Measurement and Beyond the Standard Model interpretations of the $t\bar{t}t\bar{t}$ production cross section from 140fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13\mathrm{TeV}$ in ATLAS

This dissertation presents the measurement of the $t\bar{t}t\bar{t}$ production cross section in $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ at the ATLAS detector at CERN, with an integrated luminosity of $140\mathrm{fb^{-1}}$. Two independent analyses are conducted: one using the single-lepton and opposite-charge dilepton final states (1LOS), and the other using the same-charge dilepton and multilepton final states (SSML). The 1LOS analysis measures the $t\bar{t}t\bar{t}$ cross section in the context of a Two Higgs Doublet Model, with $t\bar{t}t\bar{t}$ cross section enhanced by contributions from $t\bar{t}H$ production, where $H$ is a Heavy Higgs boson. The SSML analysis measures the $t\bar{t}t\bar{t}$ cross section and extracts limits on the Beyond the Standard Model (BSM) Higgs Oblique parameter, $\hat{H}$. Both analyses use signal regions with high particle jet and b-tagged jet multiplicity, as well as multivariate classifiers to isolate the $t\bar{t}t\bar{t}$ signal. Statistical analysis is performed using a profile likelihood method to obtain the cross section and the uncertainty on it, as well as the signal significance. The SSML analysis measures $22.5^{+4.7}_{-4.3}$(stat)$^{+4.6}_{-3.4}$(syst) $ = 22.5^{+6.6}_{-5.5}\mathrm{fb}$ for the $t\bar{t}t\bar{t}$ cross section, with an observed significance of 6.1 standard deviations above the background-only hypothesis, and a significance of 4.3 assuming a Standard Model cross section. This constitutes the first discovery of $t\bar{t}t\bar{t}$ production. The extracted $\hat{H}$ parameter is $\hat{H}=0.09$ with the upper end of the 95\% CL region at $\hat{H}=0.2$, which is also the upper limit of the model's validity. The 1LOS analysis has not yet been unblinded, but fits to Asimov data place upper limits at the 95\% CL on the $t\bar{t}t\bar{t}$ cross section in association with a Heavy Higgs of mass $m_H$. The limits range from $40-50\mathrm{fb}$ for $m_H = 400\mathrm{GeV}$ to $10-20\mathrm{fb}$ for $m_H = 1000\mathrm{GeV}$.