Neutrino Mixing in the Seesaw Model

We study the neutrino mixing matrix (the MNS matrix) in the seesaw model. Byassuming a large mass hierarchy for the heavy right-handed Majorana mass, weshow that, in the diagonal Majorana base, the MNS matrix is determined by aunitary matrix, $S$, which transforms the neutrino Yukawa matrix, $y_{\nu}$,into a triangular form, $y_{\triangle}$. The mixing matrix of light leptons is$V_{KM} S^{\prime *}$, where $V_{KM} \equiv {V_{Le}}^{\dagger} V_{L \nu}$%$V_{Le}$ and $V_{L\nu}$ diagonalize %the Yukawa matrices of charged leptons,%$y_e y_e^{\dagger}$, and neutrinos, $y_{\nu} y_{\nu}^{\dagger}$,%respectively, and $S^{\prime *} \equiv {V_{L\nu}}^{\dagger} S^*$. Large mixingmay occur without fine tuning of the matrix elements of $y_{\triangle}$ even ifthe usual KM-like matrix $V_{KM}$ is given by $V_{KM} =1$. This large mixingnaturally may satisfy the experimental lower bound of the mixing implied by theatmospheric neutrino oscillation.Comment: 14 pages, 4 figures, PTPTe