Collider implications of universal extra dimensions

We consider the universal extra dimensions scenario of Appelquist, Cheng, and Dobrescu, in which all of the standard model fields propagate into one extra compact dimension, estimated therein to be as large as $\ensuremath{\sim}(350 \mathrm{GeV}{)}^{\ensuremath{-}1}.$ Tree-level Klauza-Klein (KK) number conservation dictates that the associated KK excitations cannot be singly produced. We calculate the cross sections for the direct production of KK excitations of the gluon, ${g}_{n}^{\ensuremath{\star}},$ and two distinct towers of quarks, ${q}_{n}^{\ifmmode\bullet\else\textbullet\fi{}}$ and ${q}_{n}\ifmmode^\circ\else\textdegree\fi{},$ in proton-antiproton collisions at the Fermilab Tevatron run I and II energies in addition to proton-proton collisions at the CERN 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 $350\char21{}400 \mathrm{GeV},$ while run II can push this up to $450\char21{}500 \mathrm{GeV}$ at its initial luminosity and $500\char21{}550 \mathrm{GeV}$ if the projected final luminosity is reached. The LHC can probe much further: The LHC will either discover universal extra dimension (UED) KK excitations of quarks and gluons or extend the mass limit to about 3 TeV.