In models with universal extra dimensions ( i.e . in which all Standard Model fields , including fermions , propagate into compact extra dimensions ) momentum conservation in the extra dimensions leads to the conservation of Kaluza–Klein ( KK ) number at each vertex .
KK number is violated by loop effects because of the orbifold imposed to reproduce the chiral Standard Model with zero modes , however , a KK parity remains at any order in perturbation theory which leads to the existence of a stable lightest KK particle ( LKP ) .
In addition , the degeneracy in the KK spectrum is lifted by radiative corrections so that all other KK particles eventually decay into the LKP .
We investigate cases where the Standard Model lives in five or six dimensions with compactification radius of TeV ^ { -1 } size and the LKP is the first massive state in the KK tower of either the photon or the neutrino .
We derive the relic density of the LKP under a variety of assumptions about the spectrum of first tier KK modes .
We find that both the KK photon and the KK neutrino , with masses at the TeV scale , may have appropriate annihilation cross sections to account for the dark matter , \Omega _ { M } \sim 0.3 .