| dc.description.abstracteng | 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 momenta of several objects are used as discriminant and to constrain uncertainties on the dominant \ttjets background. To obtain a reliable background prediction, scale factors are derived which are applied to the nominal Monte Carlo prediction in the fitted regions. A profile likelihood fit is used to measure a signal strength of $\mu_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}=\frac{\sigma_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}}{\sigma^\text{SM}_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}}\,=\,1.7^{+0.7}_{-0.7}\text{(stat)}\,^{+1.0}_{-0.7}\text{(syst)}$ with an (expected) significance of 1.76 (1.08) standard deviations. The result is still in agreement with the Standard Model prediction. Due to the excess observed in events with multiple leptons and two leptons of same charge, the combined result of $\mu_{\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}}=\,2.0^{+0.4}_{-0.4}\text{(stat)}\,^{+0.3}_{-0.3}\text{(syst)}$ shows an excess and first evidence of the $\text{t}\bar{\text{t}}\text{t}\bar{\text{t}}$ process with a significance of 4.71 (2.9) $\sigma$. | de |