Observable contributions of new exotic quarks to quark mixing
  Models with new vector-like quarks can produce observable quark mixing effects which are forbidden in the Standard Model. We classify all such models and write down the effective Lagrangian that results from integrating out the new quarks. We study the relations between neutral and charged currents and discuss how to distinguish among the different possibilities. 
F. del Aguila, M. Perez-Victoria and J. Santiago