The first observation of the $t\overline{t}H$ process in a single Higgs boson decay channel with the full reconstruction of the final state ($H\to \gamma \gamma$) is presented, with a significance of 6.6 standard deviations ($\sigma$). The $CP$ structure of Higgs boson couplings to fermions is measured, resulting in an exclusion of the pure $CP$-odd structure of the top Yukawa coupling at $3.2\sigma$. The measurements are based on a sample of proton-proton collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected by the CMS detector at the LHC, corresponding to an integrated luminosity of $137{\mathrm{fb}}^{-1}$. The cross section times branching fraction of the $t\overline{t}H$ process is measured to be ${\sigma }_{t\overline{t}H}{\mathcal{B}}_{\gamma \gamma }={1.56}_{-0.32}^{+0.34}\mathrm{fb}$, which is compatible with the standard model prediction of ${1.13}_{-0.11}^{+0.08}\mathrm{fb}$. The fractional contribution of the $CP$-odd component is measured to be $f_{CP}^{Htt}=0.00\pm 0.33$.

• Received 24 March 2020
• Accepted 19 June 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.061801

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