CP tests of Higgs couplings in tt¯h semileptonic events at the LHC
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Abstract
The $CP$ nature of the Higgs coupling to top quarks is addressed in this paper, in single charged lepton final states of $t\overline{t}h$ events produced in proton-proton collisions at the LHC. Pure scalar ($h=H$) and pseudoscalar ($h=A$) Higgs boson signal events, generated with MadGraph5_aMC@NLO, are fully reconstructed using a kinematic fit. Angular distributions of the decay products, as well as $CP$-sensitive asymmetries, are exploited to separate and gain sensitivity to possible pseudoscalar components of the Higgs boson and reduce the contribution from the dominant irreducible background $t\overline{t}b\overline{b}$. Significant differences are found between the pure $CP$-even and -odd signal hypotheses as well as with respect to the Standard Model background, in particular the $t\overline{t}b\overline{b}$ contribution. Such differences survive the event reconstruction, allowing one to define optimal observables to extract the Higgs couplings parameters from a global fit. A dedicated analysis is applied to efficiently identify signal events and reject as much as possible the expected Standard Model background. The results obtained are compared with a similar analysis in the dilepton channel. We show that the single lepton channel is more promising overall and can be used in combination to study the $CP$ nature of the Higgs coupling to top quarks.