Z and Higgs boson decays with doubly-charged scalars at one-loop: current constraints, future sensitivities, and application to lepton-triality models

TL;DR

This paper investigates how Z and Higgs boson decays mediated by doubly-charged scalars can constrain lepton-triality models, exploring current limits and future collider sensitivities to understand their impact on lepton flavor violation.

Contribution

It provides the first constraints from Higgs decays on the Higgs portal sector of lepton-triality models and analyzes future collider prospects for probing these models.

Findings

Current constraints limit the parameter space of lepton-triality models.

Future colliders like HL-LHC, ILC, CEPC, FCC can significantly probe the models.

Constraints from Z decays reduce viable parameter space for lepton flavor violation.

Abstract

We analyse the $Z$ and Higgs boson decays $Z\to \ell^+ \ell^- $ ($\ell = e, \mu,\tau$), $H\to \gamma\gamma$ and $H\to Z\gamma$ that are induced at one-loop level in models with a doubly-charged isosinglet scalar. After discussing current constraints, we derive the parameter space that will be probed by the HL-LHC and the possible future colliders the ILC, CEPC and FCC. We then apply those constraints to lepton triality models which are based on a discrete $Z_3$ family symmetry and were recently studied in the context of charged-lepton flavour-violating processes at Belle II and the proposed $\mu^+ \mu^+$ and $\mu^+ e^-$ collider known as $\mu$TRISTAN. We find that the future constraints that can be imposed by $Z \to \ell^+ \ell^-$ on the lepton flavour conserving couplings of the triality models reduce the viable parameter space to probe lepton flavour violating processes. The constraints from Higgs boson decays are the first on the Higgs portal sector of the triality models.