OLYMPUS Experiment Sheds Light on Inner Workings of Protons

Seven-year study explains how packets of light are exchanged when protons meet electrons.

The OLYMPUS detector, shown from the downstream end, during installation at the DORIS storage ring in the German Electron Synchrotron, let scientists precisely measure the results of electron beams colliding with protons.

The Science

To understand the structure of matter in the visible universe, scientists need to determine the structure and behavior of protons. In studying the electric charge and magnetic moments inside the proton, the results varied. Scientists disagreed as to why two different types of beams yielded two different results. The OLYMPUS team carried out a precise experiment that showed why. They proved that a proton and electron exchange two packets of light, one intense and one not. That is, they validated the theoretical description of the basic scattering process, known as the elastic lepton-proton scattering process, that occurs during the collision.

The Impact

Scientists thought that interpreting the collision data required either a small or a significant correction factor. If a significant correction was needed, it called into question the scientific description of the proton. The OLYMPUS data showed that the small corrections are accurate. Further, the OLYMPUS data validated that the description used for over half a century to determine proton structure was correct.  


The OLYMPUS experiment was conceived at the Massachusetts Institute of Technology (MIT) in 2007 and carried out by an international collaboration from 2010 to 2013 at the DORIS particle storage ring at DESY, Hamburg, Germany. It was motivated by the claim that significant new two-photon contributions were essential to describe elastic lepton-proton scattering at multi-GeV beam energies. Positron and electron beams of 2-GeV energy were sequentially scattered from an internal hydrogen gas target. The elastically scattered leptons and protons were detected in coincidence in a large acceptance toroidal spectrometer that was designed and constructed at MIT. The analysis involved a careful evaluation of systematic uncertainties arising from experimental effects as well as from radiative corrections. They used a measured ratio of positron-proton elastic scattering to electron proton elastic scattering as a function of scattering angle to determine the high-energy two-photon contribution, up to momentum transfers of 2.5 (GeV/c)2. The OLYMPUS data validate that the single photon approximation is a good description of elastic lepton-proton scattering to +/- 1 percent up to these momentum transfers. The OLYMPUS data are not well described by the theoretical prediction based on two-photon exchange. Confirmation of the explanation of the discrepancy will require further precision measurements at higher energies beyond those studied with OLYMPUS. The accurate and precise theoretical description of elastic lepton-nucleon scattering is a cornerstone of the scientific program at a possible future electron-ion collider.


Richard Milner
Massachusetts Institute of Technology
milner@mit.edu, 617-258-5439


U.S. Department of Energy, Office of Science, Office of Nuclear Physics; National Science Foundation; Ministry of Education and Science of Armenia; the Deutsche Forschungsgemeinschaft; the European Community-Research Infrastructure Activity; the United Kingdom Science and Technology Facilities Council and the Scottish Universities Physics Alliance; the Ministry of Education and Science of the Russian Federation; and the Alexander von Humboldt Foundation, Germany.


B.S. Henderson, et al. (The OLYMPUS Collaboration), “Hard two-photon contribution to elastic lepton-proton scattering determined by the OLYMPUS experiment.” Physical Review Letters 118, 092501 (2017). [DOI: 10.1103/PhysRevLett.118.092501]

Related Links

Physics Today article: Proton structure comes to light

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Program: NP

Performer: University

Additional: Collaborations , Non-DOE Interagency Collaboration , International Collaboration