Whilst conducting a new series of experiments involving the analysis of protons, scientists at the US Department of Energy's (DOE) Thomas Jefferson National Accelerator Facility (Jefferson Lab) discovered that the companions protons had inside the atomic nuclei changed the positive particles' internal structures. Details of the find appear in the November 13 issue of the scientific journal Physical Review Letters, e! Science News reports.

The EMC Effect is the name given in the scientific community to the differences that appear between the way the elementary particles known as quarks are organized inside protons, the positively charged particles inside atomic nuclei. Until now, a large number of possible explanations for the effect existed, including those arguing that the mass or density of the nuclei in which the protons resided were the main elements triggering the quark differences. In order to test these ideas, the Jefferson Lab team set up the new series of experiments, which primarily dealt with light isotopes, such as 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,” the experiments' spokesperson, John Arrington, says of the findings. The scientist is also a nuclear physicist at DOE's Argonne National Lab. The observations, he adds, rule out the idea that the mass of the atomic nuclei has something to do with the quark distribution.

The next step was to check and see if density had something to do with the Effect. The team compared the quark differences in beryllium atoms, as opposed to carbon atoms. The two have the same weight, but beryllium is half as dense as carbon. The Effect was found to be the same in both atoms, which rules out density as a potential explanation. “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. So, if both of these pictures are wrong, what's really going on?” Arrington says.

“We want to isolate the quark structure during the moment when the proton and neutron are very close together. If we find a large effect in such a small and simple nucleus by looking when the proton and neutron are closest together, it will demonstrate that the EMC effect does not require a large, dense nucleus – it simply requires two nucleons coming into extremely close contact,” he adds. The nucleus of an atom is made up of protons and neutrons. The latter are not electrically charged.