Transverse spin asymmetries and the electron Yukawa coupling at an FCC-ee

TL;DR

This paper proposes using transverse spin asymmetries at an FCC-ee collider to improve the measurement sensitivity of the electron Yukawa coupling, potentially enabling its observation within the Standard Model.

Contribution

It introduces a novel method leveraging transverse spin asymmetries and quantum interference to enhance the sensitivity to the electron Yukawa coupling at future colliders.

Findings

Significance can be increased by up to three times with transverse polarization.

Additional polarization of positrons can boost significance by a factor of five.

Method is applicable to various $WW$ and $ZZ$ final states.

Abstract

We show that measurements of single transverse-spin asymmetries at an $e^+e^-$ collider can enhance the sensitivity to the electron Yukawa coupling, possibly enabling observation of the Standard Model value for this quantity. We demonstrate that the significance in both the $b\bar{b}$ and semi-leptonic $WW$ final states can be enhanced by factors of up to three compared to inclusive cross section determinations of this coupling for transversely-polarized electrons. If the positrons can be simultaneously longitudinally polarized, even at the level of 30\%, the significance can be enhanced by a factor of five or more. The method utilizes quantum interference between the Higgs signal and the continuum background and is also applicable in other $WW$ and $ZZ$ final states.