Possible deviation from the tri-bimaximal neutrino mixing in a seesaw model

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 V0. 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 θ12 from its initial value 35.3° given by V0, and the slight breaking of the mass degeneracy between the second and third generations of heavy Majorana neutrinos results in a small mixing angle θ13 and a tiny departure of the atmospheric neutrino mixing angle θ23 from 45°. It turns out that a normal hierarchy of the light neutrino mass spectrum is favored in this seesaw scenario.