Fermilab Tevatron mass limits for heavy quarks decaying via flavor-changing neutral currents.

The dimuon and dielectron data from the Fermilab Tevatron [ital [bar p]p] collider are used to probe for heavy quarks, which decay dominantly via flavor-changing neutral currents (FCNC's). Depending on whether the FCNC decay occurs at the tree or loop level, one gets a lower mass limit of 85 or 75 GeV. The former applies to singlet, vector doublet, and mirror-type quarks while the latter applies to a left-handed quark doublet of the fourth generation.