Light Radion Research Articles

A new way of calculating the effective potential for a light radion

We address again the old problem of calculating the radion effective potential in Randall-Sundrum scenarios, with the Goldberger-Wise stabilization mechanism. Various prescriptions have been used in the literature, most of them based on heuristic derivations and then applied in some approximations. We define rigorously a light radion 4D effective action by using the interpolating field method. For a given choice of the interpolating field, defined as a functional of 5D fields, the radion effective action is uniquely defined by the procedure of integrating out the other fields, with the constrained 5D equations of motion always satisfied with help of the Lagrange multipliers. Thus, for a given choice of the interpolating fields we obtain a precise prescription for calculating the effective potential. Different choices of the interpolating fields give different prescriptions but in most cases very similar effective potentials. We confirm the correctness of one prescription used so far on a more heuristic basis and also find several new, much more economical, ways of calculating the radion effective potential. Our general considerations are illustrated by several numerical examples. It is shown that in some cases the old methods, especially in models with strong back-reaction, give results which are off even by orders of magnitude. Thus, our results are important e.g. for estimation of critical temperature in phase transitions.

Discussing 125 GeV and 95 GeV excess in light radion model

Even if the LHC observations are consistent with the Standard model (SM), current LHC results are not precise enough to rule out the presence of new physics. Taking a contrarian view of the SM Higgs fandom, we look out for a more suitable candidate for the 125 GeV boson observed at the LHC. At the same time, a recent result from CMS hints toward an excess near 95 GeV in the diphoton ($\ensuremath{\gamma}\ensuremath{\gamma}$) channel. Given these aspects, we revisit the Higgs-radion mixing model to explore the viability of the radion mixed Higgs to be the 125 GeV boson along with the presence of a light radion (to be precise, Higgs mixed radion) that can show up in future experiments in the $\ensuremath{\gamma}\ensuremath{\gamma}$ channel. We find that the mixed radion-Higgs scenario gives a better fit than the SM, with the radion mixed Higgs as a more suitable 125 GeV scalar candidate. It also gives rise to a diphoton excess from the light radion, consistent with the LHC observations.

The light radion window

Inspired by the Contino-Pomarol-Rattazzi mechanism we explore scenarios with a very light (1 keV to 10 GeV) radion which could be associated with the suppression of the electroweak contribution to vacuum energy. We construct explicit, realistic models that realize this mechanism and explore the phenomenological constraints on this class of models. Compared with axion-like particles in this mass range, the bounds from SN 1987a and from cosmology can be much weaker, depending on the mass of the radion and its coupling to other particles. With couplings suppressed by a scale lower than 100 TeV, much of the mass window from 100 keV to 10 GeV is still open.

The radion as a dark matter candidate

I study a class of Randall–Sundrum (RS) models with Spontaneous Breaking of Scale Invariance (SBSI). This class of models implements the Contino–Pomarol–Rattazzi (CPR) mechanism to achieve SBSI through the small running of an external close-to-marginal scale-breaking operator that leads to a light dilaton/radion with couplings to matter suppressed by the small running. I show that for radion masses [Formula: see text] KeV, it can serve as a dark matter (DM) candidate, with a lifetime longer than the age of the universe, and show that the experimental bounds from LHC, non-Newtonian gravity and Axion-Like Particle (ALP) searches allow for the existence of such a radion. In spite of the small relic abundance of the light radion produced in this model, we show that it could be possible to obtain the required abundance through additional assumptions, an issue we postpone to the future.

KK graviton resonance and cascade decays in warped gravity

We consider the Kaluza–Klein (KK) graviton and its decays in the presence of the light radion, the modulus of extra dimension, appearing naturally light due to classical scale invariance in warped gravity. Due to the presence of a sizable wave function overlap in the extra dimension between the KK graviton and the radion, the KK graviton can decay largely into a pair of radions, each of which decays into a pair of collimated photons or photon-jets. Focusing on the setup where the KK graviton has suppressed couplings to the Higgs boson and fermions, we discuss the bounds on the light radion in the parameter space where the photon-jets are relevant. Moreover, we pursue the possibility of distinguishing the scenario from the case with direct photon contributions by the angular distributions of photons in the LHC Run 2. Roles of KK graviton and radion as mediators of dark matter interactions, extensions with brane kinetic terms and the model with a holographic composite Higgs are also addressed.

Probing the light radion through diphotons at the Large Hadron Collider

A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if the Kaluza-Klein excitation modes of the graviton turn out to be in the multi-TeV region. The discovery of such a light radion would be a gateway to new physics. We show how the two-photon mode of decay can enable us to probe a radion in the mass range 60--110 GeV. We take into account the diphoton background, including fragmentation effects, and include cuts designed to suppress the background to the maximum possible extent. Our conclusion is that, with an integrated luminosity of $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ or less, the next run of the Large Hadron Collider should be able to detect a radion in this mass range, with a significance of 5 standard deviations or more.

Production and decay of radion in Randall–Sundrum model at a photon collider

A warped extra dimension model predicts an extra scalar particle beyond the Standard Model (SM) which is called a radion. Although interactions of the radion are similar to those of the Higgs boson in the SM, a relatively light radion (≲100 GeV) is not severely constrained from the Higgs search experiments at the LHC. In this paper we study discovery potential of the radion at a photon collider as an option of ILC. Owing to the trace anomaly of the energy–momentum tensor, both a production of radion in γγ collision and its decay to gluon pair are enhanced sizably. We find that the photon collider has a sensitivity for discovering the radion in low-mass region up to Λϕ~3 TeV, where Λϕ is a scale parameter which suppresses the interactions of radion to the SM particles.

Dynamics of a stabilized radion and duality

We construct the effective theory of the graviscalar radion in the Randall-Sundrum scenario, taking into account effects arising from the stabilization of the extra dimension through the Goldberger-Wise mechanism. We explore the conditions under which the radion can remain light, and determine the corrections to its couplings to Standard Model (SM) states when the effects of stabilization are taken into account. We show that in the theories of interest for electroweak symmetry breaking that have a holographic dual, the presence of a light radion in the spectrum is not a robust prediction of the framework, but is in fact associated with mild tuning. We find that corrections to the form of the radion couplings to Standard Model particles arising from effects associated with brane stabilization are suppressed by the square of the ratio of the radion mass to the Kaluza-Klein scale. These corrections are small in theories where the radion is light, and are generally subleading, except in the case of couplings to the SM gluons and photon, when they can sometimes dominate. The AdS/CFT correspondence relates the radion in Randall-Sundrum models to the dilaton in theories where a strongly coupled conformal symmetry is spontaneously broken. We show that the discrepancies in the literature between the results for the dilaton and the radion can be traced to the omission of self-interaction terms that would otherwise dominate the potential for the Goldberger-Wise scalar near the infrared brane. In the dual picture, this corresponds to neglecting the corrections to the scaling behavior of the operator that breaks conformal symmetry when it grows large. With the inclusion of these self-interaction terms, we find good agreement between the results on the two sides of the correspondence.

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

We show that a bulk Higgs with a mass saturating the Breitenlohner-Freedman bound can naturally generate and stabilize an exponential hierarchy on a nearly AdS background. The physical Higgs boson in this class of models emerges as the lightest eigenstate of the Higgs/radion system and has a mass strictly lighter than the Kaluza-Klein scale. These theories are dual to strongly coupled CFTs deformed by a marginally relevant Higgs mass operator. On the 5D side, the marginally relevant nature of the Higgs mass operator implies that the Higgs VEV is maximally spread in the bulk. This feature significantly decreases the lower bound on the new physics scale in models that address the SM flavor problem. The collider phenomenology interpolates between Randall-Sundrum scenarios with a heavy Higgs and a light radion, and composite Higgs models.

Promising diphoton signals of the little radion at hadron colliders

In Little Randall-Sundrum models, the bulk couplings of the radion to massless gauge fields can yield a greatly enhanced diphoton signal at hadron colliders. We examine the implications of the Tevatron data for the Little radion and also show that the 7 TeV run at the Large Hadron Collider will have an impressive reach in this channel. The diphoton signal is crucial in the search for a light radion, or the dual dilaton, and can potentially probe the ultraviolet scale of the theory.

B-decay signatures of warped top-condensation

We point out that the light radion ϕ in a recently proposed Warped Top-Condensation Model (WTCM), can provide distinct signatures in b→sϕ, where the on-shell ϕ can decay with displaced vertices. We find that some of the parameter space of these models is constrained by B-meson and astrophysical data. Future B-decay measurements can lead to the discovery of the WTCM.

Quantization of scalar perturbations in brane-world inflation

We consider a quantization of scalar perturbations about a de Sitter brane in a 5-dimensional anti-de Sitter (AdS) bulk spacetime. We first derive the second order action for a master variable $\ensuremath{\Omega}$ for 5-dimensional gravitational perturbations. For a vacuum brane, there is a continuum of normalizable Kaluza-Klein (KK) modes with $m>3H/2$. There is also a light radion mode with $m=\sqrt{2}H$ which satisfies the junction conditions for two branes, but is non-normalizable for a single brane model. We perform the quantization of these bulk perturbations and calculate the effective energy density of the projected Weyl tensor on the barne. If there is a test scalar field perturbation on the brane, the ${m}^{2}=2{H}^{2}$ mode together with the zero-mode and an infinite ladder of discrete tachyonic modes become normalizable in a single brane model. This infinite ladder of discrete modes as well as the continuum of KK modes with $m>3H/2$ introduce corrections to the scalar field perturbations at first-order in a slow-roll expansion. We derive the second order action for the Mukhanov-Sasaki variable coupled to the bulk perturbations which is needed to perform the quantization and determine the amplitude of scalar perturbations generated during inflation on the brane.

THE EFFECT OF A LIGHT RADION ON THE TRIVIALITY BOUND ON HIGGS MASS

In this paper we study how the triviality bound on Higgs mass in the context of the Standard Model is modified by a light stabilized radion of the Goldberger–Wise variety. Our approach is inherently perturbative. Including the radion contribution to β(λ) and β(gt) to one-loop we evolve the Higgs self coupling λ from the cutoff Λ(=<ϕ>) down to the electroweak scale μ0=v. The triviality bound is obtained by requiring that [Formula: see text] which is the perturbative limit. We also study the effect of small changes in the UVBC on the triviality bound both in the presence and absence of a light radion.

Scalar perturbations from brane-world inflation

We investigate the scalar metric perturbations about a de Sitter brane universe in afive-dimensional anti-de Sitter bulk. We compare the master variable formalism, describingmetric perturbations in a five-dimensional longitudinal gauge, with results ina Gaussian normal gauge. For a vacuum brane (with constant brane tension)there is a continuum of normalizable Kaluza–Klein modes, with , which remain in the vacuum state. A light radion mode, with , satisfies the boundary conditions for two branes but is not normalizable in thesingle-brane case. When matter is introduced (as a test field) on the brane, this mode,together with the zero-mode and an infinite ladder of discrete tachyonic modes, becomesnormalizable. However, the boundary condition requires the self-consistent four-dimensionalevolution of scalar field perturbations on the brane and the dangerous growing modesare not excited. These normalizable discrete modes introduce corrections at firstorder to the scalar field perturbations computed in a slow-roll expansion. Onsuper-Hubble scales, the correction is smaller than slow-roll corrections to the de Sitterbackground. However, on small scales the corrections can become significant.

Holographic theories of electroweak symmetry breaking without a Higgs boson

Recently, realistic theories of electroweak symmetry breaking have been constructed in which the electroweak symmetry is broken by boundary conditions imposed at a boundary of higher dimensional spacetime. These theories have equivalent 4D dual descriptions, in which the electroweak symmetry is dynamically broken by non-trivial infrared dynamics of some gauge interaction, whose coupling g and size N satisfy g^2N > 16pi^2. Such theories allow one to calculate electroweak radiative corrections, including the oblique parameters S, T and U, as long as g^2N/16pi^2 and N are large. We study how the duality between the 4D and 5D theories manifests itself in the computation of various physical quantities. In particular, we study a warped 5D theory where the electroweak symmetry is broken by boundary conditions at the infrared brane. We show that S exceeds the experimental bound if the minimal theory is in a weakly coupled regime. This requires either an extension of the model or departure from weak coupling. An interesting scenario is obtained if the gauge couplings in the 5D theory take the largest possible values -- the value suggested by naive dimensional analysis. We argue that such a theory can provide a potentially consistent picture for dynamical electroweak symmetry breaking: corrections to the electroweak observables are sufficiently small while realistic fermion masses are obtained without conflicting with bounds from flavor violation. The theory contains only the standard model quarks, leptons and gauge bosons below \sim 2 TeV, except for a possible light radion. At \sim 2 TeV increasingly broad string resonances appear. An analysis of top-quark phenomenology and flavor violation is also presented, which is applicable to both the weakly-coupled and strongly-coupled cases.

Implications of a light radion on the RG evolution of Higgs self coupling in the Randall–Sundrum model

In this Letter we determine how the beta function of the Higgs self coupling λ at one loop order is modified by a light stabilized radion in the Randall–Sundrum model. We then use the modified beta function to derive a lower bound on the radion vev 〈φ〉, both for perturbative and non-perturbative values of λ at the ultra violet cut off Λ. The lower bound on 〈φ〉 is obtained by demanding that the renormalized coupling λ(μ) at μ=114 GeV be consistent with the present experimental bound of 114 GeV on the Higgs mass from LEPII searches. We also show that important bounds on 〈φ〉 can be derived from the sign of β(λ) in the infrared regime.

Phenomenology of the radion in the Randall–Sundrum scenario at colliders

Phenomenology of a radion ( φ) that stabilizes the modulus in the Randall–Sundrum scenario is considered. The radion couples to the trace of energy momentum tensor of the standard model (SM) with a strength suppressed only by a new scale ( Λ φ ) of an order of the electroweak scale. In particular, the effective coupling of a radion to two gluons is enhanced due to the trace anomaly of QCD. Therefore, its production cross section at hadron colliders could be enhanced, and the dominant decay mode of a relatively light radion is φ→ gg, unlike the SM Higgs boson case. We also present constraints on the mass m φ and the new scale Λ φ from the Higgs search limit at LEP and perturbative unitarity bound.