Radion mediated flavor changing neutral currents

Abstract

In the context of a warped extra-dimension with Standard Model fields in the bulk, we obtain the general flavor structure of the Radion couplings to fermions and show that the result is independent on the particular nature of the Higgs mechanism (bulk or brane localized). These couplings will be generically misaligned with respect to the fermion mass matrix when the fermion bulk mass parameters are not all degenerate. When the Radion is light enough, the generic size of these treelevel flavor changing couplings will be strongly constrained by the experimental bounds on ∆F = 2 processes. At the LHC the possibility of a heavier Radion decaying into top and charm quarks is then considered as a promising signal to probe the flavor structure of both the Radion sector and the whole scenario. PACS numbers:Introducing a warped extra-dimension in such a way as to create an exponential scale hierarchy between the two boundaries of the extra dimension [1] has generated a lot of attention in the recent years as a novel approach to solve the hierarchy problem. By placing the Standard Model (SM) fermions in the bulk of the extra dimension it was then realized that one can simultaneously address the fermion mass hierarchy puzzle [2]. In this context the main constraints come from precision electroweak bounds as well as from low energy flavor violating processes [3,4,5,6], pushing the scale of new physics (the mass of the lowest KK excitations) to several TeV. In these scenarios, the metric fluctuations contain a scalar degree of freedom -the Radion, whose mass and couplings could make it the first new physics state to be discovered at the LHC. In the original RS1 setup [1], the Radion phenomenology was extensively studied and analyzed including the possibility of some amount of mixing with the Higgs scalar [7,8,9]. But it wasn't until relatively recently [10,11,12] that Radion interactions with bulk SM fields were fully considered. In this letter we want to extend these last investigations to include the full fermion flavor structure to the Radion couplings and show that as opposed to the original RS1 scenario, there is a prediction for generic flavor violating Radion couplings to fermions. The spacetime we consider takes the usual Randall-Sundrum form [1]:with the UV (IR) branes localized at z = R (z = R ′ ). The Radion can be parametrized by the following scalar perturbation of metric:Demanding that the perturbed metric solves the Einstein equation and that the Radion field is canonically normalized, we getwhere r(x) is the corresponding canonically normalized Radion graviscalar with its associated interaction scale Λ r = √ 6 R R ′ M P l . We assume that some unknown dynamics (e.g. the Goldberger-Wise mechanism [13]) will fix the inter-brane distance and give a positive mass squared to the Radion, and that it gives negligible back-reaction to the metric. The couplings between bulk SM fermions and the Radion are calculated in [11] in the case of one generation, with a brane localized Higgs. We are interested here in the flavor...

Higgs phenomenology in warped extra dimensions with a fourth generation

“…(39) and (40). Some differences observed can nevertheless be explained, (see also [38]) so that one can still be confident in the generic size of the flavor violating couplings predicted in the flavor anarchy paradigm in RS type scenarios with four generations.…”

“…However, we observe that for the third and fourth generation quarks, the corrections to the diagonal Yukawa couplings are always negative (suppressions) if Y 1 = Y 2 and are larger than the previous estimates. This point was argued in [38] and we address it again the next subsection for completeness.…”

“…In this section, following the same procedure as in [38], we estimate the off-diagonal couplings of Higgs boson to SM fermions and then we do a numerical scan over anarchical Yukawa couplings to support our estimates.…”

“…The inclusion of the fourth generation will also affect low-energy precision observables, as well as limits on rare decays. In the lines of [38], we propose to explore here the effect of FCNC Higgs couplings with a fourth generation in a simple warped extra dimensional model. …”

“…For instance, recently it was pointed out that warped extra-dimensional models introduce new flavor-violating operators in the Higgs sector. In a composite Higgs sector with strong dynamics, flavor changing neutral currents (FCNC) can arise at tree level, generated by a misalignment between the Higgs Yukawa matrices and the fermion mass matrices [36][37][38]. The full set of operators responsible for the misalignment has been thoroughly analyzed, showing that the effect is generically large and phenomenologically important [38] and even could alter considerably the couplings of Higgs to gluons [39,40], affecting thus the main production mechanism of the Higgs at hadron colliders.…”