; Luo, Shu
; Xing, Zhi-zhong
; Zhou, Shun] Chinese Acad Sci, Inst High Energy Phys, Beijing 100049, Peoples R China
A natural extension of the standard SU(2)(L) x U(1)(Y) gauge model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a 6 x 6 neutrino mass matrix which contains three 3 x 3 submatrices, M-L, M-D and M-R. We show that three light Majorana neutrinos (i.e., the mass eigenstates of nu(e), nu(mu), and nu(tau)) are exactly massless in this model, if and only if M-L=(MDMR-1MDT) exactly holds. This no-go theorem implies that small but nonvanishing neutrino masses may result from a significant but incomplete cancellation between M-L and (MDMR-1MDT) terms in the Type-II seesaw formula, provided three right-handed Majorana neutrinos are of O(1) TeV and experimentally detectable at the LHC. We propose three simple Type-II seesaw scenarios with the Lambda(4) x U(1)(X) flavor symmetry and its explicit breaking 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 x 3 neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.