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

## TL;DR

This paper investigates beam-beam electromagnetic effects at FCC-ee that impact the precision of luminosity measurements, proposing methods to quantify and correct these effects to achieve the desired experimental accuracy.

## Contribution

It provides a detailed analysis of beam-beam effects on luminosity measurement and introduces correction methods to improve measurement precision at FCC-ee.

## Key findings

- Beam-beam electromagnetic effects significantly bias luminosity measurements.
- Proposed correction methods can mitigate these biases.
- Achieving 10^-4 luminosity precision is feasible with these corrections.

## Abstract

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. If not corrected for, these effects would lead, at the Z pole, to a systematic bias of the measured luminosity that is more than one order of magnitude larger than the desired precision. In this note, these effects are studied in detail, and methods to measure and correct for them are proposed.