Search for Long-Lived Parents of Z Bosons in pbar-p Collisions at   sqrt{s}=1.8 TeV
  We search for new long-lived particles which decay to Z bosons by looking for Z->e+e- decays with displaced vertices. We find no evidence for parent particles of the Z with long lifetimes in 90 pb-1 of data from the CDF experiment at Fermilab. We set a cross section limit as a function of the lifetime of the parent particle for both a generic Z parent and a fourth-generation, charge -1/3 quark that decays into Z+b. 
F. Abe, et al. (CDF Collaboration)