# Beam-beam effects on the luminosity measurement at LEP and the number of light neutrino species

## TL;DR

This paper investigates how beam-beam electromagnetic effects at LEP bias luminosity measurements, leading to a correction that impacts the inferred number of light neutrino species from Z boson data.

## Contribution

It quantifies the bias caused by beam-beam effects on luminosity measurements at LEP and demonstrates its impact on the determination of light neutrino species.

## Key findings

- Bias causes about 0.1% underestimation of luminosity
- Correcting the bias alters the neutrino species count
- Beam effects significantly influence precision electroweak measurements

## Abstract

In $e^+ e^-$ collisions, electromagnetic effects caused by large charge density bunches modify the effective acceptance of the luminometer system of the experiments. These effects consequently bias the luminosity measurement from the rate of low-angle Bhabha interactions $e^+ e^- \to e^+ e^- $. Surprisingly enough, the magnitude of this bias is found to yield an underestimation of the integrated luminosity measured by the LEP experiments by about 0.1%, significantly larger than the reported experimental uncertainties. When accounted for, this effect modifies the number of light neutrino species determined at LEP from the measurement of the hadronic cross section at the Z peak.