2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/179601The dimuon and dielectron data from the Tevatron $\bar pp$ collider are used to probe for heavy quarks, which decay dominantly via flavour changing neutral current. 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 lefthanded quark doublet of the fourth generation.13 pages, TIFR/TH/92-58, Two figures to be supplied on requestHigh Energy Physics - Phenomenologytext