UC Berkeley

During Run 2 of the Large Hadron Collider, the ATLAS experiment recorded proton--proton collision events with a center of mass energy of 13 TeV, the highest energy ever achieved in a collider. Analysis of this dataset, which corresponds to an integrated luminosity of 139 fb$^{-1}$, has provided new opportunities for precision measurements of the Higgs boson. The interaction between the Higgs boson and the top quark, the heaviest known particle, is a fundamental probe of the Higgs mechanism, which describes how Standard Model particles obtain mass.

The coupling of the Higgs boson to the top quark can be directly probed through the production of a Higgs boson in association with a top-antitop quark pair ($t\bar{t}H$) at the LHC. A statistical combination of ATLAS searches in multiple Higgs boson decay channels using 79.8 fb$^{-1}$ of the Run 2 dataset yielded the first observation of $t\bar{t}H$ production at the level of $6.3\sigma$.

The Higgs to diphoton ($\gamma\gamma$) decay channel is among the most sensitive for $t\bar{t}H$ measurements due to the excellent diphoton mass resolution of the ATLAS detector and the clean signature of this decay. A powerful multivariate analysis was developed to select $t\bar{t}H(\gamma\gamma)$ events based on the momenta of jets, photons, and leptons in the final state. The $t\bar{t}H$ cross section times $H\rightarrow\gamma\gamma$ branching fraction was measured to be $\sigma_{t\bar{t}H}\times B_{\gamma\gamma} = 1.59 \; _{-0.39}^{+0.43}$ fb using the full Run 2 ATLAS dataset. Top quarks were reconstructed with high probability in selected $t\bar{t}H(\gamma\gamma)$ events.

Following the observation of the $t\bar{t}H$ process, a direct measurement of the CP properties of the Higgs-top interaction was carried out in the $H\rightarrow\gamma\gamma$ decay channel. A CP-sensitive multivariate categorization was developed using reconstructed top quark variables, yielding an observed $t\bar{t}H$ significance of 5.2$\sigma$ and an upper limit on the cross section of single-top-plus-Higgs ($tH$) production of 11.6 times the SM expectation. The observed data excludes a fully CP odd Higgs-top coupling at the level of 3.9$\sigma$, and the CP mixing angle is constrained to $|\alpha|>43^\circ$ at 95\% confidence level.