Neutrino mixing in the seesaw model

We study the neutrino mixing matrix (the MNS matrix) in the seesaw model. By assuming a large mass hierarchy for the heavy right-handed Majorana mass, we show that, in the diagonal Majorana base, the MNS matrix is determined by a unitary matrix, S, which transforms the neutrino Yukawa matrix, y_ν, into a triangular form, yΔ. The mixing matrix of light leptons is V_KM-S^′*, where V_KM ≡ V_Le^†V_Lν and S^′* ≡ V_Lν^†S*. Large mixing may occur without fine tuning of the matrix elements of yΔ even if the usual KM-like matrix V_KM is given by V_KM = 1. This large mixing naturally may satisfy the experimental lower bound of the mixing implied by the atmospheric neutrino oscillation.