Exact identification of the radion and its coupling to the observable sector

Abstract

Braneworld models in extra dimensions can be tested in laboratory by the coupling of the radion to the standard model fields. The identification of the radion as a canonically normalized field involves a careful general relativity treatment: if a bulk scalar is responsible for the stabilization of the system, its fluctuations are entangled with the perturbations of the metric and they also have to be taken into account (similarly to the well-developed theory of scalar metric perturbations in 4D cosmology with a scalar field). Extracting a proper dynamical variable in a warped geometry/scalar setting is a nontrivial task, performed so far only in the limit of negligible backreaction of the scalar field on the background geometry. We perform the general calculation, diagonalizing the action up to second order in the perturbations and identifying the physical eigenmodes of the system for any amplitude of the bulk scalar. This computation allows us to derive a very simple expression for the exact coupling of the eigenmodes to the standard model fields on the brane, valid for an arbitrary background configuration. As an application, we discuss the Goldberger-Wise mechanism for the stabilization of the radion in the Randall-Sundrum-type models. The existing studies, limited to small amplitude of the bulk scalar field, are characterized by a radion mass which is significantly below the physical scale at the observable brane. We extend them beyond the small backreaction regime. For intermediate amplitudes, the radion mass approaches the electroweak scale, while its coupling to the observable brane remains nearly constant. At very high amplitudes, the radion mass instead decreases, while the coupling sharply increases. Severe experimental constraints are expected in this regime.

Gravitational waves from warped spacetime

“…According to Ref. [24], the radion couplings are expected to be large in this case. This could indeed imply the possibility of testing the strength of the transition at colliders.…”

“…When a vev is too big, there can be large corrections to the GW potential from KK modes and large corrections to the radion kinetic term due to mixing with the GW scalar. According to [24], the last two sources can be acceptable however. That is, when the full mass matrix and kinetic matrix are diagonalized, one can still find dominance of the light radion.…”

“…[24], where the backreaction parameter is defined as l = 4πv 0 /N . To guarantee that the radion and KK modes are heavy enough and not too strongly coupled to SM fields on the TeV brane, they need l ∼ < 10 i.e.…”

A natural hierarchy and a low new physics scale from a bulk Higgs

“…Yet, numerical simulations of the full backreacted system show that the physics of the lightest scalar eigenstate -our Higgs boson -is remarkably well approximated by a leading approximation, like the one performed here, even for φ 2 /M 3 = O(1) or bigger [11] [12]. In particular, it turns out that the mass of the lightest scalar is given to a good approximation by…”

Emergent gravity from a mass deformation in warped spacetime

“…The derivation of this action is quite involved (we extend the computation of Ref. [35], performed for the covariant case), but the result is relatively simple. The computation of the action supports the choice of the (generalized) Fierz-Pauli mass term for the perturbations.…”