Precise Calculation of the Relic Density of Kaluza-Klein Dark Matter in Universal Extra Dimensions

TL;DR

This work delivers a precise relic-density calculation for Kaluza-Klein dark matter in Universal Extra Dimensions by incorporating coannihilations with all level-1 KK states and keeping full mass dependence. The authors implement MUED in CompHEP, use a temperature-dependent g_*(T) and relativistic corrections, and derive σ_eff with mass splittings Δ_i to quantify coannihilation effects. They show that Accounting for all coannihilations raises the preferred LKP mass to about 500–600 GeV in MUED and demonstrate how non-minimal mass spectra can further modify Ω h^2, sometimes allowing higher LKP masses. The results provide a comprehensive, generalizable framework for relic-density calculations in UED-like models and have direct implications for collider and astroparticle searches targeting KK dark matter.

Abstract

We revisit the calculation of the relic density of the lightest Kaluza-Klein particle (LKP) in the model of Universal Extra Dimensions. The Kaluza-Klein (KK) particle spectrum at level one is rather degenerate, and various coannihilation processes may be relevant. We extend the calculation of hep-ph/0206071 to include coannihilation processes with all level one KK particles. In our computation we consider a most general KK particle spectrum, without any simplifying assumptions. In particular, we do not assume a completely degenerate KK spectrum and instead retain the dependence on each individual KK mass. As an application of our results, we calculate the Kaluza-Klein relic density in the Minimal UED model, turning on coannihilations with all level one KK particles. We then go beyond the minimal model and discuss the size of the coannihilation effects separately for each class of level 1 KK particles. Our results provide the basis for consistent relic density computations in arbitrarily general models with Universal Extra Dimenions.