Single-spin measurements and heavy new physics in the $e^{+}{e}^{-}\to t\bar{t}$ process at an FCC-ee

We investigate the potential of single-spin components of the spin-density matrix in the $e^{+}{e}^{-}\to t\bar{t}$ process at a future FCC-ee for probing heavy new physics parametrized using the SMEFT framework. We consider the full spectrum of spin observables and the complete angular decomposition of the $t\bar{t}$ production process in our study. We find that single-spin measurements generically provide stronger probes of SMEFT Wilson coefficients than measurements where the $t\bar{t}$ spins are correlated, and that single-spin observables are important for resolving flat directions that can appear in the Wilson-coefficient parameter space.