Flavor Changing Effects in Theories with a Heavy Z' Boson with Family Non-Universal Couplings

TL;DR

This work investigates heavy Z' bosons with family non-universal couplings and analyzes how quark and lepton flavor mixing induces FCNCs and potential CP violation via Z–Z' mixing. A general formalism is developed to map Z' couplings onto effective four-fermion operators and is applied to a broad set of flavor-changing and CP-violating processes, including Z decays, lepton decays, meson decays, mass splittings, and CP observables; three representative coupling patterns are studied, with CKM-like mixing used as a reference. The results show that, for TeV-scale Z' masses, experimental constraints—especially from coherent $\mu$–$e$ conversion and neutral K-system observables—strongly limit non-universal couplings, though some models with universality in the first two generations can still be viable depending on mixing. The analysis emphasizes the model-dependence of constraints, arising from unknown left and right mixing matrices, and highlights the potential of upcoming rare decays and precision measurements to further probe these scenarios.

Abstract

There are theoretical and phenomenological motivations that there may exist additional heavy Z' bosons with family non-universal couplings. Flavor mixing in the quark and lepton sectors will then lead to flavor changing couplings of the heavy Z', and also of the ordinary Z when Z-Z' mixing is included. The general formalism of such effects is described, and applications are made to a variety of flavor changing and CP-violating tree and loop processes. Results are described for three specific cases motivated by a specific heterotic string model and by phenomenological considerations, including cases in which all three families have different couplings, and those in which the first two families, but not the third, have the same couplings. Even within a specific theory the results are model dependent because of unknown quark and lepton mixing matrices. However, assuming that typical mixings are comparable to the CKM matrix, processes such as coherent mu-e conversion in a muonic atom, K^0-\Bar{K}^0 and B-\Bar{B} mixing, epsilon, and epsilon'/epsilon lead to significant constraints on Z' bosons in the theoretically and phenomenologically motivated range M_{Z'} \sim 1 TeV.