On bottom mixing with exotic quarks

In this paper we present a calculation of the effects of bottom mixing with new exotic quarks in the forward-backward and left-right asymmetries, the bottom branching ratio and the QCD coupling constant. A global fit with the recent data on these quantities is done and stringent bounds are obtained. We discuss the effects of different isospin signatures for the new possible exotic quarks. The consequences for superstrig-inspired $E_6$ models are discussed. Constraints on the bottom mixing with the isosinglet quarks of the fundamental 27-plet are presented.