The Purely Leptonic Decays D+ -> mu+ nu and Ds+-> lepton+ nu at CLEO
Abstract
We update our previous results by increasing the luminosity, the efficiency, and for the Ds+ the number of tags. We determine fD+=(205.8 +/- 8.5 +/- 2.5) MeV, and an interim preliminary value of fDs+=(267.9 +/- 8.2 +/- 3.9) MeV, where both results are radiatively corrected. We agree with the recent most precise unquenched Lattice-QCD calculation for the D+, but are in disagreement for the Ds+. Several consequences are discussed, including the possibility of physics beyond the Standard Model.
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