ScienceDaily (Nov. 20, 2009) — A recent experiment at the Department of Energy's Thomas Jefferson National Accelerator Facility has found that a proton's nearest neighbors in the nucleus of the atom may modify the proton's internal structure.

Visualization of helium-4 and beryllium nuclei. (Credit: Peter Mueller (Argonne National Lab))

When comparing large nuclei to small nuclei, past measurements have shown a clear difference in how the proton's constituent particles, called quarks, are distributed. This difference is called the EMC Effect.

Many models of the EMC Effect predict that it is caused by the mass or density of the nucleus in which the proton resides. To test these predictions, experimenters made precise new measurements of the EMC effect in a variety of light nuclei, such as isotopes of helium.

"What we found is that there is a large modification of the quark structure in helium-4, and there was a much smaller effect in helium-3. And even though they were both light nuclei, they had a very different EMC Effect," said John Arrington, a spokesperson for the experiment and a nuclear physicist at DOE's Argonne National Lab.

The results, Arrington added, rules out the idea that the size of the EMC effect scales with the mass of the nucleus.

Next, the experimenters turned their attention to density. They compared the EMC Effect in beryllium to various other nuclei. Beryllium has a mass similar to carbon but a much lower density, roughly the same as helium-3. They found that the size of the EMC Effect in beryllium is similar to that of carbon, which is twice as dense.

"So you have one set of data that tells you the mass-dependence picture doesn't work and another that tells you the density-dependence picture doesn't work," Arrington explained. "So, if both of these pictures are wrong, what's really going on?"

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