Direct measurement of the pseudoscalar decay constant 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 = 3.773 GeV 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 + ν μ are observed. Those yield the branching fraction of BF ( D + → μ + ν μ ) = ( 0.122 −0.053 +0.111 ± 0.010 ) % , and a corresponding value of the pseudoscalar decay constant f D + = ( 371 −119 +129 ± 25 ) MeV .