Radion Phenomenology in Realistic Warped Space Models

TL;DR

This work analyzes the radion phenomenology in realistic warped-space models where SM fields propagate in the bulk, focusing on how radion couplings to bulk fermions and gauge bosons arise and how they manifest in collider signatures. It provides a detailed calculation of radion couplings to both massive and massless gauge fields, including tree-level bulk terms, brane kinetic terms, and loop-induced contributions, with a clear CFT interpretation via conformal symmetry breaking. The study finds that the radion can enhance the diphoton channel relative to the SM Higgs in many regions of parameter space, but this enhancement is sensitive to brane-localized kinetic terms; for heavier radions, the 4-lepton channel offers discovery potential. Overall, the paper establishes the radion as a viable, testable state in RS-type models with bulk SM fields and clarifies how to distinguish it from the SM Higgs through branching fractions, widths, and potential KK-mode observations.

Abstract

We investigate the phenomenology of the Randall-Sundrum radion in realistic models of electroweak symmetry breaking with bulk gauge and fermion fields, since the radion may turn out to be the lightest particle in such models. We calculate the coupling of the radion in such scenarios to bulk fermion and gauge modes. Special attention needs to be devoted to the coupling to massless gauge fields (photon, gluon), since it is well known that loop effects may be important for these fields. We also present a detailed explanation of these couplings from the CFT interpretation. We then use these couplings to determine the radion branching fractions and discuss some of the discovery potential of the LHC for the radion. We find that the gamma-gamma signal is enhanced over most of the range of the radion mass over the gamma-gamma signal of a SM Higgs, as long as the RS scale is sufficiently low. However, the signal significance depends strongly on free parameters that characterize the magnitude of bare brane-localized kinetic terms for the massless gauge fields. In the absence of such terms, the signal can be be enhanced over the traditional RS1 models (where all standard model fields are localized on the IR brane), but the signal can also be reduced compared to RS1 if the brane localized terms are sizeable. We also show that for larger radion masses, where the gamma-gamma signal is no longer significant, one can use the usual 4 lepton signal to discover the radion.

Figures (7)

• Figure 1: Triangle diagrams
• Figure 2: The free gauge propagator of the CFT
• Figure 3: In these plots, we show the branching fractions of the radion into gluons and photons (the solid lines), comparing with RS1 scenario (the dashed curves). For each graph, the solid curves represent the branching fractions in the presence of different combinations of tree level brane localized kinetic terms for the gluon and photon. The magnitudes of the localized terms are given on the top of every plot individually in units of the one-loop corrections to the $\tau^{\rm UV}_i$. We have set $\Lambda_r=2$ TeV, corresponding to $1/R'=816$ GeV.
• Figure 4: In this plot, we show the branching fractions of the radion into on-shell final states. Again, the dashed curves represent the branching fractions in the RS1 scenario. We show the effect of introducing tree level brane localized kinetic terms for the photon and gluon, choosing different combinations for each graph. We set $\Lambda_r=2$ TeV, corresponding to an IR brane scale of $1/R'=816$ GeV.
• Figure 5: In the plot on the left,we show a comparison of the ratio of discovery significance for a radion vs. a Higgs of the same mass, $R^{\gamma\gamma}_S$, with the scenario where the SM fields are all localized on the IR brane (the dashed curves). In the plot on the right we show the ratio of discovery significance $R^{4l}_S$. We assume that there are no tree level brane localized kinetic terms for the gluon or photon. For the displayed values of $\Lambda_r$, the corresponding values of $1/R'$ are $408$, $816$, $2041$, and $4082$ GeV.