Measurement of D S-→ τ-ν̄ Τ and a new limit for B-→ Τ̄ ν̄ Τ L 3 Collaboration

Using a data sample of 1475 000 Z --f q4( y) events collected during 1994 with the L3 detector at LEP, we have studied the purely leptonic decays of heavy flavour mesons, D; + r-F, and B--+ Fi&. A signal is observed in the invariant mass distribution M(yDL) corresponding to the decay sequence D:---) rD;, DF -+ r-fir, 7-+ I-?/z+. The branching fraction for D; -+ 7-& decays is measured to be B(D; + T-Y,) = 0.074% O.O28(stat) f O.O16(syst) IIZ O.O18(norm). No signal of B--f T-Y, decays is observed in the data, corresponding to an upper limit on the branching fraction n(B-+ 7-i;,) < 5.7 x 10e4 at 90% CL. @ 1997 Elsevier Science B.V.