Probing the CP nature of the Higgs coupling in tt¯h events at the LHC

The determination of the CP nature of the Higgs coupling to top quarks is addressed in this paper, using tt-h events produced in s=13 TeV proton-proton collisions at the LHC. Dileptonic final states are employed, with two oppositely charged leptons and four jets, corresponding to the decays t→bW+→b+ν, t-→b-W-→b--ν-, and h→bb-. Pure scalar (h=H), pure pseudoscalar (h=A), and CP-violating Higgs boson signal events, generated with MadGraph5-aMC@NLO, are fully reconstructed through a kinematic fit. We furthermore generate samples that have both a CP-even and a CP-odd component in the tt-h coupling in order to probe the ratio of the two components. New angular distributions of the decay products, as well as CP angular asymmetries, are explored in order to separate the scalar from the pseudoscalar components of the Higgs boson and reduce the contribution from the dominant irreducible background, tt-bb. Significant differences between the angular distributions and asymmetries are observed, even after the full kinematic fit reconstruction of the events, allowing to define the best observables for a global fit of the Higgs couplings parameters. (Less)