Search for four-top-quark production in the single-lepton and opposite-sign dilepton final states in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector

A search for four-top-quark production, $t\overline{t}t\overline{t}$, is presented. It is based on proton-proton collision data with a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider during the years 2015 and 2016, corresponding to an integrated luminosity of $36.1{\mathrm{fb}}^{-1}$. Data are analyzed in both the single-lepton and opposite-sign dilepton channels, characterized by the presence of one or two isolated electrons or muons with high-transverse momentum and multiple jets. A data-driven method is used to estimate the dominant background from top-quark pair production in association with jets. No significant excess above the Standard Model expectation is observed. The result is combined with the previous same-sign dilepton and multilepton searches carried out by the ATLAS Collaboration and an observed (expected) upper limit of 5.3 (2.1) times the four-top-quark Standard Model cross section is obtained at 95% confidence level. Additionally, an upper limit on the anomalous four-top-quark production cross section is set in the context of an effective field theory model.

Figure 1

Schematic view of the different analysis regions in (a) the single-lepton and (b) the dilepton channels. The three axes represent the jet multiplicity, the $b$-tagged jet multiplicity and the mass-tagged RCLR jet multiplicity. The efficiency extraction region in each channel is defined inclusively in the mass-tagged RCLR jet multiplicity.

Figure 2

(a), (b) The jet multiplicity and (c), (d) the $b$-jet multiplicity distributions after preselection for the total predicted background with the $t\overline{t}+\text{jets}$ background estimated via MC simulation (shaded histogram) and the signal scenarios considered in this search in the single-lepton (a), (c) and the dilepton (b), (d) channels. The signals shown correspond to four-top-quark production with SM kinematics (solid) and $t\overline{t}t\overline{t}$ production involving a four-fermion contact interaction (dashed). The distributions are normalized to unit area. The last bin in each distribution contains the overflow.

Figure 3

(a), (b) The mass-tagged RCLR jet multiplicity distributions and (c), (d) the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions after preselection for the total predicted background with the $t\overline{t}+\text{jets}$ background estimated via MC simulation (shaded histogram) and signals for the single-lepton (a), (c) and the dilepton (b), (d) channels. The signals shown correspond to four-top-quark production with SM kinematics (solid) and $t\overline{t}t\overline{t}$ production involving a four-fermion contact interaction (dashed). The distributions are normalized to unit area. The last bin in each distribution contains the overflow.

Figure 4

Values of the per-jet $b$-tagging probability for $t\overline{t}+\text{jets}$ events as a function of the jet transverse momentum ($p_{\mathrm{T}}^{\mathrm{jet}}$) and the minimum $\mathrm{\Delta }R$ between the considered jet and the other jets in the event, scaled by the jet multiplicity ($\mathrm{\Delta }{R}_{\min }^{\text{jet},\text{jet}}\times N^{\text{jet}}$), as measured in data requiring the presence of at least two $b$-jets (${ϵ}_b^{\ge 2b}$) and least three $b$-jets (${ϵ}_b^{\ge 3b}$). $b$-tagging probabilities are shown separately for single-lepton and dilepton events. The vertical error bars indicate the statistical uncertainties, while the shaded (hatched) areas indicate the combined statistical and systematic uncertainties. In the dilepton case, a constant $b$-tagging probability is assumed as a function of $\mathrm{\Delta }{R}_{\min }^{\text{jet},\text{jet}}\times N^{\text{jet}}$ for ${ϵ}_b^{\ge 3b}$.

Figure 5

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the single-lepton validation regions prior to the combined fit to data in the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal. An arrow indicates that the point is off scale.

Figure 6

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the dilepton validation regions prior to the combined fit to data in the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal. An arrow indicates that the point is off scale.

Figure 7

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the single-lepton signal regions after the combined fit to data in both the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal scaled by the best-fit signal strength. An arrow indicates that the point is off scale.

Figure 8

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the dilepton signal regions after the combined fit to data in both the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal scaled by the best-fit signal strength. An arrow indicates that the point is off scale.

Figure 9

(a) Summary of the 95% C.L. upper limits on $\sigma (t\overline{t}t\overline{t})$ relative to the SM prediction in the individual channels and for the combination. The observed limits (solid black lines) are shown together with the expected limits in the background-only hypothesis (dashed black lines) and in the SM signal-plus-background hypothesis case (dashed red lines). One- and two-standard-deviation uncertainty bands around the expected limits in the background-only hypothesis are also shown. (b) Summary of the signal-strength measurements in the individual channels and for the combination. The statistical uncertainties are evaluated from a fit to the data performed with all the nuisance parameters associated with systematic uncertainties fixed to their postfit values from the nominal fit.

Figure 10

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the dilepton validation regions after the combined fit to data in the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal scaled by the best-fit signal strength. An arrow indicates that the point is off scale.

Figure 11

Comparison between data and prediction of the $H_{\mathrm{T}}^{\mathrm{had}}$ distributions in the single-lepton validation regions after the combined fit to data in the single-lepton and dilepton channels. The $t\overline{t}+\text{jets}$ background is estimated with the data-driven method. The $t\overline{t}+V$ and $t\overline{t}+H$ processes are denoted $t\overline{t}+H/V$. Contributions from $W/Z+\text{jets}$, single-top, diboson and multijet backgrounds are combined into a single background source referred to as “$\mathrm{Non}\text{−}t\overline{t}$”. The hashed area represents the combined statistical and systematic uncertainties of the prediction. The last bin in all figures contains the overflow. The lower panel shows the ratio between the data and the total prediction, including the SM $t\overline{t}t\overline{t}$ signal scaled by the best-fit signal strength. An arrow indicates that the point is off scale.