Composite Higgs-Mediated FCNC

TL;DR

The paper analyzes how higher-dimensional Higgs interactions can misalign SM fermion masses from their Higgs couplings, inducing flavor-violating Higgs interactions and tree-level $\Delta F=2$ FCNCs. It develops a model-independent operator framework and then applies it to composite Higgs models with partial compositeness, showing that for a light, strongly coupled Higgs, Higgs exchange can impose bounds on the compositeness scale $\Lambda$ comparable to heavy vector exchange; in the pseudo-Goldstone Higgs case, shift symmetry enforces alignment and greatly weakens these bounds. It further predicts potentially observable rare decays like $t \to c h$ and $h \to t c$ with BRs around $10^{-4}$–$5\times 10^{-3}$, and maps the results to 5D Randall-Sundrum scenarios via AdS/CFT. Collectively, the work clarifies how flavor structure and Higgs nature in composite Higgs theories shape FCNCs and collider signatures, providing guidance for flavor and Higgs phenomenology at the LHC and beyond.

Abstract

We discuss how, in the presence of higher-dimensional operators, the Standard Model (SM) fermion masses can be misaligned in flavor space with the Yukawa couplings to the Higgs boson, even with only one Higgs doublet. Such misalignment results in flavor-violating couplings to the Higgs and hence flavor-changing neutral current (FCNC) processes from tree-level Higgs exchange. We perform a model-independent analysis of such an effect. Specializing to the framework of a composite Higgs with partially composite SM gauge and fermion fields, we show that the constraints on the compositeness scale implied by epsilon_K can be generically as strong as those from the exchange of heavy spin-1 resonances if the Higgs is light and strongly coupled to the new states. In the special and well motivated case of a composite pseudo-Goldstone Higgs, we find that the shift symmetry acting on the Higgs forces an alignment of the fermion mass terms with their Yukawa couplings at leading order in the fermions' degree of compositeness, thus implying much milder bounds. As a consequence of the flavor-violating Higgs couplings, we estimate BR(t -> c h) ~ 10^{-4} and BR(h -> tc) ~ 5 x 10^{-3} both for a pseudo-Goldstone (if t_R is fully composite) and for a generic composite Higgs. By virtue of the AdS/CFT correspondence, our results directly apply to 5-dimensional Randall-Sundrum compactifications.