Boosted Top Quark Signals for Heavy Vector Boson Excitations in a Universal Extra Dimension Model

Abstract

In view of the fact that the $n = 1$ Kaluza-Klein (KK) modes in a model with a Universal Extra Dimension (UED), could mimic supersymmetry signatures at the LHC, it is necessary to look for the $n = 2$ KK modes, which have no analogues in supersymmetry. We discuss the possibility of searching for heavy $n = 2$ vector boson resonances -- especially the $g_2$ -- through their decays to a highly-boosted top quark-antiquark pair using recently-developed top-jet tagging techniques in the hadronic channel. It is shown that $t\bar{t}$ signals from the $n = 2$ gluon resonance are as efficient a discovery mode at the LHC as dilepton channels from the $γ_2$ and $Z_2$ resonances.

Figures (8)

• Figure 1: One-loop diagrams giving rise to vertices of the form $q$--$\bar{q}$--$V_2$ in the UED-5 model. Here $q$ stands for any quark, and $V$ stands for any vector boson. If $V_2 = g_2$, the diagrams on the fourth row are absent.
• Figure 2: Couplings of the $g_2$ to ($a$) $u$ quarks and ($b$) $d$ quarks, as a function of the cutoff parameter $\Lambda R$. The width of the curves, indicated by hatching, indicates the weaker variation induced by changing $R^{-1}$, which acts through the changed running of $\alpha_s$. It is worth noting that the $g_2$ couplings to quark pairs are considerably smaller than the corresponding couplings of a normal gluon.
• Figure 3: Illustrating ($a$) the summed resonant cross section for $t\bar{t}$ production at the LHC through $V_2 = \gamma_2, Z_2$ or $g_2$, as a function of $R^{-1}$. Solid (dotted) lines in the lower half correspond to $\Lambda R$ = 20 (5) and the horizontal dotted line indicates the projected luminosity reach of the 7 TeV run at the LHC LHCwebsite. The QCD background is indicated by broken lines in the upper half. In ($b$), the partial contributions of $\gamma_2, Z_2$ and $g_2$ are shown, for $\sqrt{s} = 7$ TeV, where again solid (dotted) lines correspond to $\Lambda R$ = 20 (5). The dominance of the $g_2$ resonance is obvious.
• Figure 4: Illustrating the kinematics in the $\phi$--$\eta$ plane of hadronic final states in $t\bar{t}$ decay for $t$ and $\bar{t}$ with 'low $p_T$' $\sim 100$ GeV (extreme left), $t$ and $\bar{t}$ with 'high $p_T$' $\sim 1$ TeV (centre), and 'high $p_T$ (blown up)' (extreme right) i.e. the region in the little box enclosing the cluster (central box) plotted with high angular resolution. The size of the plotted points is indicative of the energy deposit expected in the calorimeter.
• Figure 5: Illustrating the efficiency fractions obtained in our implementation of the FastJet algorithm. The solid line indicates the fraction of $t$-quark jets which are identified as such, while the broken line indicates the fraction of light quark or gluon jets which are mistagged as $t$-jets.