Search for new quarks and leptons from Z0 decay at LEP

This paper reports on a search for Z0 decays into pairs of new heavy quarks (t and b'), a new heavy charged leptons (L±), stable heavy neutral leptons (vL) and unstable heavy neutral leptons (L0) which is performed on data collected by the ALEPH detector corresponding to 11,550 events of Z0 → hadrons. The limits on the masses of the heavy quarks are Mt > 45.8 GeV and Mb' > 46.0 GeV, allowing for both charged-current and flavor-changing neutral-current decays of the b'. If an L± decays into a stable vL, then for MvL < 42.7 GeV, the mass of L± is excluded for all values of ML± > MvL. Finally, while the mass of the stable vL is excluded up to 42.7 GeV, the mass of the unstable L0 is excluded up to 45.7 GeV with the mixing parameters |UlLO|2 down to 10-13 at this mass. For 25.0 GeV < ML0 < 42.7 GeV, all values of |UlL0|2 are excluded. All limits are given at 95% C.L