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# 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 $2.984 \pm0.008$
 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