Direct bound on the minimal Universal Extra Dimension model from the $t\bar{t}$ resonance search at the Tevatron

Abstract

In the minimal Universal Extra Dimension (mUED) model, the second Kaluza-Klein (KK) gluon $g^{(2)}$ has loop-induced vertices with the standard model quarks, mediated by the first KK modes of the quark and the gluon/Higgs boson. With a top quark pair, this vertex is enhanced by the cooperation of the strong coupling of a gluon and the large Yukawa coupling of a top quark, leading to substantial branching ratio of $Br(g^{(2)} \to t \bar{t}) \approx 7%-8%$. As the $g^{(2)}$ coupling with two gluons appears via dimension-6 operator, $q\bar{q}\to g^{(2)}\to t\bar{t}$ is the golden mode for the mUED model. Hence the best channel is the $t\bar{t}$ resonance search in $p\bar{p}$ collisions. The recent Tevatron data at $\sqrt{s}=1.96$ TeV with an integrated luminosity of 8.7 fb$^{-1}$ are shown to give the first direct bound on the $g^{(2)}$ mass above 800 GeV. The implication and future prospect at the LHC are discussed also.