Electroweak Constraints on Effective Theories with U(2)xU(1) Flavor Symmetry

Abstract

In a previous analysis presented in hep-ph/0412166, electroweak constraints were given on arbitrary linear combinations of a set of dimension-6 operators. Flavor universality and thus U(3)^5 flavor symmetry were assumed for the operators. In this article, we expand the analysis to account for the flavor-dependent theories that distinguish the third generation of fermions from the light two generations. We still assume flavor universality for the light two generations to avoid large FCNCs. Consequently, the U(3)^5 flavor symmetry is relaxed to [U(2)xU(1)]^5, and therefore more operators are added. We calculate the corrections to electroweak precision observables, assuming arbitrary coefficients for the operators. The corrections are combined with the standard model predictions and the experimental data to obtain the chi^2 distribution as a function of the operator coefficients. We apply our result to constrain two flavor-dependent extensions of the standard model: the simplest little Higgs model and a model with an SU(2)xSU(2)xU(1) gauge group.