A definitive determination of the contribution of multi-photon exchange to elastic lepton-nucleon scattering

点击翻译按钮获取中文摘要

This project aims at a precision measurement of the e+p/e-p cross section ratio in elastic lepton proton scattering. It will directly test the contribution of multi-photon effects. Recent determinations of the proton electric to magnetic form factor ratio from polarization transfer measurements at JLab indicate an unexpected and dramatic discrepancy with the form factor ratio obtained using the Rosenbluth separation technique in unpolarized cross section measurements. The possible explanation for this discrepancy is a significant contribution of multi-photon exchange beyond the usual one-photon exchange approximation in the calculation of the elastic electron-proton scattering cross section. Since most of our understanding on the structure of the proton and atomic nuclei is based upon lepton scattering analyzed in terms of the single photon approximation, it is essential to definitively verify the contribution of multi-photon exchange. The current, dramatic discrepancy between recoil polarization and Rosenbluth measurements of the proton elastic form factor ratio constitutes a serious challenge to our understanding of the structure of the proton. The widely accepted explanation in terms of multi-photon exchange demands a definitive confirmation. The aim of the OLYMPUS-experiment at DESY is the determination of this contribution with a precision of 1%.