Z', NEW FERMIONS, AND FLAVOR-CHANGING PROCESSES - CONSTRAINTS ON E6 MODELS FROM MU-]EEE

We study a new class of flavor-changing interactions, which can arise in models based on extended gauge groups (rank > 4) when new charged fermions are present together with a new neutral gauge boson. We discuss the cases in which the flavor-changing couplings in the new neutral current coupled to the Z' are theoretically expected to be large, implying that the observed suppression of neutral flavor-changing transitions must be provided by heavy Z' masses together with small Z-Z' mixing angles. Concentrating on E6 models, we show how the tight experimental limit on mu --> eee implies serious constraints on the Z' mass and mixing angle. We conclude that if the value of the flavor-changing parameters is assumed to lie in a theoretically natural range, in most cases the presence of a Z' much lighter than 1 TeV is unlikely.