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

TL;DR

This paper explores how single-spin measurements in top quark pair production at future FCC-ee colliders can effectively probe heavy new physics described by SMEFT, offering improved sensitivity over spin correlation measurements.

Contribution

It demonstrates that single-spin observables are more sensitive to SMEFT Wilson coefficients and help resolve parameter space ambiguities in top quark studies at FCC-ee.

Findings

Single-spin measurements outperform spin correlation measurements in probing SMEFT.

Single-spin observables help resolve flat directions in Wilson coefficient space.

The study emphasizes the importance of full angular and spin analysis in top quark physics.

Abstract

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.