Eur. Phys. J. C 21, 281-289 (2001)
DOI: 10.1007/s100520100723

Neutrino mixing in the seesaw model

T.K. Kuo¹, S.-H. Chiu^{2, 3} and G.-H. Wu<sup>4

1</sup>  Department of Physics, Purdue University, West Lafayette, IN 47907, USA
²  Department of Physics, Rochester Institute of Technology, Rochester, NY 14623, USA
³  Department of Physics, Frostburg State University, Frostburg, MD 21532, USA
⁴  Institute of Theoretical Science, University of Oregon, Eugene, OR 97403, USA

(Received: 16 March 2001 / Revised version: 17 May 2001 / Published online: 19 July 2001 -© Springer-Verlag / Società Italiana di Fisica 2001 )

Abstract
In the seesaw model with hierarchical Dirac masses, the neutrino mixing angle exhibits the behavior of a narrow resonance. In general, the angle is strongly suppressed, but it can be maximal for special parameter values. We delineate the small regions in which this happens for the two-flavor problem. On the other hand, the physical neutrino masses are hierarchical, in general, except in a large part of the region in which the mixing angle is sizable, where they are nearly degenerate. Our general analysis is also applicable to the RGE of the neutrino mass matrix, where we find analytic solutions for the running of the physical parameters, in addition to a complex RGE invariant relating them. It is also shown that, if one mixing angle is small, the three-neutrino problem reduces to two two-flavor problems.

© Società Italiana di Fisica, Springer-Verlag 2001