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

We show that measurements of single transverse-spin asymmetries at an <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mi>e</a:mi><a:mo>+</a:mo></a:msup><a:msup><a:mi>e</a:mi><a:mo>−</a:mo></a:msup></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>b</c:mi><c:mover accent="true"><c:mi>b</c:mi><c:mo stretchy="false">¯</c:mo></c:mover></c:math> and semileptonic <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>W</g:mi><g:mi>W</g:mi></g:math> final states can be enhanced by factors of up to 3 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 5 or more. The method utilizes quantum interference between the Higgs signal and the continuum background and is also applicable in other <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>W</i:mi><i:mi>W</i:mi></i:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>Z</k:mi><k:mi>Z</k:mi></k:math> final states. Published by the American Physical Society 2024