Top-quark decay in models with Higgs triplets

It is well known that if the charged Higgs scalar (H^±) is lighter than the top quark (t) then the decay t→bH⁺ will dominate over the usual charged-current decays via the W in the two-Higgs-doublet extension of the standard model. We show that in models with Higgs fields in triplet representations, the fine-tuning of the ratio of triplet to doublet vacuum expectation values required by constraints on the ρ parameter naturally leads to competitive charged-Higgs-boson and W decay modes for the t quark if its mass is below M_W+m_b≃85 GeV. In these models we show that the branching fraction for t→bτν is always enhanced whereas the corresponding branching fraction for t→blν is suppressed. Universality violation is one of the best probes of this scenario.