Measurement of the branching fraction of D⁺→ τ⁺ν_τ

By analyzing e⁺e⁻ collision data with an integrated luminosity of 7.9 fb⁻¹ collected with the BESIII detector at the center-of-mass energy of 3.773 GeV, the branching fraction of D⁺→ τ⁺ν_τ is determined as B = (9.9 ± 1.1_stat± 0.5_syst) × 10⁻⁴. Using the most precise result B(D⁺→ μ⁺ν_μ) = (3.981 ± 0.079_stat± 0.040_syst) × 10⁻⁴ [1], we determine R_τ/μ = Γ(D⁺ → τ⁺ν_τ)/Γ(D⁺ → μ⁺ν_μ) = 2.49 ± 0.31, achieving a factor of two improvement in precision compared to the previous BESIII result. This measurement is in agreement with the standard model prediction of lepton flavor universality within one standard deviation.