Light charged Higgs boson scenario in 3-Higgs doublet models

Abstract

The constraints from the measurements of the [Formula: see text] decay rate on the parameter space of 3-Higgs Doublet Models (3HDMs), where all the doublets have nonzero vacuum expectation values, are studied at the next-to-leading order in QCD. In order to naturally avoid the presence of flavour changing neutral currents at the tree level, we impose two softly-broken discrete [Formula: see text] symmetries. This gives rise to five independent types of 3HDMs that differ in their Yukawa couplings. We show that in all these 3HDMs (including the case of type-II-like Yukawa interactions) both masses of the two charged Higgs bosons [Formula: see text] and [Formula: see text] can be smaller than the top mass [Formula: see text] while complying with the constraints from [Formula: see text]. As an interesting phenomenological consequence, the branching ratios of the charged Higgs bosons decay into the [Formula: see text] final states can be as large as [Formula: see text] when their masses are taken to be below [Formula: see text] in two of the five 3HDMs (named as Type-Y and Type-Z). This light charged Higgs boson scenario provides a hallmark 3HDM signature that cannot be realised in [Formula: see text] symmetric 2-Higgs doublet models. We find that in the Type-Y and Type-Z 3HDMs the scenario with [Formula: see text], [Formula: see text] is ruled out by the direct searches at the LHC, but in the Type-Y 3HDM [Formula: see text] and [Formula: see text] is allowed by [Formula: see text] and direct searches at LEP2, Tevatron and LHC due to the reduced sensitivity of these searches to the degenerate case [Formula: see text]. The cases where only one or both charged Higgs bosons are above the top quark mass are also naturally allowed in the both Type-Y and Type-Z 3HDMs.