High Energy Physics Seminar - Vincent Lee (UC San Diego) "The Dark Dimension, Revisited"

Motivated by the swampland program, it has been proposed that a tower of massive Kaluza-Klein (KK) gravitons from a compactified flat extra dimension can be dark matter, with the heavier modes constantly cascading to lighter ones. The phenomenology crucially depends on the rate at which a massive KK mode decays to lighter modes. We show that, up to mild assumptions on the UV theory, the decay rate is suppressed by a factor of (q/m)^4 compared to the one commonly used in the literature, where q is the momentum of the decay products and m is the mass of the decaying KK mode. This suggests that the proposed scenario is ruled out by CMB observations by many orders of magnitude. Remarkably, the same suppression opens up a vastly different parameter space in which KK gravitons from a strongly warped extra dimension (e.g. a Randall-Sundrum setup) are a viable dark matter candidate, either as the cascading gravitons themselves or as a stable component of the tower.