A search for ttbar resonances in lepton+jets events with highly boosted top quarks collected in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This ATLAS study searches for resonant tt̄ production in 2.05 fb^-1 of 7 TeV pp collisions using a boosted-top strategy in the lepton+jets final state. By reconstructing the hadronic top as a fat jet with substructure criteria and combining it with a semileptonic top, the analysis forms the tt̄ invariant mass spectrum to look for bumps from new states. No significant deviation from the Standard Model is observed, and 95% CL limits are set on σ×BR for narrow Z' and broad KK gluon resonances, excluding Z' masses 0.6–1.15 TeV and KK gluons below 1.5 TeV; the boosted-top approach provides improved sensitivity in the 1–2 TeV region relative to prior results. Overall, the results constrain extensions of the SM that predict high-mass tt̄ resonances and demonstrate the efficacy of boosted-object reconstruction at the LHC.

Abstract

A search for resonant production of high-mass top-quark pairs is performed on 2.05 fb^-1 of proton-proton collisions at sqrt(s) = 7 TeV collected in 2011 with the ATLAS experiment at the Large Hadron Collider. This analysis of the lepton+jets final state is specifically designed for the particular topology that arises from the decay of highly boosted top quarks. The observed ttbar invariant mass spectrum is found to be compatible with the Standard Model prediction and 95% credibility level upper limits are derived on the ttbar production rate through new massive states. An upper limit of 0.7 pb is set on the production cross section times branching fraction of a narrow 1 TeV resonance. A Kaluza-Klein gluon with a mass smaller than 1.5 TeV is excluded.

Figures (7)

• Figure 1: Event display for a $t \bar{t}$ candidate event with large $t \bar{t}$ invariant mass: $m_{t\bar{t}}=2.5$Te V. The left panel displays a transverse view of the charged particle tracks and calorimeter energy deposits. An $\eta-\phi$ view of the same event is shown in the upper right panel. Jets reconstructed with $R= 0.4$ are indicated in green, jets with $R= 1.0$ in red (colour online).
• Figure 2: Estimate from Monte Carlo simulation of the selection efficiency for the leptophobic $Z^\prime$ benchmark model. Only events with the targeted final state are considered ($t \bar{t} \rightarrow W^+bW^{-}\bar{b} \rightarrow \ell \nu_{\ell} b \bar{b} j j$, where $\ell$ is either an electron or a muon, corresponding to approximately 30% of $t\bar{t}$ events). The error bars shown correspond to the Monte Carlo statistical uncertainty.
• Figure 3: Estimate from Monte Carlo simulation of the reconstructed $t \bar{t}$ invariant mass distribution for the leptophobic $Z^\prime$. The natural width of the resonance is small ($\Gamma/m = 1.2$%) compared to the experimental mass resolution. For the highest mass points a considerable tail towards smaller mass appears due to the convolution with the rapidly falling parton luminosity.
• Figure 4: The reconstructed $t \bar{t}$ invariant mass distribution of candidate events in the $W$+jets enriched control region. The $e$+jets and $\mu$+jets channels are combined. The scale factors derived in this section have been applied to the $W$+jets Monte Carlo distribution. The hatched area indicates the normalization uncertainty on the Standard Model prediction, but the shape uncertainty is not included.
• Figure 5: Comparison of the data and the Standard Model prediction for two kinematic distributions: (a) transverse momentum and (b) jet mass of the fat $R= 1.0$ jets selected as the hadronically decaying top quark candidate. The $e$+jets and $\mu$+jets channels are combined. The shaded band indicates the normalization uncertainty on the Standard Model prediction, but does not include the shape uncertainty or the impact of uncertainties on reconstructed objects.