<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math> Properties of Higgs Boson Interactions with Top Quarks in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>t</mml:mi><mml:mover accent="true"><mml:mi>t</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover><mml:mi>H</mml:mi></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>t</mml:mi><mml:mi>H</mml:mi></mml:math> Processes Using <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>H</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>γ</mml:mi><mml:mi>γ</mml:mi></mml:math> with the ATLAS Detector

A study of the charge conjugation and parity ($CP$) properties of the interaction between the Higgs boson and top quarks is presented. Higgs bosons are identified via the diphoton decay channel ($H\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$), and their production in association with a top quark pair ($t\overline{t}H$) or single top quark ($tH$) is studied. The analysis uses $139\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of proton--proton collision data recorded at a center-of-mass energy of $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ with the ATLAS detector at the Large Hadron Collider. Assuming a $CP$-even coupling, the $t\overline{t}H$ process is observed with a significance of 5.2 standard deviations. The measured cross section times $H\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ branching ratio is ${1.64}_{\ensuremath{-}0.36}^{+0.38}(\text{stat}{)}_{\ensuremath{-}0.14}^{+0.17}(\text{sys})\text{ }\text{ }\mathrm{fb}$, and the measured rate for $t\overline{t}H$ is ${1.43}_{\ensuremath{-}0.31}^{+0.33}(\text{stat}{)}_{\ensuremath{-}0.15}^{+0.21}(\text{sys})$ times the Standard Model expectation. The $tH$ production process is not observed and an upper limit on its rate of 12 times the Standard Model expectation is set. A $CP$-mixing angle greater (less) than 43 $(\ensuremath{-}43)\ifmmode^\circ\else\textdegree\fi{}$ is excluded at 95% confidence level.

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