Abstract
A natural extension of the standard model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a 6×6 neutrino mass matrix. We show that three light Majorana neutrinos (i.e., the mass eigenstates of νe, νμ and ντ) are exactly massless, if and only if [Formula: see text] exactly holds in this seesaw model. We propose three simple Type-II seesaw scenarios with broken A4 × U(1)x flavor symmetry to interpret the observed neutrino mass spectrum and neutrino mixing pattern. Such a TeV-scale neutrino model can be tested in two complementary ways: (1) searching for possible collider signatures of lepton number violation induced by the right-handed Majorana neutrinos and doubly-charged Higgs particles; and (2) searching for possible consequences of unitarity violation of the 3 × 3 neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.
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