New flavor changing interactions in extended gauge models
  A new class of flavor changing (FC) neutral interactions can arise in models based on extended gauge groups (rank $>$4) when new charged fermions are present together with new neutral gauge bosons. In some cases the FC couplings to a new $Z_1$ are expected to be sizeable, implying that the $Z_1$ mass must be large enough as to explain the observed suppression of FC transitions. Concentrating on E$_6$ models and assuming for the FC parameters a theoretically natural range of values, I show that in most cases the presence of a $Z_1$ much lighter than 1 TeV is unlikely. 
Enrico Nardi