Flavor violation and warped geometry

TL;DR

The paper presents a compelling framework in which the fermion mass hierarchy and CKM structure arise from flavor-dependent localization of bulk fields in a warped extra dimension, rather than hierarchically tuned Yukawas. With the Higgs on the TeV-brane and a KK scale around $M_{KK}\sim 10\,\mathrm{TeV}$, electroweak precision data and flavor constraints can be satisfied, while KK gauge boson exchange induces flavor-violating processes that remain within current bounds but yield potentially observable signals in channels like muon–electron conversion in next-generation experiments. A statistical approach over random 5D Yukawas shows the observed masses and mixings can be accommodated by natural fermion localizations, and non-unitarity of the CKM arises mainly from KK mixing yet stays within experimental limits. Non-renormalizable operators are generally suppressed by geometry, though proton decay remains a challenge requiring tiny couplings or extra symmetries. Overall, the warped setup reconciles the gauge hierarchy problem with realistic flavor physics and predicts distinctive, testable flavor signatures distinct from flat extra-dimensional or supersymmetric scenarios.

Abstract

Extra dimensions have interesting consequences for flavor physics. We consider a setup where the standard model fermions and gauge fields reside in the bulk of a warped extra dimension. Fermion masses and mixings are explained by flavor dependent fermion locations, without relying on hierarchical Yukawa couplings. We discuss various flavor violating processes induced by (Kaluza-Klein) gauge boson exchange and non-renormalizable operators. Experimental constraints are satisfied with a Kaluza-Klein scale of about 10 TeV. Some processes, such as muon-electron conversion, are within reach of next generation experiments.

Figures (3)

• Figure 1: The wave functions of the left-handed quark zero modes $q_i^{(0)}$ and the first KK state, $q_1^{(1)}$, of $Q_1$ for the parameters of eq. (\ref{['qlocations']}).
• Figure 2: Logarithmic distributions of $m_u$ (a) and $|V_{ub}|$ (b) for Yukawa coupling distributions $2/3<|l_{ij}|<4/3$ (solid lines), $0<|l_{ij}|<2$ (dashed dotted lines) and $|l_{ij}|=1$ (solid lines). The vertical dashed lines indicate the experimental uncertainties.
• Figure 3: Gauge couplings of the Z boson (a) and its first KK state relative to the SM value as a function of the fermion location.