Single-spin measurements and heavy new physics in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>−</mml:mo> </mml:msup>

We investigate the potential of single-spin components of the spin-density matrix in the <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:mo stretchy="false">→</a:mo> <a:mi>t</a:mi> <a:mover accent="true"> <a:mi>t</a:mi> <a:mo stretchy="false">¯</a:mo> </a:mover> </a:math> process at a future FCC-ee for probing heavy new physics parametrized using the Standard Model effective field theory (SMEFT) framework. We consider the full spectrum of spin observables and the complete angular decomposition of the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mi>t</f:mi> <f:mover accent="true"> <f:mi>t</f:mi> <f:mo stretchy="false">¯</f:mo> </f:mover> </f:math> production process in our study. We find that single-spin measurements generically provide stronger probes of SMEFT Wilson coefficients than measurements where the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mi>t</j:mi> <j:mover accent="true"> <j:mi>t</j:mi> <j:mo stretchy="false">¯</j:mo> </j:mover> </j:math> spins are correlated, and that single-spin observables are important for resolving flat directions that can appear in the Wilson-coefficient parameter space.