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A search for events containing four top quarks (ttbar-ttbar) is reported from proton-proton collisions recorded by the CMS experiment at sqrt(s) = 13 TeV and corresponding to an integrated luminosity of 2.6 inverse femtobarns The analysis considers the single-lepton (e or mu)+jets and the opposite-sign dilepton (mu+mu-, mu+/- e-/+, or e+e- )+jets channels. It uses boosted decision trees to combine information on the global event and jet properties to distinguish between ttbar-ttbar and ttbar production. The number of events observed after all selection requirements is consistent with expectations from background and standard model signal predictions, and an upper limit is set on the cross section for ttbar-ttbar production in the standard model of 94 fb at 95% confidence level (10.2 times the prediction), with an expected limit of 118 fb. This is combined with the results from the published CMS search in the same-sign dilepton channel, resulting in an improved limit of 69 fb at 95% confidence level (7.4 times the prediction), with an expected limit of 71 fb. These are the strongest constraints on the rate of ttbar-ttbar production to date.

This thesis presents the search for the production of $t\bar{t}t\bar{t}$ from $pp$ collision at $\sqrt{s} = 13$ TeV. The analysed dataset has been collected by the ATLAS detector during the LHC Run 2, corresponding to an integrated luminosity of $139$ fb$^{-1}$. The analysis uses events containing either one lepton or two leptons with opposite-sign charges, at large jet and $b$-jet multiplicities. The output of a Boosted Decision Tree is used as a discriminating variable in a profile likelihood fit for the measurement of the $t\bar{t}t\bar{t}$ signal strength $\mu_{t\bar{t}t\bar{t}} = \sigm...

The search for the production of $\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}$ in $pp$ collisions at $\sqrt{s}\,=\,13\,$TeV is presented in this thesis for the full \lhc \runii dataset of 139$\,$fb$^{-1}\,$collected by the ATLAS experiment at CERN. With an expected cross-section of $\sigma_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}\,=\,11.97^{+18\%}_{-21\%}\,\text{fb}$, the process is one of the rarest being studied at the \lhc. This document focuses on high jet and b-jet multiplicities with one or two leptons of opposite charge. A Boosted Decision Tree and the sum of the transverse mo...