Solving cosmological problem in universal extra dimension models by introducing Dirac neutrino

Universal extra dimension (UED) models with right-handed neutrinos are studied. The introduction of the neutrinos makes us possible not only to describe Dirac neutrino masses but also to solve the cosmological problem called the KK graviton problem. This problem is essentially caused by the late time decay of a KK photon into a KK graviton and a photon, and it distorts the spectrum of the cosmic microwave background or the diffuse photon. We point out that, once we introduce right-handed neutrinos to UED models, the KK photon decays dominantly into neutrinos and does not emit a photon. We also discuss sub-dominant modes with a photon in the decay quantitatively, and show that their branching ratios are so small that the spectra are not distorted.