Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector

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 $\sqrt{s}=13\mathrm{TeV}$ corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$. 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 $Z^{\prime }$ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the $Z^{\prime }$ 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 to 0.9 TeV and from 2.0 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.

Figure 1

Representative Feynman diagrams for leading-order production in the selected signal models: (a) $Z^{\prime }$, (b) $g_{\mathrm{KK}}$ and (c) $G_{\mathrm{KK}}$. The details of each signal model are described in the text.

Figure 2

Schematic diagram of the event categorization in the boosted analysis. (a) Events selected in the boosted analysis are classified into nine categories based on the number of tight $b$-tagged jets ($n_b$) and $L_{{\tau }_{32}}$, i.e., Loose, Medium and Tight regions for $n_b=0$, 1 and 2. At least two loose $b$-tagged jets are already required in the preselection. The region $0.35\le L_{{\tau }_{32}}<1.0$ is referred to as Inclusive. (b) In each category, events are further classified into three regions, R1, R2 and CR1–4, according to the leading and sub-leading large-$R$ jet masses.

Figure 3

Comparison between data and predicted background after the fit (“Post-Fit”) in events satisfying the criteria for the Tight $L_{{\tau }_{32}}$ requirement and one (a),(c) or two (b),(d) $b$-tagged jets in the boosted analysis. Shown are (a),(b) the mass of the leading reconstructed top-quark candidate, and (c),(d) the mass of the sub-leading reconstructed top-quark candidate. The background components are shown as stacked histograms and the shaded areas around the histograms indicate the total systematic uncertainties after the fit. The lower panel of the distribution shows the ratio of data to the background prediction. The multijet contribution also contains all other small non-$t\overline{t}$ backgrounds.

Figure 4

Normalized $m_{t\overline{t}}^{\mathrm{reco}}$ distributions for simulated signal samples of (a) $pp\to Z_{\mathrm{TC}2}^{\prime }\to t\overline{t}$, (b) $pp\to G_{\mathrm{KK}}\to t\overline{t}$ and (c) $pp\to g_{\mathrm{KK}}\to t\overline{t}$. The benchmark signals with masses of 0.75, 1 or 1.5 TeV reconstructed in region D of the resolved analysis, and with masses of 2 and 3 TeV reconstructed in the $\mathrm{R}1(\text{T}\text{ight},2b)$ region of the boosted analysis are shown. The 3 TeV $g_{\mathrm{KK}}$ signal has a broader $m_{t\overline{t}}^{\mathrm{reco}}$ distribution without an apparent peak at the generated mass because the $g_{\mathrm{KK}}$ signal is much wider than other signals and the lower mass region is further enhanced by the parton luminosity effect.

Figure 5

Acceptance times selection efficiency as a function of $m_{t\overline{t}}$ for all regions A–D in the resolved analysis and the combination of all SRs in the boosted analysis. The momenta of top and antitop quarks evaluated at the generator level before final state radiation are used to define $m_{t\overline{t}}$. The efficiency calculation includes the branching fractions of the $t\overline{t}$ system into all possible final states. (a) is $Z_{\mathrm{TC}2}^{\prime }$, (b) is $G_{\mathrm{KK}}$, and (c) is $g_{\mathrm{KK}}$.

Figure 6

Observed $m_{t\overline{t}}^{\mathrm{reco}}$ distributions in the multijet-enriched regions (a) A, (b) B, (c) C and (d) the main signal region D after the fit (“Post-Fit”) under the background-only hypothesis for the resolved analysis. The shaded areas around the histograms indicate the total uncertainties in the background. The lower panel of the distribution shows the ratio of data to the fitted background prediction. The distributions before the fit are shown by the dashed lines and the background components are shown as stacked histograms. The multijet contribution also contains all other small non-$t\overline{t}$ backgrounds.

Figure 7

Observed $m_{t\overline{t}}^{\mathrm{reco}}$ distributions in (a) Medium R1 1b (b) Medium R1 2b (c) Tight R1 1b and (d) Tight R1 2b after the fit (“Post-Fit”) under the background-only hypothesis for the boosted analysis. The shaded areas around the histograms indicate the total uncertainties in the background. The lower panel of the distribution shows the ratio of data to the fitted background prediction. The open triangles indicate that the ratio values are outside the plotted range. The distributions before the fit are shown by the dashed lines and the background components are shown as stacked histograms. The multijet contribution also contains all other small non-$t\overline{t}$ backgrounds.

Figure 8

Observed $m_{t\overline{t}}^{\mathrm{reco}}$ distributions in (a) Medium R2 1b (b) Medium R2 2b (c) Tight R2 1b and (d) Tight R2 2b after the fit (“Post-Fit”) under the background-only hypothesis for the boosted analysis. The shaded areas around the histograms indicate the total uncertainties in the background. The lower panel of the distribution shows the ratio of data to the fitted background prediction. The open triangles indicate that the ratio values are outside the plotted range. The distributions before the fit are shown by the dashed lines and the background components are shown as stacked histograms. The multijet contribution also contains all other small non-$t\overline{t}$ backgrounds.

Figure 9

Observed and expected upper limits on the cross-section times branching fraction of $Z_{\mathrm{TC}2}^{\prime }$ decaying into $t\overline{t}$ as a function of the $Z_{\mathrm{TC}2}^{\prime }$ mass. The theory predictions of the cross sections for the $Z_{\mathrm{TC}2}^{\prime }$ with $\mathrm{\Gamma }=1\%$ and 3% are shown by the dotted and dashed lines at NLO and by the solid line with $\mathrm{\Gamma }=1.2\%$ at LO, respectively. The results from the resolved and boosted analyses are shown to the left and right of the vertical dashed line, respectively.

Figure 10

Observed and expected 95% C.L. upper limits on the cross-section times branching fraction of $Z_{\mathrm{med}}^{\prime }$ decaying into $t\overline{t}$ as a function of the $Z_{\mathrm{med}}^{\prime }$ mass. The theoretical predictions of the cross sections for the $Z_{\mathrm{med}}^{\prime }$ in the (a) A1 axial-vector mediator and (b) V1 vector mediator scenarios of the benchmark DM models are shown by the solid lines. The resolved and boosted analyses are shown to the left and right of the vertical dashed line, respectively.

Figure 11

Observed and expected 95% C.L. upper limits on the cross-section times branching fraction of $G_{\mathrm{KK}}$ decaying into $t\overline{t}$ as a function of the $G_{\mathrm{KK}}$ mass. The theoretical prediction of the cross section for the $G_{\mathrm{KK}}$ in the bulk RS model with $k/{\overline{M}}_{\mathrm{Pl}}=1.0$ is shown by the solid line. The resolved and boosted analyses are shown to the left and right of the vertical dashed line, respectively.

Figure 12

Observed and expected 95% C.L. upper limits on the cross-section times branching fraction of $g_{\mathrm{KK}}$ decaying into $t\overline{t}$ as a function of the $g_{\mathrm{KK}}$ mass with $\mathrm{\Gamma }=30\%$. The theoretical prediction of the cross section for the $g_{\mathrm{KK}}$ in the RS model with a single warped extra dimension is shown by the solid line. The resolved and boosted analyses are shown to the left and right of the vertical dashed line, respectively.

Figure 13

Observed and expected 95% C.L. upper limits on cross-section times branching fraction of $g_{\mathrm{KK}}$ decaying into $t\overline{t}$ as a function of the width of $g_{\mathrm{KK}}$ for masses of (a) 0.5 TeV and (b) 1 TeV (using the resolved analysis) and (c) 1.5 TeV (d) 2.0 TeV and (e) 5.0 TeV (using the boosted analysis). The width refers to the decay width of a resonance divided by the resonance mass.