### Abstract:

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 \(\sqrt{s}=13\text{}\text{}\mathrm{TeV}\) collected by the CMS detector at the LHC, corresponding to an integrated luminosity of \(137\text{}\text{}{\mathrm{fb}}^{-1}\). The cross section times branching fraction of the \(t\overline{t}H\) process is measured to be \({\sigma }_{t\overline{t}H}{B}_{\gamma \gamma }={1.56}_{-0.32}^{+0.34}\text{}\text{}\mathrm{fb}\), which is compatible with the standard model prediction of \({1.13}_{-0.11}^{+0.08}\text{}\text{}\mathrm{fb}\). The fractional contribution of the \(CP\)-odd component is measured to be \({f}_{CP}^{Htt}=0.00±0.33\).