Displaced vertex signatures of doubly charged scalars in the type-II seesaw and its left-right extensions

Abstract

The type-II seesaw mechanism with an isospin-triplet scalar ${\Delta }_L$ provides one of the most compelling explanations for the observed smallness of neutrino masses. The triplet contains a doubly-charged component $H_L^{\pm \pm }$, 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 $H_L^{\pm \pm }$ 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 $H_L^{\pm \pm }\to {\ell }_{\alpha }^{\pm }{\ell }_{\beta }^{\pm }$ could probe a broad parameter space with $10^{-10}\lesssim |f_L|\lesssim 10^{-6}$ and 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 500 GeV at a future 100 TeV proton-proton collider. Similar conclusions apply for the right-handed triplet $H_R^{\pm \pm }$ 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 oscillation and Møller scattering.

Comments: 49 pages, 25 figures and 2 tables, minor changes, version to appear in JHEP