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

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.