Large Mixing Angle MSW Solution in an SU(3)_LxU(1)_N Gauge Model with Two-loop Radiative Mechanism

Abstract

An SU(3)_L x U(1)_N gauge model, where E^{-i} (i=1,2,3) as heavy leptons are placed in the third member of lepton triplets, is shown to provide the large mixing angle (LMA) MSW solution to the solar neutrino problem. By introducing approximate conservation of L_e-L_mu-L_tau (\equiv L') into the model, we have estimated \Delta m_\odot^2 \sim \sqrt{2}\epsilon \Delta m_{atm}^2m_{E^1}/(m_{E^2}+m_{E^3}), where \epsilon represents L'-breaking effects and m_{E^i} is a mass of E^{-i}. Since \Delta m_{atm}^2 \sim 3 x 10^{-3} eV^2, the LMA solution is obtained for \epsilon\sim 0.01-0.1 in the case of m_{E^1} \sim m_{E^{2,3}}. This mechanism utilizes two-loop radiative mechanism, which contains a triplet version of the singly charged Zee boson, h^+, given by \xi = (\xi^{++}, {\bar \xi}^+, h^+)^T. Almost bimaximal mixing is dynamically realized by the approximate equality between the \xi-couplings of the first family to the second and third families.