Resurrecting Kaluza-Klein Dark Matter with Low-Temperature Reheating

Abstract

In Universal Extra Dimension (UED) scenarios, the lightest Kaluza-Klein (KK) particle is naturally stable due to a remnant discrete symmetry, KK parity, arising from extra-dimensional compactification. This stability requires no ad hoc symmetry and renders Kaluza-Klein dark matter a well-motivated candidate, provided it reproduces the observed relic abundance. The minimal UED (mUED) framework being highly predictive is strongly constrained by the combined requirements of relic density and collider searches under standard cosmological assumptions. We revisit the dark matter phenomenology of mUED in the presence of a nonstandard cosmological history featuring a low reheating temperature driven by prolonged inflaton decay. Solving the coupled Boltzmann equations for dark matter, radiation, and inflaton energy densities, we show that entropy injection during reheating can dilute the relic abundance by orders of magnitude, reopening large regions of parameter space previously ruled out. We further demonstrate that the revived parameter space is consistent with current collider, direct-detection, and indirect-detection constraints, while remaining testable by upcoming experiments.

Figures (2)

• Figure 1: Summary of the dark matter analysis in the MUED framework under nonstandard cosmological conditions. Top: $\Gamma_{\phi}$ vs $\Omega h^2$ for different $R^{-1}$ for $\Lambda R = 5$ TeV. Middle: Constraints From Direct Detection. Bottom: Constraints From Indirect Detection.
• Figure 2: Summary of the Present and expected future constraints on the MUED parameter space.