Enhancing $CP$ Measurement of the Yukawa Interactions of Top-Quark at $e^{-}e^{+}$ Collider

TL;DR

This paper proposes a novel analysis method to measure $CP$ violation in top-quark Yukawa interactions at $e^{-}e^{+}$ colliders, utilizing lepton spin correlations without reconstructing top-quarks, and finds significant enhancement effects in specific reference frames.

Contribution

It introduces a universal, frame-independent analysis strategy for probing $CP$ violation in top-quark Yukawa interactions without top-quark reconstruction, enhancing sensitivity at future colliders.

Findings

Large enhancement of $CP$-odd and $CP$-even observables in the Higgs rest frame.

Method does not require top-quark reconstruction, simplifying experimental analysis.

Universal formal expressions for observables across reference frames.

Abstract

It is well known that $CP$-violating coupling of the Yukawa interaction of top-quark is a promising candidate of a new source of $CP$ violation effect. Precisely measurement of its $CP$ properties is crucial for understanding new physics above the electroweak scale. In this paper, we introduce a complete analysis method for probing $CP$ violation effects in the associated production of top-quark pair and Higgs boson at $e^{-}e^{+}$ collider. Reconstructions of the top-quarks are not needed in our strategy. The observables are defined based on spin correlation effects of the leptons emerging from decays of top-quarks, and their formal expressions are universal at any reference frames. Two reference frames, the rest frame of the top-quark pair and the rest frame of Higgs boson, are examined. Importantly, large enhancement effects for both $CP$-odd and $CP$-even observables are observed in the rest frame of Higgs boson. This can be essential for probing $CP$ violation effects at future $e^{-}e^{+}$ collider.