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Inclusive branching ratios involving b->t transitions are measured in
approximately four million hadronic Z decays collected by the ALEPH detector at
LEP. The fully-inclusive branching ratio b->tnX and the semi-inclusive
branching ratio b->tnD±X are measured to be (2.43±0.20±0.25)% and (0.88±0.31±0.28)%, in agreement with the standard model
predictions. Upper limits on the branching fractions b->tn
and b->stn are set to 8.3x10-4 and 6.4x10-4 at the
90% C.L. These results
allow a 90% C.L. lower limit of 0.40 (GeV/c2)-1 to be set on the
tan b/mH± ratio, in the framework of type-II two-Higgs-doublet models.
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