Abstract
The Z peak cross section sigma^0_had can be measured at FCC-ee with hadronic and dimuon events produced at s = 91.2GeV, with a potential relative statistical precision of O(1e-6). Together with the ratio Rl, this quantity allows the determination of the number of light neutrino types with a precision of 0.0004. A limiting systematic uncertainty comes from the absolute determination of the integrated luminosity. A determination with low-angle Bhabha scattering is likely to be limited by a relative theoretical precision of O(1e−4), but that might not be the case for large angle diphoton production, e+e− → γγ (to be checked with actual full two-loop calculations). The requirements on the detector design to measure the absolute luminosity with diphoton events, and in particular to separate these events from the large angle Bhabha background, will be studied, in synergy with the lepton acceptance studies required for Rl.