Observation of the beam-size effect at HERA
Krzysztof Piotrzkowski
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.
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