Search for the production of four top quarks in proton-proton collisions at $\sqrt{s}=13$ TeV in the single lepton and opposite-sign dilepton channels with the ATLAS detector at the Large Hadron Collider

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}} = \sigma_{t\bar{t}t\bar{t}} / \sigma_{t\bar{t}t\bar{t}}^\text{SM} $. The estimation of the dominant $t\bar{t}$+jets background is given by the data-driven t$\bar{\text{t}}$TRF method. The measured signal strength in the single lepton and opposite-sign dilepton channel is $\mu_{t\bar{t}t\bar{t}}^\text{1L+OS2L} = 0.9^{+0.5}_{-0.5}~\text{(stat.)}^{+1.0}_{-0.9}~\text{(syst.)}= 0.9^{+1.1}_{-1.0}$, compatible with the Standard Model expectation and corresponding to a deviation of $0.9$ ($1.0$) standard deviations from the background-only hypothesis. This result is combined with a complementary measurement, using events with two leptons with same-sign charges or three leptons in the final state. The combination provides a measurement of the $t\bar{t}t\bar{t}$cross-section of $24\pm6$ fb compatible with the Standard Model prediction within $1.8$ standard deviations. The measured significance of the $t\bar{t}t\bar{t}$ process over the background of $4.4$ standard deviations, providing evidence of the $t\bar{t}t\bar{t}$ production process.