Enhancement of the $H^\pm W^\mp Z$ vertex in the three scalar doublet model

TL;DR

This paper calculates one-loop induced vertices involving charged Higgs bosons in a three scalar doublet model, revealing potential enhancements in the $H^\u00b0 W^\u00b0 Z$ vertex and sizable decay modes for light charged Higgs bosons, with implications for collider searches.

Contribution

It provides the first detailed analysis of the $H^\u00b0 W^ Z$ vertex in a three doublet model, showing non-decoupling effects can significantly enhance this vertex compared to two Higgs doublet models.

Findings

$|F_Z|^2$ can be an order of magnitude larger than in two Higgs doublet models.

Branching ratios for $H^\u00b0 o W^\u00b0 Z$ and $H^\u00b0 o W^\u00b0 \gamma$ can reach 10% and 1%, respectively.

Light $H^\u00b0$ with mass below top quark is phenomenologically viable and detectable at the LHC.

Abstract

We compute one-loop induced trilinear vertices with physical charged Higgs bosons $H^\pm$ and ordinary gauge bosons, i.e., $H^\pm W^\mp Z$ and $H^\pm W^\mp \gamma$, in the model with two active plus one inert scalar doublet fields under a $Z_2(\text{unbroken})\times \tilde{Z}_2(\text{softly-broken})$ symmetry. The $Z_2$ and $\tilde{Z}_2$ symmetries are introduced to guarantee the stability of a dark matter candidate and to forbid the flavour changing neutral current at the tree level, respectively. The dominant form factor $F_Z$ of the $H^\pm W^\mp Z$ vertex can be enhanced by non-decoupling effects of extra scalar boson loop contributions. We find that, in such a model, $|F_Z|^2$ can be one order of magnitude larger than that predicted in two Higgs doublet models under the constraints from vacuum stability, perturbative unitarity and the electroweak precision observables. In addition, the branching fraction of the $H^\pm \to W^\pm Z$ $(H^\pm \to W^\pm \gamma)$ mode can be of order 10~(1)\% level when the mass of $H^\pm$ is below the top quark mass. Such a light $H^\pm$ is allowed by the so-called Type-I and Type-X Yukawa interactions which appear under the classification of the $\tilde{Z}$ charge assignment of the quarks and leptons. We also calculate the cross sections for the processes $H^\pm \to W^\pm Z$ and $H^\pm \to W^\pm \gamma$ onset by the top quark decay $t\to H^\pm b$ and electroweak $H^\pm$ production at the LHC.