Polarization and Luminosity Coupling  Concept: Use Bremsstrahlung ep  ep  as reference cross section Use Bremsstrahlung ep  ep  as reference cross section  normally only single photon counting  Hera: reached 1-2% systematic uncertainty  eRHIC BUTs:  with 10 33 cm -2 s -1 one gets on average of 23 bremsstrahlungs photons/bunch for proton beam  A-beam Z 2 -dependence  this will challenge single photon measurement under 0 o  coupling between polarization measurement uncertainty and uncertainty achievable for lumi-measurement  no experience no polarized ep collider jet  have started to calculate a with the help of  have started to calculate a with the help of Vladimir Makarenk (NC PHEP BSU, Minsk), did these calculations for ZEUS and is now at CERN to work on CLIC-QED calculations  hopefully a is small E.C.
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