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 ${{\mathit \nu}_{{3}}}$ .
Number from ${{\mathit e}^{+}}{{\mathit e}^{-}}$ Colliders
Number of Light ${{\mathit \nu}}$ Types   $2.996 \pm0.007$  
Number of Light ${{\mathit \nu}}$ Types from Direct Measurement of Invisible ${{\mathit Z}}$ Width   $2.92 \pm0.05$  (S = 1.2)
Limits from Astrophysics and Cosmology
Effective Number of Light ${{\mathit \nu}}$ Types
Number Coupling with Less Than Full Weak Strength