Measurements of $t\overline tH$ Production and the CP Structure of the Yukawa Interaction between the Higgs Boson and Top Quark in the Diphoton Decay Channel

The first observation of the t t ¯ H process in a single Higgs boson decay channel with the full reconstruction of the final state ( H → γγ ) is presented, with a significance of 6.6 standard deviations ( σ ). 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 3.2σ . The measurements are based on a sample of proton-proton collisions at a center-of-mass energy s=13 TeV collected by the CMS detector at the LHC, corresponding to an integrated luminosity of 137 fb–1 . The cross section times branching fraction of the t t ¯ H process is measured to be σt t ¯ H B γγ=1.56–0.32+0.34 fb , which is compatible with the standard model prediction of 1.13–0.11+0.08 fb . The fractional contribution of the CP -odd component is measured to be fCPHtt=0.00±0.33 .