We analyze collider signatures of massive color-octet bosons whose couplings to quarks are suppressed. Gauge invariance forces the octets to couple at tree level only in pairs to gluons, with a strength set by the QCD gauge coupling. For a spin-1 oct
et, the cross section for pair production at hadron colliders is larger than that for a quark of equal mass. The octet decays into two jets, leading to a 4-jet signature with two pairs of jets forming resonances of the same mass. For a spin-0 octet the cross section is smaller, and the dominant decay is into bbar{b}, or tbar{t} if kinematically allowed. We estimate that discovery of spin-1 octets is possible for masses up to 330 GeV at the Tevatron, and 1 TeV at the LHC with 1 fb^{-1}, while the reach is somewhat lower for spin-0 octets.
We show that a top-prime quark as heavy as 600 GeV can be discovered at the Tevatron, provided it is resonantly pair-produced via a vector color octet. If the top-prime originates from a vectorlike quark, then the production of a single top-prime in
association with a top may also be observable, even through its decay into a Higgs boson and a top. A color octet with mass of about 1 TeV, which decays into a top-prime pair, may account for the CDF excess of semileptonic (Wj)(Wj) events.
We study methods of extracting new physics signals in final states with a top-quark pair plus large missing energy at the LHC. We consider two typical examples of such new physics: pair production of a fermionic top partner (a $T$ in Little Higgs mod
els for example) and of a scalar top partner (a $tilde{t}$ in SUSY). With a commonly-adopted discrete symmetry under which non Standard Model particles are odd, the top partner is assumed to decay predominantly to a top quark plus a massive neutral stable particle $A^0$. We focus on the case in which one of the top quarks decays leptonically and the other decays hadronically, $pp to {tt} A^0A^0 X to bj_1j_2 bar bell^- bar u A^0A^0 X + c.c.$, where the $A^0$s escape detection. We identify a key parameter for the signal observation: the mass splitting between the top partner and the missing particle. We reconstruct a transverse mass for the lepton-missing transverse energy system to separate the real $W$ background from the signal and propose a definition for the reconstructed top quark mass that allows it to take unphysical values as an indication of new physics. We perform a scan over the two masses to map out the discovery reach at the LHC in this channel. We also comment on the possibility of distinguishing between scalar and fermionic top partners using collider signatures.
