Possible Deviation from the Tri-bimaximal Neutrino Mixing in a Seesaw Model

Abstract

We propose a simple but suggestive seesaw model with two phenomenological conjectures: three heavy (right-handed) Majorana neutrinos are degenerate in mass in the symmetry limit and three light Majorana neutrinos have the tri-bimaximal mixing pattern $V_0^{}$. We show that a small mass splitting between the first generation and the other two generations of heavy Majorana neutrinos is responsible for the deviation of the solar neutrino mixing angle ${\theta }_{12}^{}$ from its initial value $35.3^{\circ }$ given by $V_0^{}$, and the slight breaking of the mass degeneracy between the second and third generations of heavy Majorana neutrinos results in a small mixing angle ${\theta }_{13}^{}$ and a tiny departure of the atmospheric neutrino mixing angle ${\theta }_{23}^{}$ from $45^{\circ }$. It turns out that a normal hierarchy of the light neutrino mass spectrum is favored in this seesaw scenario.

Comments: RevTex 12 pages (2 EPS figures included). More discussions and references added