NATURAL NEUTRINO MASS MATRIX - ART. NO. 093002

The naturalness of the neutrino mass hierarchy and mixing is studied. First we select a few patterns from among 12 neutrino mixing patterns, which could form the natural neutrino mass matrix. Further we show th at if the Dirac neutrino mass matrix is taken as the natural one in th e quark sector, then only two mixing patterns without large mixing lea d to the natural right-handed Majorana mass matrix. The rest of the ch osen patterns with three degenerate mass solutions lead to the unnatur al right-handed Majorana mass matrix in the seesaw mechanism. Notice, however, that for the chosen two natural patterns there could be a hug e mass hierarchy such as O(10(4)-10(6)) in order to reproduce the inve rse mass hierarchy of the light neutrinos. [S0556-2821(98)00819-4].