Resonances from two universal extra dimensions

Standard model gauge bosons propagating in two universal extra dimensions give rise to heavy spin-1 and spin-0 particles. The lightest of these, carrying Kaluza-Klein numbers (1,0), may be produced only in pairs at colliders, whereas the (1,1) modes, which are heavier by a factor of $\sqrt{2}$, may be singly produced. We show that the cascade decays of (1,1) particles generate a series of closely-spaced narrow resonances in the $t\overline{t}$ invariant mass distribution. At the Tevatron, $s$-channel production of (1,1) gluons and electroweak bosons will be sensitive to $t\overline{t}$ resonances up to masses in the 0.5--0.7 TeV range. Searches at the LHC for resonances originating from several higher-level modes will further test the existence of two universal extra dimensions.