Ultraheavy resonances at the LHC: beyond the QCD background

We study the theory and some experimental hints of ultraheavy resonances at the LHC. The production of an ultraheavy narrow particle may have a larger rate than the QCD background even when the final state includes only hadronic jets. We consider two classes of models that lead to 4-jet signals. In the first class a diquark scalar decays into two vectorlike quarks. In the second one a coloron decays into two color-octet scalars or into a pair of vectorlike quarks. We show that a diquark as heavy as 11.5 TeV, or a coloron as heavy as 8.5 TeV may be discovered at the LHC. We point out that a CMS 4-jet event may be due to an 8 TeV resonance decaying into two secondary particles, each with a mass of 1.8 TeV. We find that the QCD background with a 4-jet mass and dijet masses that equal or exceed those of the CMS event is approximately $5\times 10^{-5}$ events in 78 fb$^{-1}$ of data, while the diquark signal could have easily produced that event. The diquark also decays directly into two jets, which may be the origin of some of the three other events of mass near 8 TeV observed by ATLAS and CMS.