Complementarity between ILC250 and ILC-GigaZ

In view of the very precise measurements on fermion couplings which will be performed at ILC250 with polarized beams, there is emerging evidence that the LEP1/SLC measurements on these couplings are an order of magnitude too imprecise to match the accuracies reachable at ILC250. This will therefore severely limit the indirect sensitivity to new resonances and require revisiting the possibility to run ILC at the Z pole with polarized electrons. This work was done as a contribution to the ESU 2018-2020.