Top quark electroweak couplings at future lepton colliders

Abstract: We perform a comparative study of the reach of future $e^+e^-$ collider options for the scale of non-resonant new physics effects in the top quark sector, phrased in the language of higher-dimensional operators. Our focus is on the electroweak top quark pair production process $e^+e^- \to Z^*/\gamma \to t\bar t $, and we study benchmark scenarios at the ILC and CLIC. We find that both are able to constrain mass scales up to the few TeV range in the most sensitive cases, improving by orders of magnitude on the forecasted capabilities of the LHC. We discuss the role played by observables such as forward-backward asymmetries, and making use of different beam polarisation settings, and highlight the possibility of lifting a degeneracy in the allowed parameter space by combining top observables with precision $Z$-pole measurements from LEP1.