Role of the t(t)over-barh rest frame in direct top-quark Yukawa coupling measurements

This paper studies new possibilities to directly measure a hypothetical CP-odd (pseudoscalar) component in the top-quark Yukawa coupling. In particular, the role of the t (t) over barh center-of-mass rest frame in the associated production of a top pair and a h boson at the LHC is explored. The h boson is assumed to have both a CP-even (scalar) and a CP-odd coupling to the top quark. The relative strength of the scalar and pseudoscalar components is regulated by an angle a. Observables sensitive to the nature of the top-quark Yukawa coupling are proposed. These observables are defined in terms of the transverse and longitudinal projections of t, (t) over bar and h momenta with respect to the beam axis in the t (t) over barh rest frame. Distributions differential with respect to those observables are evaluated up to next-to-leading order in QCD. These distributions are found to be sensitive to the CP nature of the coupling. Dileptonic final states of the t (t) over barh system (with h -> b (b) over bar) are used, after fast DELPHES detector simulation and full event reconstruction through a kinematic fit, as a case study to test the observables' sensitivity to the CP nature of the coupling. Confidence levels are presented as a function of the total integrated LHC luminosity for the case of exclusion of a pure CP-odd coupling against the Standard Model CP-even hypothesis. By using observables evaluated in the t (t) over barh system, the luminosity needed to directly probe the CP properties of the top-quark Yukawa coupling at the High-Luminosity run of the LHC can be decreased by a few hundred inverse femtobarns, when compared to analyses that use observables in the laboratory rest frame. In addition, transverse momentum distributions of the h boson and top quarks are found to provide no more discriminant power than a counting experiment.