Upper limit for the decay B−→τ−ν̄τ

The branching ratio for the leptonic decay of charged B mesons (B−→τ−ν̄τ) has been measured using selected leptonic τ−→ℓ−ντν̄ℓ and hadronic τ−→ντX decays in Z→bb̄ decays recorded by DELPHI at LEP1 in 1992–1995. The result, BR(B−→τ−ν̄τ)<1.1×10−3 at the 90% confidence level, is consistent with standard model expectations and puts a constraint on the ratio tanβ/MH±<0.46 (GeV/c2)−1 in the framework of models with two Higgs doublets (type II Higgs doublet model). From the missing energy distribution in Z→bb̄ decays without identified leptons, the b→τν̄τX branching ratio has been measured in the hadronic channel τ→ντX′. The result, BR(b→τν̄τX)=(2.19±0.24 (stat)±0.39 (syst))%, is consistent with the Standard Model prediction and with previous experimental measurements.