The Zb(b)over-bar couplings at future e+e- colliders

Many new physics models predict sizable modifications to the SM Zb (b) over bar couplings, while the corresponding measurements at LEP and SLC exhibit some discrepancy with the SM predictions. After updating the current results on the Zb (b) over bar coupling constraints from global fits, we list the observables that are most important for improving the Zb (b) over bar coupling constraints and estimate the expected precision reach of three proposed future e(+)e(-) colliders, CEPC, ILC and FCC-ee. We consider both the case that the results are SM-like and the one that the Zb (b) over bar couplings deviate significantly from the SM predictions. We show that, if we assume the value of the Zb (b) over bar couplings to be within 68% CL of the current measurements, any one of the three colliders will be able to rule out the SM with more than 99.9999% CL (5 sigma). We study the implications of the improved Zb (b) over bar coupling constraints on new physics models, and point out their complementarity with the constraints from the direct search of new physics particles at the LHC, as well as with Higgs precision measurements. Our results provide a further motivation for the construction of future e(+)e(-) colliders.