A Search for New Quarks and Leptons From $\Z^0$ Decay

A search for Z 0 decays into pairs of possible new heavy quarks (t and b′), new heavy charged leptons (L ± ), stable heavy neutral leptons ( ν L ) and unstable heavy neutral leptons (L 0 ) is performed on data collected by the ALEPH detector corresponding to 11 550 events of Z 0 →hadrons. The limits on the masses of the heavy quarks are M t > 45.8 GeV and M b′ > 46.0 GeV, allowing for both charged-current and flavor-changing neutral-current decays of the b′. If an L ± decays into a stable ν L , then for M ν L M νL . Finally, while the mass of the stable ν L is excluded up to 42.7 GeV, the mass of the unstable L 0 is excluded up to 45.7 GeV with the mixing parameters | U lL 0 | 2 down to 10 −13 at this mass. For 25.0 GeV < M L 0 < 42.7 GeV, all values of | U lL 0 | 2 are excluded. All limits are given at 95% CL.