Resurrecting Kaluza-Klein Dark Matter with Low-Temperature Reheating

TL;DR

This paper explores how low-temperature reheating in the early universe can dilute Kaluza-Klein dark matter relic abundance, expanding viable parameter space and aligning with current experimental constraints.

Contribution

It introduces a nonstandard cosmological scenario with low reheating temperature that alters dark matter relic calculations in minimal UED models.

Findings

Entropy injection during reheating dilutes relic abundance.

Revived parameter space is consistent with collider and detection constraints.

Upcoming experiments can test the new viable regions.

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.