Pseudo-Dirac Neutrinos in the New Standard Model

Understand

The addition of gauge singlet fermions to the Standard Model Lagrangian renders the neutrinos massive and allows one to explain all that is experimentally known about neutrino masses and lepton mixing by varying the values of the Majorana mass parameters M for the gauge singlets and the neutrino Yukawa couplings.

• Here we explore the region of parameter space where M values are much smaller than the neutrino Dirac masses.
• In this region, neutrinos are pseudo-Dirac fermions.
• We find that current solar data constrain M values to be less than at least 1E-9 eV, and discuss the sensitivity of future experiments to tiny gauge singlet fermion masses.