Lab-frame observables for probing the top-Higgs interaction

We investigate methods to explore the CP nature of the $t\bar{t}h$ coupling at the LHC, focusing on associated production of the Higgs with a $t \bar{t}$ pair. We first discuss the constraints implied by low-energy observables and by the Higgs-rate information from available LHC data, emphasizing that they cannot provide conclusive evidence on the nature of this coupling. We then investigate kinematic observables that could probe the $t\bar{t}h$ coupling directly, in particular quantities that can be constructed out of just lab-frame kinematics. We define one such observable by exploiting the fact that $t \bar{t}$ spin correlations do also carry information about the CP-nature of the $t\bar{t}h$ coupling. Finally, we introduce a CP-odd quantity and a related asymmetry, able to probe CP violation in the $t\bar{t}h$ coupling and likewise constructed out of lab-frame momenta only.

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