Measurement of B(Ds+→μ+νμ)

We present a measurement of the branching fraction B(D-s(+)->mu(+)nu(mu)) using a 548 fb(-1) data sample collected by the Belle experiment at the KEKB e(+)e(-) collider. The D-s momentum is determined by reconstruction of the system recoiling against DK gamma X in events of the type e(+)e(-)->(DsDKX)-D-*, D-s(*)-> D-s gamma, where X represents additional pions or photons from fragmentation. This full-reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that achieved by experiments at the tau-charm factories. We obtain the branching fraction B(D-s(+)->mu(+)nu(mu))=[6.44 +/- 0.76(stat)+/- 0.57(syst)]x10(-3), implying a D-s decay constant of f(Ds)=[275 +/- 16(stat)+/- 12(syst)] MeV.