The international OLYMPUS Collaboration this week published their first results in the journal Physical Review Letters. In 2012, the OLYMPUS detector made measurements at the DORIS storage ring to study a problem observed in electron-proton scattering. “The publication marks the culmination of a seven year research project to resolve a puzzling discrepancy in measurements of the proton form factors: G_E and G_M, which describe the electric and magnetic charge distributions inside the proton,” says Douglas Hasell from the Massachusetts Institute of Technology (MIT) in Boston, who is the spokesperson for some 55 OLYMPUS scientists from 13 institutions. The experiment produced precise measurements of the ratio between positron-proton and electron-proton elastic scattering to investigate the role of two-photon exchange in electron-proton scattering.

The form factors examined by the OLYMPUS group are determined by the distribution of the quarks inside the proton. Scientists have been measuring these form factors for the past 60 or so years; in the 1960s and 1970s, they were also carried out at the DESY accelerator. Measurements made at Jefferson Lab in the USA in the early 2000s revealed deviations from older experiments by studying the collisions of polarised electrons and protons. One possible explanation could be that in some collisions instead of just one photon, several photons are exchanged between the two particles. In order to test this hypothesis, the 50-tonne OLYMPUS detector was installed at the DORIS storage ring. Most of it came from the BLAST detector, which was used at MIT from 2002 to 2005, adapted for the DORIS storage ring that also had to be modified.

The big advantage of this combination was that DORIS could alternate between high intensity beams of electrons and their antiparticles, positrons, incident on the protons in a hydrogen gas target. In multi-photon exchange, differences arise depending on whether the protons were struck with electrons or positrons. “Using DORIS, we were able to switch very rapidly between electron and positron operation, which considerably reduces the systematic error in the measurements,” explains Uwe Schneekloth, a researcher at DESY who is the deputy spokesperson for the collaboration. “Thanks to the amazing support of DESY’s accelerator team, which kept DORIS up and running over the Christmas break and even implemented the top-up mode of operation for DORIS, we were able to collect a large amount of valuable data over our short operating time in spite of some technical challenges.”

Overall, the scientists collected data for just over three months. In the course of the subsequent analysis, the researchers found that two different processes contribute to the assumed exchange of two photons during a collision. Whereas the dominant process can be described very well in theoretical terms, the distinctly smaller effect still poses certain riddles. It is markedly weaker than previous, less precise experiments, led scientists to believe. The OLYMPUS results indicate that this so-called “hard two-photon exchange” can explain the discrepancy between the two form factors. Although they agree with a general description of the phenomenon, existing model-dependent calculations still need to be modified in order to describe it. “To achieve a more precise understanding of the process, it would be helpful to conduct similar experiments at higher collision energies and with substantially higher collision frequencies. However, at the moment there is no suitable tailor-made solution for this, as we had in the case of the OLYMPUS detector at DORIS,” explains Schneekloth.

“The findings from OLYMPUS will lead to a marked advance in our understanding of the proton,” explains Joachim Mnich, Director for particle and astroparticle physics at DESY. “I would like to congratulate the OLYMPUS Collaboration, whose experiment has supplied the most accurate data on this effect that will be available for the foreseeable future.”

Original publication