Abstract
A measurement of four-top-quark production using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 139 fb−1 is presented. Events are selected if they contain a single lepton (electron or muon) or an opposite-sign lepton pair, in association with multiple jets. The events are categorised according to the number of jets and how likely these are to contain b-hadrons. A multivariate technique is then used to discriminate between signal and background events. The measured four-top-quark production cross section is found to be {26}_{-15}^{+17} fb, with a corresponding observed (expected) significance of 1.9 (1.0) standard deviations over the background-only hypothesis. The result is combined with the previous measurement performed by the ATLAS Collaboration in the multilepton final state. The combined four-top-quark production cross section is measured to be {24}_{-6}^{+7} fb, with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over the background-only predictions. It is consistent within 2.0 standard deviations with the Standard Model expectation of 12.0 ± 2.4 fb.
Highlights
Each bin in these regions is represented by a Poisson probability term for the observed data, with the expectation value provided by the Monte Carlo (MC) prediction with the corrections to tt+jets
The expected yield depends on the signal strength, μ, defined as the ratio of the measured ttttproduction cross section to that predicted by the SM, σtttt/σtStMtt, and the systematic uncertainties, θ
Systematic uncertainties are taken into account in the likelihood function, each as a nuisance parameter constrained by a Gaussian prior
Summary
The ATLAS experiment [17] at the LHC is a multipurpose particle detector with a forwardbackward symmetric cylindrical geometry and a near 4π coverage in solid angle. It consists of an inner tracking detector (ID) surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic (EM) and hadron calorimeters, and a muon spectrometer (MS). The ATLAS experiment [17] at the LHC is a multipurpose particle detector with a forwardbackward symmetric cylindrical geometry and a near 4π coverage in solid angle.. The ATLAS experiment [17] at the LHC is a multipurpose particle detector with a forwardbackward symmetric cylindrical geometry and a near 4π coverage in solid angle.2 It consists of an inner tracking detector (ID) surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field, electromagnetic (EM) and hadron calorimeters, and a muon spectrometer (MS). The inner tracking detector covers the pseudorapidity range |η| < 2.5. It consists of silicon pixel, silicon microstrip, and transition radiation tracking detectors. A steel/scintillator-tile hadron calorimeter covers the central pseudorapidity range (|η| < 1.7). An extensive software suite [19] is used for real and simulated data reconstruction and analysis, for operation and in the trigger and data acquisition systems of the experiment
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