Measurement of B ( D + s → μ + ν μ ) / B ( D + s → φ μ + ν μ ) and Determination of the Decay Constant f

We have observed 23.2±6.0 −0.9 purely-leptonic decays of D + s → μ νμ from a sample of muonic one prong decay events detected in the emulsion target of Fermilab experiment E653. Using the D+ s → φμ νμ yield measured previously in this experiment, we obtain B(D+ s → μ νμ)/B(D + s → φμ νμ) = 0.16 ± 0.06 ± 0.03. In addition, we extract the decay constant fDs = 194 ± 35 ± 20 ± 14 MeV . 1) Aichi University of Education, Kariya 448, JAPAN 2) University of California (Davis), Davis, CA 95616, USA