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⁺,ξ⁰) 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⁺J⁰ and ξ⁰→ZJ⁰, where J⁰ is the massless Goldstone boson (Majoron), are also possible, but of special importance is the decay ξ⁰→J⁰J⁰ 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.