The little hierarchy in Universal Extra Dimensions

Abstract

Abstract:In the standard model in universal extra dimensions (UED) the mass of the Higgs field is driven to the cutoff of the higher-dimensional theory. This re-introduces a small hierarchy since the compactification scale 1/R should not be smaller than the weak scale. In this paper we study possible solutions to this problem by considering five-dimensional theories where the Higgs field potential vanishes at tree level due to a global symmetry. We consider two avenues: a Little Higgs model and a Twin Higgs model. An obstacle for the embedding of these four-dimensional models in five dimensions is that their logarithmic sensitivity to the cutoff will result in linear divergences in the higher dimensional theory. We show that, despite the increased cutoff sensitivity of higher dimensional theories, it is possible to control the Higgs mass in these two scenarios. For the Little Higgs model studied, the phenomenology will be significantly different from the case of the standard model in UED. This is due to the fact that the compactification scale approximately coincides with the scale where the masses of the new states appear. For the case of the Twin Higgs model, the compactification scale may be considerably lower than the scale where the new states appear. If it is as low as allowed by current limits, it would be possible to experimentally observe the standard model Kaluza-Klein states as well as a new heavy quark. On the other hand, if the compactification scale is higher, then the phenomenology at colliders would coincide with the one for the standard model in UED.

KK parity in warped extra dimension

“…One should keep in mind that the flatness of profiles in UED is not natural and reflects the fact that electroweak symmetry breaking is not addressed but just postulated. A model of dynamical symmetry breaking in UED would typically spoil the flatness of the Higgs profile and constraints on the KK scale would have to be reexamined accordingly (a somewhat related discussion on the little hierarchy problem in UED is presented in [28]). The virtue of UED is that mass scales of new particles are allowed to be very close to the electroweak scale at a few hundreds GeV, allowing for easy access at the LHC, even though the model addresses neither the Planck-weak nor the fermion mass hierarchy as it stands in the literature.…”

Status report on universal extra dimensions after LHC8

“…Anyhow, these scenarios generally share the same problems as models where the Higgs is a tt condensate, 3 for a related recent discussion). Another attempt to address the little hierarchy problem in UED was provided in 4 by embedding either a Little Higgs or Twin Higgs model in UED. Moreover, to fully address the hierarchy problem requires a mechanism that dynamically selects the compactification scale to be near the TeV scale, to connect the compactification scale and electroweak symmetry breaking (Ref.…”

Two Universal Extra Dimensions

“…It was recently pointed in Ref. [9] that the SM in 5D UED has a "little hierarchy" problem since the Higgs mass still receives quadratically divergent contributions. Solving this problem in the 5D scenario by assuming the Higgs is a pseudo-Nambu-Goldstone boson, results in the cutoff dependence becoming linear.…”