Search for four-top-quark production in final states with one charged lepton and multiple jets using 3.2 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector at the LHC

A search for four-top-quark production, $t\bar{t}t\bar{t}$, is presented based on the proton-proton collision data taken at $\sqrt{s} = 13$ TeV and collected with the ATLAS detector at the Large Hadron Collider during 2015, corresponding to an integrated luminosity of 3.2 fb$^{-1}$. Data are analysed in the single-lepton channel, characterised by an isolated high transverse momentum electron or muon and multiple jets. The search exploits the high jet multiplicity, high $b$-tagged jet multiplicity and large total sum of jet transverse momenta which characterise signal events and provide good discrimination against the dominant background, namely the top-quark pair production in association with jets. Events are categorised according to their jet and $b$-tagged jet multiplicities in order to improve the sensitivity of the search and reduce the impact of systematic uncertainties. No significant excess of events above the background expectation is found and an observed (expected) upper limit of 21 (16) times the four-top-quark Standard Model cross-section is obtained at 95% confidence level. Additionally, upper bounds on four-top-quark production are set in different scenarios of physics beyond the Standard Model.