Probing the top Yukawa coupling at the LHC via associated production of single top and Higgs

TL;DR

This paper investigates the associated production of single top quarks and Higgs bosons at the LHC to probe the top Yukawa coupling and potential CP violation through interference effects, asymmetries, and polarization measurements.

Contribution

It introduces an analytical framework for helicity amplitudes including CP phases and proposes new asymmetry and polarization observables to test CP violation in top-Higgs interactions.

Findings

Azimuthal asymmetry reveals CP phase effects.

Opposite signs of asymmetries for top and anti-top.

Potential for direct CP violation tests at the LHC.

Abstract

We study Higgs boson production associated with single top or anti-top via $t$-channel weak boson exchange at the LHC. The process is an ideal probe of the top quark Yukawa coupling, because we can measure the relative phase of $htt$ and $hWW$ couplings, thanks to the significant interference between the two amplitudes. By choosing the emitted $W$ momentum along the polar axis in the $th\,(\bar{t}h)$ rest frame, we obtain the helicity amplitudes for all the contributing subprocesses analytically, with possible CP phase of the Yukawa coupling. We study the azimuthal asymmetry between the $W$ emission and the $Wb\,(\bar{b})\to t(\bar{t})\,h$ scattering planes, as well as several $t$ and $\bar{t}$ polarization asymmetries as a signal of CP violating phase in the $htt$ coupling. Both the azimuthal asymmetry and the polarization perpendicular to the scattering plane are found to have the opposite sign between the top and anti-top events. We identify the origin of the sign of asymmetries, and propose the possibility of direct CP violation test in $pp$ collisions by comparing the top and anti-top polarization at the LHC.