$CP$ Properties of Higgs Boson Interactions with Top Quarks in the $t\overline{t}H$ and $tH$ Processes Using $H\to \gamma \gamma$ 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 ($H\to \gamma \gamma$), and their production in association with a top quark pair ($t\overline{t}H$) or single top quark ($tH$) is studied. The analysis uses $139{\mathrm{fb}}^{-1}$ of proton–proton collision data recorded at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ 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 $H\to \gamma \gamma$ branching ratio is ${1.64}_{-0.36}^{+0.38}(\text{stat}{)}_{-0.14}^{+0.17}(\text{sys})\mathrm{fb}$, and the measured rate for $t\overline{t}H$ is ${1.43}_{-0.31}^{+0.33}(\text{stat}{)}_{-0.15}^{+0.21}(\text{sys})$ 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 $(-43)°$ is excluded at 95% confidence level.

Figure 1

Left: two-dimensional BDT distribution in the selected data events ($m_{\gamma \gamma }\in [105,160]\mathrm{GeV}$) from the Had region showing the Background Rejection BDT and $CP$ BDT. The inner (outer) contours capture 25% (50%) of the $t\overline{t}H$ and $tH$ signal events for $CP$-even (blue) and $CP$-odd (red) hypotheses. Right: projections onto the background rejection and $CP$ BDT axes. Contributions from $CP$-even (blue) and $CP$-odd (red) $t\overline{t}H/tH$ processes and the data (black) are shown and normalized to unit area. The error bars on data are statistical.

Figure 2

Distribution of reconstructed primary top quark mass versus reconstructed Higgs boson mass in the data events. The right panels show the projections onto the Higgs boson mass and primary top quark mass axes. In the upper panel, the fitted continuum background (blue), the total background including non-$t\overline{t}H/tH$ Higgs boson production (green), and the total fitted signal plus background (red) are shown. The error bars on data are statistical.

Figure 3

Two-dimensional likelihood contours for ${\kappa }_t\cos (\alpha )$ and ${\kappa }_t\sin (\alpha )$ with ggF and $H\to \gamma \gamma$ constrained by the Higgs boson coupling combination.