barν_τ$ with an integrated luminosity of 1 ab$^{-1}$ at the center-of-mass energy of $4.009$ GeV at a future Super Tau Charm Facility (STCF). Under the help of the fast simulation software package, the statistical sensitivity for the absolute branching fraction of $D_s^+ \to τ^+ ν_τ$ is determined to be $2\times10^{-4}$. Combining with our previous prospect of $D_s^+ \to μ^+ ν_μ$, the ratio of the branching fractions for $D_s^+ \to τ^+ ν_τ$ over $D_s^+ \to μ^+ ν_μ$ can achieve a relative statistical precision of 0.5%, which will provide the most stringent test of the $τ$-$μ$ lepton flavor universality in heavy quark decays. Taking the decay constant $f_{D_s^+}$ from lattice QCD calculations or the CKM matrix element $|V_{cs}|$ from the CKMfitter group as an input, the relative statistical uncertainties for $|V_{cs}|$ and $f_{D_s^+}$ are estimated to be 0.3% and 0.2%, respectively.