The Purely Leptonic Decays D + → μ + ν and D + s → l + ν at CLEO

(1) where GF is the Fermi constant, MD+ is theD + mass, ml the final state charged-lepton mass, and Vcd is a CKM matrix element, taken equal to Vus. Thus, in the SM measurement of purely leptonic decays allow a determination the decay constant, fD+ of the D + meson and similarly fDs of the D + s meson. Meson decay constants in the B system are used to translate measurements of BB̄ mixing to CKMmatrix elements. Currently, it is not possible to determine fB accurately from leptonic B decays, so theoretical calculations of fB must be used. Since the B 0 s meson does not have lν decays, it will never be possible to determine fBs experimentally, so again theory must be relied upon. If calculations disagree on D mesons, they may be questionable on B mesons. If, on the other hand new physics is present, it is imperative to understand how it effects SM based predictions of the B decay constants. Decay constants can be calculated using Lattice-QCD techniques. Recently, Follana et al. using an unquenched lattice technique predicted fD+ = (207± 4) MeV and fDs = (241± 3) MeV. [1] In these analyses we exploit the reactions ee → DD, and ee → D s D + s or D − s D ∗+ s . The D + is studied at 3770 MeV using 818 pb. D s is studied at 4170 MeV, using 400 pb for the μν, and τν, τ → πν final states, and 300 pb for τ → eνν. (Eventually CLEO will present results using 600 pb.)