Measurement of the Absolute Branching Fraction of D+s → τ+ vτ Decay

Using a sample of tagged D + s decays collected near the D *± s D ± s peak production energy in e + e - collisions with the CLEO-c detector, we study the leptonic decay D + s → r + v τ via the decay channel τ + → e + v e v τ . We measure B(D + s → τ + v τ ) = (6.17 ± 0.71 ± 0.34)%, where the first error is statistical and the second systematic. Combining this result with our measurements of D + s → μ + v μ and D + s → τ + v τ (via τ + → π + v τ ), we determine f Ds = (274 ± 10 ± 5) MeV.