  In the universal extra dimensions (UED) scenario, the tree level masses of
the first level Kaluza-Klein (KK) excitations of Standard Model particles are
essentially degenerate. Radiative corrections will, however, lift this
degeneracy, allowing the first level excitations to decay to the lightest KK
particle (LKP), which is the gamma^*. KK number conservation implies that the
LKP is stable. Then, since the SM particles radiated during these decays are
rather soft, the observation of KK excitations production and decay in collider
experiments will be quite difficult. We propose to add to this model KK number
violating interactions mediated by gravity, which allow the gamma^* to decay to
a photon and a KK graviton. For a variety a models and a large range of
parameters, these decays will occur within the detector. Thus, pair production
of KK excitations will give rise to a striking collider signal, consisting of
two hard photons plus large missing energy (due to escaping gravitons). We
evaluate the cross-section for these signals at the Tevatron and LHC, and
derive the reach of these colliders in the search for universal extra
dimensions.
