Searches for New Quarks and Leptons in Z Boson Decays

Searches for the decay of Z bosons into pairs of new quarks and leptons in a data sample including 455 hadronic Z decays are presented. The Z bosons were produced in electon-positron annihilations at the SLAC Linear Collider (SLC) operating in the center-of-mass energy range from 89.2 to 93.0 GeV, and the data collected using the Mark II detector. The Standard Model provides no prediction for fermion masses and does not exclude new generations of fermions. The existence and masses of these new particles may provide valuable information to help understand the pattern of fermion masses, the presence of generations, and physics beyond the Standard Model. Specific searches for top quarks and sequential fourth generation charge -1/3 (b ^') quarks are made considering a variety of possible standard and non-standard decay modes. In addition, searches for sequential fourth generation massive neutrinos nu_4 (Dirac and Majorana) and their charged lepton partners L^ {-} are pursued. The nu _4 may be stable or decay through mixing to the lighter generations. The data sample is examined for new particle topologies of events with high-momentum isolated tracks, high-energy isolated photons, spherical event shapes, and detached vertices. Measurements of the Z boson resonance parameters that provide crucial indicators of new particle production are also considered. No evidence is observed for the production of new quarks and leptons. 95% confidence level lower mass limits of 40.7 GeV/c^2 for the top quark and 42.0 GeV/c^2 for the b^'-quark mass are obtained regardless of the branching fractions to the considered decay modes. A significant range of mixing matrix elements of nu_4 to other generation neutrinos for a nu_4 mass from 1 GeV/c^2 to 43 GeV/c ^2 is excluded at 95% confidence level. Measurements of the upper limit of the invisible width of the Z exclude additional values of the nu_4 mass and mixing matrix elements, and also permit the exclusion of a region in the L^{-} mass versus nu_4 mass plane. These results substantially extend previous limits, and exclude a large fraction of the mass range available for Z decays into top quarks and fourth generation quarks and leptons.