Evidence for the leptonic decay D→μν

Purely leptonic decays of the charged D meson have been studied using the reaction e + e − →D ∗+ D − at a center of mass energy of 4.03 GeV. A search was performed for D→μνμ recoiling against a D0 or D+ which had been reconstructed from its hadronic decay products. A single event candidate was found in the reaction e + e − →D ∗+ D − , where D ∗+ →π + D 0 with the D0→K−π+, and the recoiling D− decaying via D − →μ − ν μ . This yields a branching fraction value B(D→μνμ)=0.08−0.05−0.02+0.16+0.05%, and a corresponding value of the pseudoscalar decay constant fD=300−150−40+180+80 MeV.