Abstract
The type-II seesaw mechanism with an isospin-triplet scalar ΔL 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 fL 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 ≲ |fL| ≲ 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 500GeV at a future 100TeV 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-antimuonium oscillation and Møller scattering.
Highlights
Similar sensitivity can be achieved at a future 1 TeV e+e− collider
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
It is worth noting that the diboson decay HL±± → W ±(∗)W ±(∗) could induce displaced vertex (DV) at high energy colliders, and the searches of the displaced W decay products are largely complementary to the dilepton DV signals discussed above, in the sense that they are sensitive to different ranges of the vacuum expectation value (VEV) vL, as implied by the BR contours in figure 1
Summary
In the type-II seesaw model [2,3,4,5,6,7], there exists a new complex scalar multiplet which transforms as a triplet under the SM SU(2)L gauge group. A non-zero VEV for the doublet field φ0 = vEW/ 2 (with vEW 246 GeV being the electroweak scale) induces a tadpole term for the scalar triplet field ∆L via the λ6 term√ in eq (2.2), thereby generating a non-zero VEV for its neutral component, δL0 = vL/ 2, and breaking lepton number by two units, which is responsible for neutrino mass generation at tree-level. The Yukawa coupling matrix fL is fixed by the active neutrino data, i.e. the observed neutrino mass squared differences and mixing angles, up to the unknown lightest neutrino mass m0, the neutrino mass hierarchy and the Dirac and Majorana CP violating phases. Note that the recent T2K [81] and NOνA [82] results indicate a mild preference for non-zero δCP, but this has not been established at 5σ level yet
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