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Universal extra dimension models with Kaluza-Klein parity provide usexcellent candidates for dark matter. We consider phenomenological universalextra dimension models where the Kaluza-Klein (KK) mass spectrum is differentfrom that of the minimal universal extra dimension model, and compute thethermal relic abundance of the first KK mode of the photon taking into accountthe production of second KK particles. It is pointed out that its thermal relicabundance depends significantly on the mass degeneracy between the KK-photonand other KK particles because of considerable coannihilation effects. Thecosmologically favored compactification scale is shown to range from around 1TeV to a few TeV even in the cases where one of the first KK particles istightly degenerate with the first KK photon in mass.

Thermal relic abundance of the lightest Kaluza-Klein particle in phenomenological universal extra dimension models