We test whether or not the τ lepton manifests the same couplings as the μ lepton by investigating the relative decay rates in purely leptonic D+ meson decays. We use 281pb-1 of data accumulated at the ψ(3770) resonance with the CLEO-c detector, to limit B(D+→τ+ν)<2.1×10-3 at 90% confidence level (C.L.), thus allowing us to place the first upper limit on the ratio R=Γ(D+→τ+ν)/Γ(D+→μ+ν). The ratio of R to the standard model expectation of 2.65 then is <1.8 at 90% C.L., consistent with the prediction of lepton universality. © 2006 The American Physical Society.
Rubin, P., Cawlfield, C., Eisenstein, B. I., Karliner, I., Kim, D., Lowrey, N., … Yelton, J. (2006). An investigation of D+→τ+ν. Physical Review D - Particles, Fields, Gravitation and Cosmology, 73(11). https://doi.org/10.1103/PhysRevD.73.112005