Massive and Massless Neutrinos on Unbalanced Seesaws

Understand

The observation of neutrino oscillations requires new physics beyond the standard model (SM).

• A SM-like gauge theory with p lepton families can be extended by introducing q heavy right-handed Majorana neutrinos but preserving its SU(2)_L x U(1)_Y gauge symmetry.
• The overall neutrino mass matrix M turns out to be a symmetric (p+q) x (p+q) matrix.
• Given p>q, the rank of M is in general equal to 2q, corresponding to 2q non-zero mass eigenvalues.