Radions

Radions are hypothetical scalar particles arising in theories with extra spatial dimensions, most prominently in the Randall-Sundrum (RS) framework. They describe fluctuations of the size, or radion field, of the extra dimension, and thus encode the dynamics of the modulus that fixes the separation between branes in RS models.

In the RS1 scenario, the 5D geometry is warped, with an infrared brane scale suppressed to the TeV level. The radion is the 4D remnant of the extra-dimensional metric degree of freedom. Its stabilization by mechanisms such as the Goldberger-Wise method gives it a nonzero mass, typically in the range from tens of GeV to a few TeV, depending on the stabilization scale and model parameters. The radion's couplings to Standard Model fields are universal in proportion to the trace of the SM energy-momentum tensor, so heavier particles couple more strongly. Couplings are suppressed by a scale Λ_r, of order the TeV scale, making the radion resemble a heavy Higgs but with distinctive features. An important difference is the contribution from the trace anomaly, which yields tree-level decays into photons and gluons that can be enhanced relative to a SM Higgs of the same mass. Mixing with the Higgs is possible, altering production rates and decay patterns.

At high-energy colliders, radions can be produced mainly via gluon fusion, with decays to WW, ZZ, hh, tt, bb, as well as γγ and gg. The branching ratios depend on mass and mixing. Constraints from LHC Higgs measurements and searches for heavy resonances limit the radion parameter space, particularly the mixing angle and the scale Λ_r. In some models, radions can act as portals to hidden sectors or dark matter.