Observation of the beam-size effect at HERA

TL;DR

This paper reports the observation of the beam-size effect at HERA, where the photon spectrum from electron-proton bremsstrahlung is suppressed at low energies due to the finite transverse size of the beams.

Contribution

First experimental observation of the beam-size effect at HERA, confirming the impact of beam geometry on bremsstrahlung photon spectra.

Findings

Reduced photon rate below 5 GeV compared to Bethe-Heitler spectrum

Confirmation of the beam-size effect due to finite beam overlap

Agreement with theoretical explanations of the effect

Abstract

A precise measurement of the spectrum of the photons from $ep$ bremsstrahlung with the ZEUS luminosity monitor at HERA is reported. The measurement shows a reduced rate compared to the Bethe-Heitler spectrum for photon energies below 5~GeV. This suppression, called the beam-size effect, is explained by the finite transverse size of the beam overlap relative to the typical impact parameter in the process of $ep$ bremsstrahlung at HERA energies.