Search for heavy particles decaying into top-quark pairs using lepton-plus-jets events in proton-proton collisions at s = 13 TeV with the ATLAS detector

Abstract

A search for new heavy particles that decay into top-quark pairs is performed using data collected from proton-proton collisions at a centre-of-mass energy of 13 $\text{TeV}$ by the ATLAS detector at the Large Hadron Collider. The integrated luminosity of the data sample is 36.1 fb ${}^{-1}$ . Events consistent with top-quark pair production are selected by requiring a single isolated charged lepton, missing transverse momentum and jet activity compatible with a hadronic top-quark decay. Jets identified as likely to contain b-hadrons are required to reduce the background from other Standard Model processes. The invariant mass spectrum of the candidate top-quark pairs is examined for local excesses above the background expectation. No significant deviations from the Standard Model predictions are found. Exclusion limits are set on the production cross-section times branching ratio for hypothetical $Z^{\text{'}}$ bosons, Kaluza-Kein gluons and Kaluza-Klein gravitons that decay into top-quark pairs.

Fig. 1

Leading-order Feynman diagrams for the signal processes studied in this search. The $Z^{\prime }$ (a) and Kaluza–Klein gluons ($g_{\mathit{KK}}$) have spin 1 (b), while the Kaluza–Klein graviton ($G_{\mathit{KK}}$) has spin 2 (c).

Fig. 2

Acceptance times efficiency ($A\times ϵ$), including the branching ratio for MC simulated BSM particles decaying into $t\overline{t}$, as a function of the $t\overline{t}$ invariant mass $m_{t\overline{t}}$ (computed before parton radiation) for simulated signal events. The signal samples shown here include events from generated masses ranging from 0.4 to 5 $\text{TeV}$. All $t\overline{t}$ decay modes are simulated. The e and $\mu$ channel efficiencies are combined

Fig. 3

Reconstructed top-quark pairs invariant mass, $m_{t\overline{t}}^{\text{reco}}$, for simulated signal events satisfying the selection criteria. The $Z^{\prime }$ in the simulated samples used here has a width of 3% of its mass. The $g_{\mathrm{KK}}$ shown here has a width of 30% of its mass and the width of the $G_{\mathrm{KK}}$ width varies between 3 and 6% of its mass. The figure shows the distribution including events that may satisfy both the boosted and resolved selections in the line marked as “before boosted-veto”. The line marked as “after boosted-veto” excludes events which satisfy both the boosted and resolved selections from the resolved selection

Fig. 4

The distribution of the transverse momentum of the lepton in the a boosted e+jets, b boosted $\mu$+jets, c resolved e+jets, and d resolved $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 5

The distribution of the $E_{\text{T}}^{\text{miss}}$ in the a boosted e+jets, b boosted $\mu$+jets, c resolved e+jets, and d resolved $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 6

The distribution of the transverse momentum of the hardest small-R jet with $\mathrm{\Delta }R(\ell ,\text{jet})<1.5$ in the a boosted e+jets, and b boosted $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 7

The distribution of the transverse momentum of the large-R jet in the a boosted e+jets, and b boosted $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 8

The distribution of the reconstructed mass of the leptonic-top candidate in the a boosted e+jets, b boosted $\mu$+jets, c resolved e+jets, and d resolved $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 9

The distribution of the mass of the large-R jet in the a boosted e+jets, and b boosted $\mu$+jets selections. The mass of the hadronic-top candidate in the c resolved e+jets, and d resolved $\mu$+jets selections. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The lower panels in each plot show the ratio of data (points) and a signal example (line) to the background expectation

Fig. 10

The $m_{t\overline{t}}^{\text{reco}}$ distribution before the likelihood fit in the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the total expectation from background processes is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel

Fig. 11

The $m_{t\overline{t}}^{\text{reco}}$ distribution before the likelihood fit in the resolved selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the total expectation from background processes is shown in the lower panel

Fig. 12

The $m_{t\overline{t}}^{\text{reco}}$ distributions, after a likelihood fit under the background-only hypothesis, for the boosted selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the final fitted expectation is shown in the lower panel, open triangles indicate that the ratio point would appear outside the panel. a e+jets b-tag category 1. b $\mu$+jets b-tag category 1. c e+jets b-tag category. 2 d $\mu$+jets b-tag category 2. e e+jets b-tag category 3. f $\mu$+jets b-tag category 3

Fig. 13

The $m_{t\overline{t}}^{\text{reco}}$ distributions, after a likelihood fit under the background-only hypothesis, for the resolved selection. The SM background components are shown as stacked histograms. The shaded areas indicate the total systematic uncertainties. The ratio of the data to the final fitted expectation is shown in the lower panel. a e+jets b-tag category 1. b $\mu$+jets b-tag category 1. c e+jets b-tag category 2. d $\mu$+jets b-tag category 2. e e+jets b-tag category 3. f $\mu$+jets b-tag category 3

Fig. 14

The observed and expected cross-section 95% CL upper limits on the $Z_{\text{TC2}}^{\prime }$ signal. The theoretical predictions for the production cross-section times branching ratio of $Z_{\text{TC2}}^{\prime }\to t\overline{t}$ at the corresponding masses are also shown

Fig. 15

The observed and expected cross-section 95% CL upper limits on the a $Z_{\text{DM,ax}}^{\prime }$ and b $Z_{\text{DM,vec}}^{\prime }$ signals. The theoretical predictions for the production cross-section times branching ratio of $Z_{\text{DM}}^{\prime }\to t\overline{t}$ at the corresponding masses are also shown

Fig. 16

The observed and expected cross-section 95% CL upper limits on the $G_{\text{KK}}$ signal. The theoretical predictions for the production cross-section times branching ratio of $G_{\text{KK}}\to t\overline{t}$ at the corresponding masses are also shown

Fig. 17

The observed and expected cross-section 95% CL upper limits on the $g_{\text{KK}}$ signal for resonance widths of a 30% and b 15%. The theoretical predictions for the production cross-section times branching ratio of $g_{\text{KK}}\to t\overline{t}$ at the corresponding masses are also shown

Fig. 18

The observed and expected cross-section 95% CL upper limits on the $g_{\text{KK}}$ signal as a function of the resonance width for masses of a 1 $\text{TeV}$, and b 5 $\text{TeV}$