Observation of Four-Top Quark Production and Measurement of Off-shell Higgs Boson Interactions with Top Quarks with ATLAS

The ATLAS experiment at CERN measures proton--proton collisions produced by the Large Hadron Collider at energies up to 13.6 TeV. Analysis of the Run 2 dataset, which totals an integrated luminosity of 140 fb$^{-1}$ at a center of mass energy of 13 \TeV, has resulted in unprecedented precision in measurements of rare processes involving the top quark and the Higgs boson, the heaviest known fundamental particles. Measuring these processes provides direct probes of the top Yukawa coupling and tests of the Standard Model fermion mass generation mechanism. The total decay width of the Higgs boson can also be accessed by comparing the rates of on- and off-shell Higgs boson production. The simultaneous production of four top quarks (tttt) is a rare process sensitive to possible contributions from heavy new particles. Additionally, tttt production is sensitive to the top Yukawa coupling through contributions from off-shell Higgs bosons. Therefore, tttt production may be used to measure the top Yukawa coupling independently of the Higgs boson's total width. The first observation of tttt production in ATLAS established the tttt process with a statistical significance of $6.1\sigma$. The observation of tttt production was performed in the same-sign dilepton and multilepton final states and uses a graph neural network to separate the small tttt signal from background processes. The measured value of the tttt production cross-section was $\sigma=22^{+6.6}_{-5.5}$ fb, which is 1.9 times the Standard Model prediction. Interpretations of the observed tttt signal were used to set limits on the coefficients of four-fermion SMEFT operators and the Higgs oblique parameter. Further interpretations set limits on the top Yukawa coupling and its CP properties. The top Yukawa coupling was constrained to $\kappa_{t} < 1.9$ at the 95% CL when considering the variation of the ttH background and $\kappa_{t} < 2.3$ when considering only the effect on tttt production. Additional limits were set on the cross-section of ttt production, which remains unobserved. Following the observation of tttt production, the results were combined with measurements of on-shell Higgs boson production and decay to produce the first limits on the total width of the Higgs boson $\Gamma_h$ based on measurements of Higgs boson couplings to fermions. The combination placed a limit of $\Gamma_h < 450$ MeV at the 95% CL when loop-induced interactions of the Higgs boson were profiled and $\Gamma_h < 160$ MeV when they were parameterized as functions of tree-level couplings to Standard Model Particles.