of freedom. Therefore, generically, one expects that a different and independent class of higher dimension operators may be localized to this singular region in the extra dimension manifold. (These are commonly denoted ‘brane terms’ even though, in most cases, they have very little to do with string theory). The brane-localized terms can, in principle, be of anarchic flavor structure and provide new flavor and CP violating sources. One important class of such operators are brane kinetic terms: their impact is somewhat similar to that of non-canonical kinetic terms which generically arise in supersymmetric flavor models. We focus on flavor physics of five dimension (5D) models, with bulk SM fields, since most of the literature focuses on this class. Furthermore, the new flavor structure that arises in 5D models captures most of the known effects of extra-dimension flavor models. Assuming a flat extra dimension, the energy range, Λ$_{5D}$ R (where Λ$_{5D}$ is the 5D effective cutoff scale and R is the extra dimension radius with the extra dimension coordinate y ∈ (0 , πR )), for which the 5D effective field theory holds, can be estimated as follows. Since gauge couplings in extra dimensional theories are dimensional, i.e. α$_{5D}$ has mass dimension − 1, a rough guess (which is confirmed, up to order one corrections, by various NDA methods) is [61] Λ$_{5D}$ ∼ 4 π/α$_{5D}$ . Matching this 5D gauge coupling to a 4D coupling of the SM, at leading order, 1 /g 2 = πR/g 2 $_{5D}$, we obtain Generically, the mass of the lightest Kaluza-Klein (KK) states, M$_{KK}$ , is of O ( R − 1 ) . If the extra dimension theory is linked to the solution of the hierarchy problem and/or directly accessible to near future experiments, then R − 1 = O ( TeV ) . This implies an upper bound on the 5D cutoff: where Λ$_{K}$ is the scale required to suppress the generic contributions to ϵ$_{K}$ , discussed above (see Table I). The above discussion ignores the possibility of splitting the fermions in the extra dimension. In split fermion models different bulk masses are assigned to different generations. Consequently fermions are localized and separated in the bulk of the extra dimension in a manner which may successfully address the SM flavor puzzle [62]. Separation in the extra dimension may suppress the contributions to ϵ$_{K}$ from the higher-dimension cut-off induced operators. As shown in Table I, the most dangerous operator is Λ$_{5D}$ R ∼ 4 α ∼ 30 . (6.1) Λ$_{5D}$ ≲ 10 $^{2}$TeV ≪ Λ$_{K}$ ∼ 2 × 10 $^{5}$TeV , (6.2) O 4 K = 1 Λ 2 5D (¯ s$_{L}$ d$_{R}$ ) (¯ s$_{R}$ d$_{L}$ ) . (6.3) 22