A GIM Mechanism from Extra Dimensions

TL;DR

This work presents a framework to realize a GIM mechanism in warped extra-dimensional models by enforcing a large bulk flavor symmetry and introducing flavor mixing exclusively on the UV brane via right-handed quark kinetic terms. It shows how neutral current FCNCs are suppressed while CKM-like mixing emerges from misalignment, enabling MFV and a variant NMFV without introducing a CP problem. The authors provide two concrete model realizations—one MFV with full flavor protection and one NMFV with third-generation emphasis—and analyze their compatibility with precision electroweak constraints, custodial symmetry, and KK scales around a few TeV. The holographic (AdS/CFT) interpretation supports the mechanism, suggesting that warped extra-dimensional flavor can reproduce CKM without large FCNCs and could yield testable signals at colliders.

Abstract

We explore how to protect extra dimensional models from large flavor changing neutral currents by using bulk and brane flavor symmetries. We show that a GIM mechanism can be built in to warped space models such as Randall-Sundrum or composite Higgs models if flavor mixing is introduced via UV brane kinetic mixings for right handed quarks. We give a realistic implementation both for a model with minimal flavor violation and one with next-to-minimal flavor violation. The latter does not suffer from a CP problem. We consider some of the existing experimental constraints on these models implied by precision electroweak tests.

Figures (2)

• Figure 1: Left panel: We show the lower bounds on $1/R'$ as a function of $c_L$ for which $|S| < 1,0.5,0.25$ (bottom to top). Right panel: $S$ in RS1 as a function of the bulk mass $c_L$ for $1/R'= 2$ and $4$ TeV. In both cases we have set $c_R=0$.
• Figure 2: The allowed region of the parameter space in Model 1 with MFV. The upper dash-dotted curve shows the perturbativity bound on the parameter $c_L$ assuming that the right handed top is strongly localized on the TeV brane (we chose $c_R=1$, and required that the Yukawa coupling is less than four). The middle solid curve shows the region where the deviation of the coupling of the left handed up-type quark is below 0.5%. The allowed region is the shaded one between these two curves. The lowest dashed curve shows the bound from the coupling of the KK mode of the $Z$ coupling to light left handed quarks. Such a coupling would generically induce four fermi operators involving leptons. The bound obtained in this plot is requiring that the $q/M_{Z'}$ ratio of coupling to mass is less than 1/(5 TeV) CCMS. We can see that this bound is irrelevant for the allowed region.