Probing Charged Higgs Bosons through $H^\pm \to W^\pm A$ Decay in the Type-I Two-Higgs Doublet Model

Abstract

A light pseudoscalar lighter than the 125 GeV Higgs boson remains allowed within the type-I Two-Higgs Doublet Model (2HDM), consistent with current collider constraints, including Higgs signal strength measurements and direct searches for additional Higgs bosons. At the Large Hadron Collider (LHC), the light pseudoscalar $A$ can be pair-produced through the undetected decay of the Standard Model-like Higgs boson, $h_{125} \to AA$, when this channel is kinematically accessible (i.e., $m_A<m_{h_{125}}/2$). For the charged scalar $H^\pm$, once open, the bosonic decay channel $H^\pm \to W^{\pm(*)} A$ naturally dominates over the fermionic $H^\pm \to \tau^\pm\nu$ and $H^\pm \to tb$ channels, while relaxing the existing $H^\pm$ limits. As a result, we propose searching for the charged Higgs boson through $H^\pm \to W^{\pm(*)} A$ in the $4b+2j+\ell\nu$ final state. This channel is particularly sensitive to moderate-to-large $\tan\beta$ values and provides an excellent complementary probe to searches based on fermionic $H^\pm$ decays at the LHC. Our collider analysis demonstrates that this channel can probe a substantial region of the type-I 2HDM parameter space at the high-luminosity LHC.