Phenomenology of the neutrino-mass-giving Higgs triplet and the low-energy seesaw violation of lepton number

Abstract

Small realistic Majorana neutrino masses can be generated via a Higgs triplet ( ξ ++, ξ +, ξ 0) without having energy scales larger than M ∗= O(1) TeV in the theory. The large effective mass scale Λ in the well-known seesaw neutrino-mass operator Λ −1( LLΦΦ) is naturally obtained with Λ∼M ∗ 2/μ, where μ is a small scale of lepton-number violation. In theories with large extra dimensions, the smallness of μ is naturally obtained by the mechanism of “shining” if the number of extra dimensions n⩾3. We study here the Higgs phenomenology of this model, where the spontaneous violation of lepton number is treated as an external source from extra dimensions. The observable decays ξ ++→ l i + l j + will determine directly the magnitudes of the { ij} elements of the neutrino mass matrix. The decays ξ +→ W + J 0 and ξ 0→ ZJ 0, where J 0 is the massless Goldstone boson (Majoron), are also possible, but of special importance is the decay ξ 0→ J 0 J 0 which provides stringent constraints on the allowed parameter space of this model. Based on the current neutrino data, we also predict observable rates of μ–e conversion in nuclei.