Probing doubly charged Higgs bosons with three-body associated production at future e + e − colliders

We study the discovery prospects for a doubly charged Higgs boson H ± ± in the 2-Higgs doublet model with type-II seesaw at future e + e − colliders. Focusing on the three-body channels e + e − → H ± ± H 1 ∓ H 1 ∓ and e + e − → H ± ± H 1 ∓ W ∓ , we scan the model parameter space subject to theoretical consistency as well as current collider, flavor, and electroweak precision observables. We find that these 2 → 3 production modes can exceed the conventional pair production rate e + e − → H + + H − − , followed by H ± ± → H 1 ± H 1 ± and H 1 ± W ± decays, over wide regions, particularly above the H ± ± → H 1 ± H 1 ± and H ± ± → H 1 ± W ± thresholds, reaching cross sections up to O ( 10 2 ) fb for s = 500 – 1500 GeV . A detector-level analysis of the 4 ℓ + E T signature, including dominant multiboson and top quark backgrounds, shows that discovery sensitivity is achievable for s = 1000 – 1500 GeV