Enhanced four-body decays of charged Higgs bosons into off-shell pseudoscalar Higgs and W^± boson pairs in a lepton-specific 2-Higgs doublet model

We study the time-honoured decay H^± → A W^± H ± → A W ± but for the first time, we do so for the case of both A and W^± W ± being off-shell, therefore computing a 1→ 4 1 → 4 body decay. We show that the corresponding decay rate not only extends the reach of H^± H ± searches to small masses of the latter but also that the results of our implementation differ significantly from the yield of the 1→ 3 1 → 3 body decay over the phase space region in which the latter is normally used. We show the phenomenological relevance of this implementation in the case of the so-called lepton-specific 2-Higgs Doublet Model (2HDM) over the mass region wherein the aforementioned 1→ 4 1 → 4 body decay can dominate just beyond the top (anti)quark mass. This mass region is accessible in the lepton-specific 2HDM as the Yukawa couplings are such that limits from b → s γ b → s γ and τ → μ ν τ ν _μ ̄ τ → μ ν τ ν μ ¯ observables on MH^± M H ± are rather mild. However, we emphasize that similar effects may occur in other 2HDM types, as the W^± H^∓ A W ± H ∓ A vertex is 2HDM type independent.