Measurement of D- s → τ- v̄τ and a new limit for B- → τ-v̄τ

Abstract Using a data sample of 1 475 000 Z → qq(γ) events collected during 1994 with the L3 detector at LEP, we have studied the purely leptonic decays of heavy flavour mesons, D s − → τ − ν τ and B − → τ − ν τ . A signal is observed in the invariant mass distribution M(γD s − ) corresponding to the decay sequence D ∗− → γD s − , D s − → τ − ν τ , τ − → l − ν l ν τ . The branching fraction for D s − → τ − ν τ decays is measured to be β (D s − → τ − ν τ ) = 0.074 ± 0.028(stat) ± 0.016(syst) ± 0.018(norm) . No signal of B − → τ − ν τ decays is observed in the data, corresponding to an upper limit on the branching franction β( B − → τ − ν τ ) −4 at 90% CL.