The effects of O(α2) initial state QED corrections to e+e- → γ⁎/Z⁎ at very high luminosity colliders
We present numerical results on the recently completed O (α2) initial state corrections to the process e+e- →γ⁎ /Z⁎, which is a central process at past and future high energy and high luminosity colliders for precision measurements of the properties of the Z-boson, the Higgs boson, and the top quark. We observe differences to an earlier result  in the non-logarithmic contributions at O (α2). The new result leads to a 4 MeV shift in the Z width considering the lower end s0 = 4 mτ2 of the radiation region, which is larger than the present accuracy. A corresponding cut to s0 / s = 0.01 only implies a shift of 0.2 MeV. We present predictions on the radiative corrections to the central processes e+e- →γ⁎ /Z⁎, e+e- → ZH and e+e- → t t ‾ planned at future colliders like the ILC, CLIC, FCC_ee and CEPC to measure the mass and the width of the Z boson, the Higgs boson and the top quark, for which the present corrections are significant.