Probing the Higgs Yukawa coupling to the top-quark at the LHC via single top + Higgs production

TL;DR

This paper investigates how single top and Higgs production at the LHC can be used to probe the top-quark Yukawa coupling, revealing polarization effects and CP violation signatures.

Contribution

It introduces a method to analyze top polarization and CP violation in single top + Higgs production, providing new observables sensitive to the top Yukawa coupling.

Findings

Top quark is produced with 100% polarization at leading order.

A polarization asymmetry sensitive to CP violation is identified.

Potential to observe CP violation effects at the LHC.

Abstract

The conjoined production at the LHC of single top and Higgs boson via $t$-channel weak boson exchange is ideal to probe the top-quark Yukawa coupling, due to a delicate cancellation between the amplitudes with the $htt$ and the $hWW$ couplings. We find that the top quark is produced with $100\%$ polarization in the leading order, and its quantum state is determined by the spin-vector direction in the $t$-quark rest frame. We relate the spin direction to the four-momenta of the top, Higgs and a jet in the helicity amplitude framework. We identify a polarization asymmetry that is sensitive to CP violation, even after partial integration over the forward jet momentum. This CP violating asymmetry may be observed at the LHC via the component of the top-quark polarization that is perpendicular to the $th$ scattering plane.