  Neutrino masses and mixings are analyzed in terms of left-handed fields and a
6x6 complex symmetric mass matrix whose singular values are the neutrino
masses. An angle theta_nu characterizes the kind of the neutrinos, with
theta_nu=0 for Dirac neutrinos and theta_nu=pi/2 for Majorana neutrinos. At
theta_nu = 0 baryon-minus-lepton number is conserved. If theta_nu is
approximately zero, the six neutrino masses coalesce into three nearly
degenerate pairs. Thus the tiny mass differences exhibited in the solar and
atmospheric neutrino experiments are naturally explained by the approximate
conservation of B-L. Neutrinos are nearly Dirac fermions.
  This B-L model leads to these predictions: neutrinos oscillate mainly between
flavor eigenfields and sterile eigenfields, and so the appearance of neutrinos
and antineutrinos is suppressed; neutrinos may well be of cosmological
importance; in principle the disappearance of the tau neutrino should be
observable; and neutrinoless double-beta decay is suppressed by an extra factor
of 10^(-5) and so will not be seen in the Heidelberg/Moscow, IGEX, GENIUS, or
CUORE experiments.
