New physics off the Z-pole: e+e− → $$ f\overline{f} $$ at Future Lepton Colliders

A bstract We explore the prospects for probing new physics (NP) beyond the Standard Model (SM) at the future lepton colliders through precision measurements of e + e − → $$ f\overline{f} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>f</mml:mi> <mml:mover> <mml:mi>f</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> observables off the Z resonance. We consider the interference between the SM contributions and those arising from the dimension-6 four-fermion effective operators that encode the effects of NP, yielding a linear dependence on the latter. This linear dependence in general increases with the collision energy offset from the Z pole. We consider a variety of asymmetries in order to enhance the NP sensitivity while reducing the experimental systematic and theoretical SM uncertainties: the inclusive above- and below- Z -resonance cross section asymmetry ( A σ ) as well as the conventional forward-backward ( A FB ) and polarization ( A pol ) asymmetries. Based on the projected statistical uncertainties at the Circular Electron-Positron Collider (CEPC), we find that the measurement of A σ could extend the sensitivity to the NP mass scale by as much as a factor of ~ 7 compared to the present reach obtained with the CERN Large Electron Positron Collider. The projected systematic theoretical SM uncertainties substantially reduce this sensitivity gain. For A FB , the experimental systematic uncertainties has a marginal impact on the gain in NP reach, whereas the SM theoretical uncertainties remain a significant barrier to realizing the full NP sensitivity. Analogous conclusions apply to the CERN Future Circular Collider (FCC-ee) and International Linear Collider (ILC).