Flavor from Minimal Flavor Violation&a Viable Randall-Sundrum Model

We present a variant of the warped extra dimension, Randall-Sundrum (RS), framework which is based on five dimensional (5D) minimal flavor violation (MFV), in which the only sources of flavor breaking are two 5D anarchic Yukawa matrices. The Yukawa matrices also control the bulk masses, which are responsible for the resulting flavor structure and mass hierarchy in the low energy theory. An interesting result of this set-up is that at low energies the theory flows to next to MFV model where flavor violation is dominantly coming from the third generation. Low energy flavor violation is further suppressed by a single parameter that dials the amount of violation in the up or down sector. There is therefore a sharp limit in which there is no flavor violation in the down type quark sector which, remarkably, is favored when we fit for the flavor parameters. This mechanism is used to eliminate the current RS flavor and CP problem even with a Kaluza-Klein scale as low as 2 TeV! Our construction also suggests that economic supersymmetric and non-supersymmetric, strong dynamic-based, flavor models may be built based on the same concepts. Introduction. The standard model (SM) agrees very well with data. However, it is widely perceived to be an incomplete theory. In particular, in the SM, the hierarchy between the Planck scale and the electroweak (EW) symmetry breaking (EWSB) scale is unnatural since the Higgs mass is ultra-violet (UV) sensitive. Solutions to the hierarchy problem therefore involve extending the SM at just above the EWSB scale which, in general, spoils the good agreement of the SM with data when trying to explain flavor as well. Given this inherent tension, it is important to identify new physics (NP) frameworks that preserve the approximate symmetries of the SM. In this letter we consider the Randall-Sundrum scenario (RS1) [1], which potentially provides an elegant solution to the hierarchy problem. In this framework, due to warped higher-dimensional spacetime, the mass scales in an effective 4D description depend on location in an extra dimension: the Higgs sector is localized at the “TeV” brane where it is protected by a low warped-down fundamental scale of order a TeV while 4D gravity is localized near the “Planck” brane which has a Planckian fundamental scale. In the original RS1 model, the entire SM was localized on the TeV brane. In this set-up, flavor issues are sensitive to the UV completion of the RS1 effective field theory: there is no understanding of the hierarchies in fermion masses or of smallness of flavor changing neutral currents (FCNCs) from higher-dimensional operators that would be too large if suppressed only by the warped-down cut-off ∼ TeV. Similar tension arises when considering the model predictions regarding EW precision tests. Allowing the SM fermions and gauge fields to propagate in the bulk gives an opportunity to explain flavor, and makes flavor issues UV-insensitive as follows. The light fermions can be localized near the Planck brane (using a 5D fermion mass parameter [2, 3]) where the effective cut-off is much higher than TeV so that FCNCs from higher-dimensional operators are suppressed [3, 4]. Moreover, this results in small 4D Yukawa couplings to the Higgs, even if there are no small 5D Yukawa couplings [3, 4]. The top quark can be localized near the TeV brane to obtain a large 4D top Yukawa coupling. Because the fermion profiles depend exponentially on the bulk masses, this provides an understanding of the hierarchy of fermion masses (and mixing) without hierarchies in fundamental (5D) parameters, solving the SM flavor puzzle. However, with bulk fermions and gauge fields, calculable FCNCs from exchange of gauge Kaluza-Klein (KK) modes are induced. The couplings of light fermions to gauge KK modes are non-universal which induce FCNCs. Unlike the flat case, there is a significant protection from a built-in RS1-GIM [5] due to the approximate flatness of the KK gauge boson wavefunctions in the UV and a hierarchy in the fermion wavefunctions in the IR; nevertheless, the resulting contributions to FCNCs are nonnegligible. In [6] it was shown that at low energies this class of models flows to next to MFV (NMFV); that is, flavor changing effects are generated primarily through mixing with the third generation. Generically, within the NMFV framework flavor violation occurs through NP sources, with a typical scale of ΛNMFV, which breaks the