Probing the 𝐶𝑃 nature of the top–Higgs Yukawa coupling in 𝑡̄𝑡𝐻 and 𝑡𝐻 events with 𝐻 → 𝑏̄𝑏 decays using the ATLAS detector at the LHC

The 𝐶𝑃 properties of the coupling between the Higgs boson and the top quark are investigated using 139 fb−1 of proton–proton collision data recorded by the ATLAS experiment at the LHC at a centre-of-mass energy of√𝑠 = 13 TeV. The 𝐶𝑃 structure of the top quark–Higgs boson Yukawa coupling is probed in events with a Higgs boson decaying into a pair of 𝑏-quarks and produced in association with either a pair of top quarks, 𝑡̄𝑡𝐻, or a single top quark, 𝑡𝐻. Events containing one or two electrons or muons are used for the measurement. Multivariate techniques are used to select regions enriched in 𝑡̄𝑡𝐻 and 𝑡𝐻 events, where dedicated 𝐶𝑃 -sensitive observables are exploited. In an extension of the Standard Model (SM) with a 𝐶𝑃 -odd admixture in the top–Higgs Yukawa coupling, the mixing angle between 𝐶𝑃 -even and 𝐶𝑃 -odd couplings is measured to be 𝛼 = 11 ◦+52◦−73◦, compatible with the SM prediction corresponding to 𝛼 = 0.