Internal structure of proton is still a mystery
Date: Saturday, March 04, 2017 @ 15:17:59 EST
Topic: Science

OLYMPUS experiment sheds light on structure of protons (from

A mystery concerning the structure of protons is a step closer to being solved, thanks to a seven-year experiment led by researchers at MIT.

For many years researchers have probed the structure of protons—subatomic particles with a positive charge—by bombarding them with electrons and examining the intensity of the scattered electrons at different angles.

In this way they have attempted to determine how the proton's electric charge and magnetization are distributed. These experiments had previously led researchers to assume that the electric and magnetic charge distributions are the same, and that one photon—an elementary particle of light—is exchanged when the protons interact with the bombarding electrons.

However, in the early 2000s, researchers began to carry out experiments using polarized electron beams, which measure electron-proton elastic scattering using the spin of the protons and electrons. These experiments revealed that the ratio of electric to magnetic charge distributions decreased dramatically with higher-energy interactions between the electrons and protons.

This led to the theory that not one but two photons were sometimes being exchanged during the interaction, causing the uneven charge distribution. What's more, the theory predicted that both of these particles would be so-called "hard," or high-energy photons.

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.

In a paper published this week in the journal Physical Review Letters, the researchers reveal the results of this experiment, which indicate that two photons are indeed exchanged during electron-proton interactions.

However, unlike the theoretical predictions, analysis of the OLYMPUS measurements suggests that, most of the time, only one of the photons has high energy, while the other must carry very little energy indeed, according to Richard Milner, a professor of physics and member of the Laboratory for Nuclear Science's Hadronic Physics Group, who led the experiment.

"We saw little if no evidence for a hard two-photon exchange," Milner says.

Having proposed the idea for the experiment in the late 2000s, the group was awarded funding in 2010...

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