Direct measurement of the pseudoscalar decay constant fD+fD+

The absolute branching fraction for the decay D+→μ+ν has been directly measured based on a data sample of about 33 pb−1 collected around s=3.773GeV with the BES-II detector at the BEPC collider. A total of 5321±149±160 D− mesons are reconstructed in nine hadronic decay modes. In the system recoiling against these singly tagged D− mesons, 2.67±1.74 purely leptonic decay events of D+→μ+νμ are observed. Those yield the branching fraction of BF(D+→μ+νμ)=(0.122−0.053+0.111±0.010)%, and a corresponding value of the pseudoscalar decay constant fD+=(371−119+129±25)MeV.