Flavour physics in the soft wall model

Abstract

We extend the description of flavour that exists in the Randall-Sundrum (RS) model to the soft wall (SW) model in which the IR brane is removed and the Higgs is free to propagate in the bulk. It is demonstrated that, like the RS model, one can generate the hierarchy of fermion masses by localising the fermions at different locations throughout the space. However, there are two significant differences. Firstly the possible fermion masses scale down, from the electroweak scale, less steeply than in the RS model and secondly there now exists a minimum fermion mass for fermions sitting towards the UV brane. With a quadratic Higgs VEV, this minimum mass is about fifteen orders of magnitude lower than the electroweak scale. We derive the gauge propagator and despite the KK masses scaling as m 2 n ∼ n, it is demonstrated that the coefficients of four fermion operators are not divergent at tree level. FCNC's amongst kaons and leptons are considered and compared to calculations in the RS model, with a brane localised Higgs and equivalent levels of tuning. It is found that since the gauge fermion couplings are slightly more universal and the SM fermions typically sit slightly further towards the UV brane, the contributions to observables such as ǫ K and ∆m K , from the exchange of KK gauge fields, are significantly reduced.

Flavor phenomenology in general 5D warped spaces

“…In that way we are able to associate a probability to a certain choice of bulk masses and hence quantify the fine tuning to achieve a given KK scale that leads to agreement with experimental constraints. The improvement of flavor/CP bounds with modified metrics has recently been noted in the context of soft wall models [14]. Here we show that a similar improvement can be obtained in the hard wall setup, and again we quantify the amount of fine tuning needed to obtain a satisfactory bound on the KK scale.…”

Neutrino mixing from Wilson lines in warped space

“…However a similar study would also apply for IR deformed metrics (as the soft-wall class of metrics [5,44]) and we expect similar conclusions to follow. In particular for brane localized Majorana masses and Yukawa couplings, as in the example we have worked out, most of the results (except for normalization factors) only depend on the total warp factor, which is metric independent while gauge breaking arguments should hold in general metrics.…”

“…quark sector in WED theories both for an AdS metric [2,3], as in the original RandallSundrum (RS) model, and for asymptotically AdS (or IR deformed) metrics [4][5][6][7]. The reason of this success is that both the quark spectrum (in the up and down sectors) and the CKM mixing angles are hierarchical, a situation which can be readily described for order unity 5D Yukawa couplings if the various quark flavors are differently localized along the fifth dimension.…”

“…gluinos) thus generating dimensionsix flavor changing neutral current (FCNC) operators. While these flavor violations are suppressed by the so-called RS-GIM mechanism [8][9][10] the KK masses have to be heavier than ∼ 20 TeV to suppress FCNC processes in the RS theory [6,11] while this bound can be lowered to 4-5 TeV in the case of IR deformed metrics [5,6].…”

“…Clearly generic brane masses again violate L. It is important to notice that besides n i = 0 also n i = ∞ conserve L. 5 In the general case of several generations of RH neutrinos we can always work in a basis where the bulk Dirac masses are diagonal. However the Majorana masses will, in general, be non-diagonal except if there is a bulk symmetry implying diagonal masses.…”