Search for resonances decaying into top-quark pairs using fully hadronic decays in pp collisions with ATLAS at sqrt(s) = 7 TeV

TL;DR

To search for resonances decaying to top-quark pairs in the fully hadronic channel, the ATLAS collaboration employed two jet-substructure taggers (HEPTopTagger and Top Template Tagger) to identify boosted hadronic top decays in 7 TeV pp collisions with 4.7 fb^-1 of data. The analysis relies on data-driven background estimates for multijet processes and MC-based predictions for SM ttbar, then compares reconstructed ttbar mass spectra to set 95% CL limits on Z' and KK gluon resonances. No significant excess was observed; the results exclude Z' masses in the ranges 0.70–1.00 TeV and 1.28–1.32 TeV, and KK gluons in 0.70–1.62 TeV, with complementary coverage from the two tagging strategies. These results extend previous ATLAS limits obtained in the lepton+jets channel and illustrate the power of jet substructure techniques for fully hadronic heavy resonance searches.

Abstract

A search for resonances produced in 7 TeV proton-proton collisions and decaying into top-quark pairs is described. In this Letter events where the top-quark decay produces two massive jets with large transverse momenta recorded with the ATLAS detector at the Large Hadron Collider are considered. Two techniques that rely on jet substructure are used to separate top-quark jets from those arising from light quarks and gluons. In addition, each massive jet is required to have evidence of an associated bottom-quark decay. The data are consistent with the Standard Model, and limits can be set on the production cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon resonance. These limits exclude, at the 95% credibility level, Z' bosons with masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with masses 0.70-1.62 TeV.

Figures (15)

• Figure 1: Distributions of (a) mean HEPTopTagger top-quark candidate mass and (b) mean reconstructed $t\bar{t}$ mass as a function of the average number of interactions per bunch-crossing, $\langle \mu \rangle$, for data and simulated $t\bar{t}$ events with the full selection applied. Only statistical uncertainties are shown.
• Figure 2: Distributions of the reconstructed $t\bar{t}$ mass predicted by MC simulations for (a) $Z^\prime$ boson and (b) KK gluon benchmark models with various mass values for the HEPTopTagger analysis with the full selection applied. For each model, $\sigma(pp \rightarrow Z^\prime/{\rm KK\:gluon}) \times BR(Z^\prime/{\rm KK\:gluon} \rightarrow t\bar{t})$ is fixed to $1\:\text{pb}$ and an integrated luminosity of $4.7\:\text{fb}^{-1}$ is assumed.
• Figure 3: The $OV_3$ distributions for the leading jets in the 2 TeV $Z^\prime \rightarrow t\bar{t}$ MC sample, a multijet-dominated 2011 data sample, and the multijet MC sample. The data and multijet MC distributions are from the samples prior to making any $b$-tagging or jet mass requirements on either jet, and so are dominated by light quark/gluon jets.
• Figure 4: Pile-up-corrected jet mass distribution in the multijet and $t\bar{t}$ MC samples for (a) the leading and (b) recoil jets. In both cases, the jet mass requirement has been applied on the opposing jet in the event. The distributions are independently normalised to unit area.
• Figure 5: The jet mass distributions for the leading (a) and for the recoil (b) jet when all other requirements have been made on the sample except the mass and $OV_3$ requirements on the jet being considered. The fits are described in the text.