Evidence for tt̄tt̄ production in the multilepton final state in proton–proton collisions at √s = 13 TeV with the ATLAS detector at the LHC

Abstract

The first evidence for simultaneous production of four top quarks is presented. The measurement is conducted by selecting events having two same-charge leptons or at least three leptons and by analysing 139 fb^-1 of proton–proton collision data at a centre-of-mass energy of &radic;s = 13 TeV provided by the LHC and collected by the ATLAS detector. In addition to simulated samples, data-driven methods are utilised for the estimation of background contributions. A multivariate discriminant is developed to separate tt&#772;tt&#772; signal from the background processes using the Boosted Decision Tree (BDT) method. The production cross-section is extracted using a binned profile-likelihood fit to all control regions and the BDT distribution in the signal region simultaneously and is measured to be <br /> &delta;_{tt&#772;tt&#772;} = 24 ± 5(stat.)⁺⁵_{− 4}(syst.) fb = 24⁺⁷_{− 6} fb, <br/> corresponding to an observed (expected) statistical significance of 4.3 (2.9) standard deviations from the background-only hypothesis. The analysis model is used to set limits on the top-quark Yukawa coupling parameter &gamma;_t . A pure CP-even (CP-odd) coupling is excluded for |&gamma;_t| > 2.2 (|&gamma;_t| > 1.6) at the 95% confidence level. No deviations from the Standard Model of particle physics prediction has been found.