Beam-beam effects on the luminosity measurement at FCC-ee

Understand

The first part of the physics programme of the integrated FCC (Future Circular Colliders) proposal includes measurements of Standard Model processes in $e^+e^-$ collisions (FCC-ee) with an unprecedented precision.

• In particular, the potential precision of the Z lineshape determination calls for a very precise measurement of the absolute luminosity, at the level of 1E-4, and the precision on the relative luminosity between energy scan points around the Z pole should be an order of magnitude better.
• The luminosity is principally determined from the rate of low-angle Bhabha interactions, $e^+e^- \to e^+e^-$, where the final state electrons and positrons are detected in dedicated calorimeters covering small angles from the outgoing beam directions.
• Electromagnetic effects caused by the very large charge density of the beam bunches affect the effective acceptance of these luminometers in a nontrivial way.