Extra quark-lepton generations and precision measurements

The existence of extra chiral generations with all fermions heavier than M- Z is strongly disfavoured by the precision electroweak data. However the da ta are fitted nicely even by a few extra generations, if one allows neutral leptons to have masses close to 50 GeV. The data allow inclusion of one ad ditional generation of heavy fermions in SUSY extension of Standard Model i f chargino and neutralino have masses close to 60 GeV with Delta m similar or equal to 1 GeV. (C) 2000 Published by Elsevier Science B.V. All rights r eserved.