Abstract
A search for four-top-quark production, $t\bar{t}t\bar{t}$, is presented. It is based on proton-proton collision data with a center-of-mass energy $\sqrt{s}$ = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during the year 2015 and 2016, corresponding to an integrated luminosity of $36.1$ 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 ATLAS 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, upper limit on anomalous four-top-quark production cross section is set in the context of an effective field theory model.
Highlights
With a mass close to the scale of electroweak symmetry breaking, the top quark, besides having a large coupling to the Standard Model (SM) Higgs boson [1,2], is predicted to have large couplings to new particles hypothesized in many models beyond the StandardModel (BSM) [3,4,5]
This paper focuses on a search for four-top-quark production via the SM processes in proton-proton collisions at the Large Hadron Collider (LHC) and the results are interpreted in the context of an effective field theory (EFT) approach where the beyond the StandardModel (BSM) contribution is represented via a four-top-quarks contact interaction [10]
In order to further suppress contributions from pileup and other soft radiation, the reclustered large-R (RCLR) jets are trimmed [49] by removing all small-R jets within a reclustered jet that have pT below 5% of the pT of the reclustered jet
Summary
With a mass close to the scale of electroweak symmetry breaking, the top quark, besides having a large coupling to the Standard Model (SM) Higgs boson [1,2], is predicted to have large couplings to new particles hypothesized in many models beyond the Standard. Searches for anomalous tttt production via an EFT model were recently performed by the ATLAS Collaboration [19,20], which set an observed (expected) upper limit of 16 fb (31 fb) on the production cross section at 95% C.L. The four-top-quark production events can give rise to different final states depending on the hadronic or semileptonic decay mode of each of the top quarks.
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