We show that recent data from the CERN ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ collider LEP on the Z line shape and decays give stringent new constraints on mixing of e and τ with exotics and Z-Z’ mixing. Even in nonstandard models, where both the visible and the invisible part of the Z width are modified, a fourth light neutrino is unlikely unless substantial mixings between neutrinos and exotics are allowed. If the gluino is detectable at the Fermilab Tevatron then the lighter-chargino mass is tightly constrained (>42 GeV).

• Received 5 February 1990

DOI:https://doi.org/10.1103/PhysRevLett.64.2870