The decay of beauty quark to τ lepton and neutrino in the DELPHI experiment at LEP

Abstract: Using 3.5 millions hadronic Z0 events collected during the LEP1 period at the DELPHI detector, the decays of b quarks to tvt have been searched for in both the inclusive and exclusive decay modes. The analysis technique is based on b-tagging, missing energy and identifications of t products. The inclusive semileptonic branching ratio was found to be BR(b --> tntX)= 2.52 ± 0.23(stat.) ± ( 0.49(syst.)%, a result that is in good agreement with the Standard Model prediction and with previous measurements by LEP experiments. No signal for the exclusive leptonic decay B- --> tn-t was observed in the data. The upper limit on the branching ratio $BR(B- --> t-nt) < 1.1x10-3 GeV-1 is set at 90% confidence level. As a consequence of this result a limit on tanb /MH± < 0.46 is also derived in the framework of any type II Higgs doublet model at 90 % confidence level.