We investigate the possibility that the universal Yukawa coupling (democratic mass matrix) with small violations of Dirac and Majorana neutrinos can induce the large mixing of neutrinos through the seesaw mechanism. The possibility can be produced by the condition that the violation parameters of Majorana neutrinos are sufficiently smaller than the violation parameters of Dirac neutrinos. The allowed regions of the violation parameters producing the observed neutrino mass hierarchy and large neutrino mixing are not so restricted at present in contrast with the violation parameters for the quark sector. In order to obtain the distinctive features in the violation parameters for the neutrino sector as a quark sector, one should take more precise data concerned with the neutrino, for example, mass values, mixing parameters of $U_{\mathrm{MNS}},$ and $\mathrm{CP}$ violation phases.

• Received 8 June 2002

DOI:https://doi.org/10.1103/PhysRevD.66.093011