| Summary: | Abstract The type-II seesaw mechanism with an isospin-triplet scalar Δ L provides one of the most compelling explanations for the observed smallness of neutrino masses. The triplet contains a doubly-charged component H L ± ± $$ {H}_{{}^L}^{\pm \pm } $$ , which dominantly decays to either same-sign dileptons or to a pair of W bosons, depending on the size of the triplet vacuum expectation value. However, there exists a range of Yukawa couplings f L of the triplet to the charged leptons, wherein a relatively light H L ± ± $$ {H}_{{}^L}^{\pm \pm } $$ tends to be long-lived, giving rise to distinct displaced-vertex signatures at the high-energy colliders. We find that the displaced vertex signals from the leptonic decays H L ± ± → ℓ α ± ℓ β ± $$ {H}_{{}^L}^{\pm \pm}\to {\ell}_{\alpha}^{\pm }{\ell}_{{}^{\beta}}^{\pm } $$ could probe a broad parameter space with 10−10 ≲ |f L | ≲ 10−6 and 45.6 GeV < M H L ± ± ≲ 200 GeV $$ 45.6\ GeV<{M}_{H_L^{\pm \pm }}\lesssim 200\ GeV $$ at the high-luminosity LHC. Similar sensitivity can also be achieved at a future 1 TeV e + e − collider. The mass reach can be extended to about 500GeV at a future 100TeV proton-proton collider. Similar conclusions apply for the right-handed triplet H R ± ± $$ {H}_{{}^R}^{\pm \pm } $$ in the TeV-scale left-right symmetric models, which provide a natural embedding of the type-II seesaw. We show that the displaced vertex signals are largely complementary to the prompt same-sign dilepton pair searches at the LHC and the low-energy, high-intensity/precision measurements, such as neutrinoless double beta decay, charged lepton flavor violation, electron and muon anomalous magnetic moments, muonium-antimuonium oscillation and Møller scattering.
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