This paper presents a search for new heavy particles decaying into a pair of top quarks using 139 fb$^{-1}$ of proton--proton collision data recorded at a centre-of-mass energy of $\sqrt{s}=13$ TeV with the ATLAS detector at the Large Hadron Collider. The search is performed using events consistent with pair production of high-transverse-momentum top quarks and their subsequent decays into the fully hadronic final states. The analysis is optimized for resonances decaying into a $t\bar{t}$ pair with mass above 1.4 TeV, exploiting a dedicated multivariate technique with jet substructure to identify hadronically decaying top quarks using large-radius jets and evaluating the background expectation from data. No significant deviation from the background prediction is observed. Limits are set on the production cross-section times branching fraction for the new $Z'$ boson in a topcolor-assisted-technicolor model. The $Z'$ boson masses below 3.9 and 4.7 TeV are excluded at 95% confidence level for the decay widths of 1% and 3%, respectively.
Acceptance and acceptance times selection efficiency as a function of $m^{gen}_{t\bar{t}}$ in SR$1b$. The acceptance is measured as the fraction of events with two leading truth-contained large-$R$ jets, both satisfying the kinematic requirements, but not containing generator-level electrons or muons, as described in the paper. The acceptance $\times$ efficiency is calculated with respect to the full analysis selections including top- and $b$-tagging requirements on the two leading large-$R$ jets. The $m^{gen}_{t\bar{t}}$ is calculated from the momenta of top and anti-top quarks at the generator level before final-state radiation. The branching fractions of the $t \bar{t}$ into all possible final states are included in the acceptance calculation.
Acceptance and acceptance times selection efficiency as a function of $m^{gen}_{t\bar{t}}$ in SR$2b$. The acceptance is measured as the fraction of events with two leading truth-contained large-$R$ jets, both satisfying the kinematic requirements, but not containing generator-level electrons or muons, as described in the paper. The acceptance $\times$ efficiency is calculated with respect to the full analysis selections including top- and $b$-tagging requirements on the two leading large-$R$ jets. The $m^{gen}_{t\bar{t}}$ is calculated from the momenta of top and anti-top quarks at the generator level before final-state radiation. The branching fractions of the $t \bar{t}$ into all possible final states are included in the acceptance calculation.
Observed $m_{t\bar{t}}^{reco}$ distributions in data for SR$1b$, shown together with the result of the fit with the three-shape-parameter function. The error bars indicate the effect of the fit parameter uncertainty on the background prediction. The bin width of the distributions is chosen to be the same as that used in the background parameterization.
A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of $\sqrt{s} = $13 TeV corresponding to an integrated luminosity of 36.1 fb$^{-1}$. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing $b$-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical $Z'$ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross-sections, the $Z'$ boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1$-$3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 TeV to 0.9 TeV and from 2.0 TeV to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.
Acceptance times selection efficiency for topcolor-assisted-technicolor Z$^{\prime}_{TC2}$ as a function of top-quark pair mass for all regions A–D in the resolved analysis and the combination of all SRs in the boosted analysis.
Acceptance times selection efficiency for Kaluza-Klein graviton as a function of top-quark pair mass for all regions A–D in the resolved analysis and the combination of all SRs in the boosted analysis.
Acceptance times selection efficiency for Kaluza-Klein gluon Γ=30% as a function of top-quark pair mass for all regions A–D in the resolved analysis and the combination of all SRs in the boosted analysis.
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