In a bid to identify this “two-photon exchange,” an international team led by researchers in the Laboratory for Nuclear Science at MIT carried out a seven-year experiment, known as OLYMPUS, at the German Electron Synchrotron (DESY) in Hamburg.
The researchers reveal the results of this experiment, which indicate that two photons are indeed exchanged during electron-proton interactions.
Having proposed the idea for the experiment in the late 2000s, the group was awarded funding in 2010.
The researchers had to disassemble the former BLAST spectrometer — a complex 125-cubic-meter-sized detector based at MIT — and transport it to Germany, where it was reassembled with some improvements.
They then carried out the experiment over three months in 2012, before the particle accelerator at the laboratory was itself decommissioned and shut down at the end of that year.
The experiment, which was carried out at the same time as two others in the U.S. and Russia, involved bombarding the protons with both negatively charged electrons and positively charged positrons, and comparing the difference between the two interactions.
The process will produce a subtly different measurement depending on whether the protons are scattered by electrons or positrons.
The collisions were run for three months, and the resulting data took a further three years to analyze.
The difference between the theoretical and experimental results means further experiments may need to be carried out in the future, at even higher energies where the two-photon exchange effect is expected to be larger.
It may prove difficult to achieve the same level of precision reached in the OLYMPUS experiment, however.