Measurement of Ds→τντ and search for B→τντ with the L3 detector

The purely leptonic decays of heavy mesons Ds→τντ and B→τντ are studied in a sample of 1.5M Z→q bar q(γ) events collected in 1994 by the L3 detector at LEP. A signal for the decay chain Ds0→γDs, Ds→τντ, τ→scr(l)νscr(l)ντ is observed in the invariant mass distribution M(γDs). From this signal, a branching ration of scr(B)(Ds→τντ)=0.074±0.028(stat)±0.016(syst)±0.018(n rm) is derived, corresponding to a decay constant fDs=309±58(stat)±33(syst)±38(norm)thinspGeV. No signal is observed for the decay B→τντ, and an upper limit of scr(B)(B→τντ)<5.7x10-4 at 90% CL is set. These results have been published in [1]. copyright 1998 American Institute of Physics