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

Abstract

We analyse the Z and Higgs boson decays Z→ℓ+ℓ− (ℓ=e,μ,τ), H→γγ and H→Zγ 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 Z3 family symmetry and were recently studied in the context of charged-lepton flavour-violating processes at Belle II and the proposed μ+μ+ and μ+e− collider known as μTRISTAN. We find that the future constraints that can be imposed by Z→ℓ+ℓ− on the lepton flavour conserving couplings of the tr..