Number of Neutrino Types  

The neutrinos referred to in this section are those of the Standard SU(2)${\times }$U(1) Electroweak Model possibly extended to allow nonzero neutrino masses. Light neutrinos are those with $\mathit m$ $<$ ${\mathit m}_{{{\mathit Z}}}$/2. The limits are on the number of neutrino mass eigenstates, including ${{\mathit \nu}_{{{1}}}}$, ${{\mathit \nu}_{{{2}}}}$, and ${{\mathit \nu}_{{{3}}}}$.