We consider the universal extra dimensions scenario of Appelquist, Cheng, and Dobrescu, in which all of the SM fields propagate into one extra compact dimension, estimated therein to be as large as ∼ (350 GeV) −1 . Tree-level KK number conservation dictates that the associated KK excitations can not be singly produced. We calculate the cross sections for the direct production of KK excitations of the gluon, g n , and two distinct towers of quarks, q • n and q • n , in proton-antiproton collisions at the Tevatron Run I and II energies in addition to proton-proton collisions at the Large Hadron Collider energy. The experimental signatures for these processes depend on the stability of the lowest-lying KK excitations of the gluons and light quarks. We find that the Tevatron Run I mass bound for KK quark and gluon final states is about 350-400 GeV, while Run II can push this up to 450-500 GeV at its initial luminosity and 500-550 GeV if the projected final luminosity is reached. The LHC can probe much further: The LHC will either discover UED KK excitations of quarks and gluons or extend the mass limit to about 3 TeV. *

We consider an asymmetric string compactification scenario in which the SM gauge bosons can propagate into one TeV −1 -size extra compact dimension. These gauge bosons have associated KK excitations that present additional contributions to the SM processes. We calculate the effects that the KK excitations of the gluons, g ⋆ 's, have on multijet final state production in proton-proton collisions at the Large Hadron Collider energy. In the case of dijet final states with very high p T , the KK signal due to the exchanges of the g ⋆ 's is several factors greater than the SM background for compactification scales as high as about 7 TeV. The high-p T effect is not as dramatic for the direct production of a single on-shell g ⋆ , which subsequently decays into q-q pairs, where the KK signal significantly exceeds the SM three-jet background for compactification scales up to about 3 TeV. We also present our results for the four-jet final state signal from the direct production of two on-shell g ⋆ 's. *

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 γ * . 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 γ * 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.

We consider in detail the possibility that the Higgs is wholly or partially excluded from propagating into one otherwise universal extra dimension. This exclusion of the Higgs from propagating into an otherwise universal extra dimension violates treelevel KaluzaKlein number conservation in the Yukawa interactions. As a consequence, there is intermode mixing between fermions. For example, zero mode fermions mix with their associated KaluzaKlein excitations. This is in contrast to the original universal extra dimensions scenario, in which conservation of KaluzaKlein number prohibits such inter mode mixing. Intermode mixing is especially significant for the top quark, since its mass (171.4 ± 2.1 GeV) is approximately onehalf the current Tevatron mass bound (~350400 GeV) for KaluzaKlein excitations of quarks propagating into universal extra dimensions. We compute the effects that mixing among the zeromodes and lowestlying KaluzaKlein excitations has on the lightest thirdgeneration charge 2/3 quark mass eigenvalue in the nonuniversal Higgs model with one otherwise universal extra dimension. Another consequence of the intermode mixing is that the KaluzaKlein excitations of the fermions can decay to a zeromode Higgs and a corresponding zeromode fermion. As a result, the pair production of KaluzaKlein excitations of the top quark would lead to two zeromode Higgs bosons plus a zeromode top quark/antiquark pair. We compute the cross section that the nonuniversal Higgs model contributes to Higgs production at the Large Hadron Collider. The effect is quite large: For example, the KaluzaKlein contribution to Higgs production is comparable to or larger than the Standard Model contribution, depending on the Higgs mass, for compactification scales up to about 600 GeV.

We consider multiple TeV -1 -size extra compact dimensions in an asymmetric string compactification scenario in which the SM gauge bosons can propagate into the TeV -1 -size extra dimensions while the SM fermions are confined to the usual SM D 3 -brane. We calculate the contributions that the KK excitations of the gluons, g*'s, make to the multijet cross sections in proton-proton collisions at the LHC energy. At very high p T , the dijet signal will either be enhanced significantly due to virtual g* exchanges or place a lower bound on the compactification scale of about 8 TeV. We find that the dijet signal is very sensitive to three parameters -the compactification scale, the string scale, and the number of extra dimensions. Thus, although the dijet signal is much more sensitive to KK effects, the dijet signal alone does not provide sufficient information to deduce the number of extra dimensions nor the compactification scale. However, the three-jet signal, which is not sensitive to the string scale, can be analyzed in conjunction with the dijet signal to extract all three parameters. For proton-proton collisions at the LHC energy, the three-jet signal can be significantly enhanced by KK contributions for a compactification scale of about 4-5 TeV.