Mixing between ordinary and exotic fermions

The results of a comprehensive analysis of the limits on mixings between ordinary fermions and possible heavy fermions with exotic SU(2)×U(1) assignments (e.g., left-handed singlets and/or right-handed doublets) are presented. A general formalism for describing such mixings is given. It is shown that a variety of constraints, including the relation between the W and Z masses and the Fermi constant, charged-current universality, limits on induced right-handed charged currents, and flavor-diagonal neutral currents suffice to limit all directions in parameter space that are not excluded by the absence of flavor-changing neutral currents. Limits on s2, the square of the mixing between ordinary and exotic fermions, are quite stringent for the L, L-, uL, and dL (s20.0020.005) if only one particle is allowed to mix at a time, but are weaker by an order of magnitude if fine-tuned cancellations between different mixings are allowed. Similar statements apply to quark mixings with heavy sequential doublets. Limits on s2 for the other light fermions (eL, eL-,eR-,R-, uR,dR) are in the range 0.020.06, while those for the s, c, b, and - are considerably weaker. Slightly stronger limits are found in specific models (e.g., E6). Implications for the masses of the heavy exotic fermions are discussed.