Physicists at the Massachusetts Institute of Technology have found an explanation for a 35-year-old mystery about why quark momenta are distributed differently inside the nuclei of atoms than in free protons and neutrons. It turned out that this is due to the fact that short-term correlations arise between nucleons. This was announced in a press release on Phys.org.
Scientists analyzed data from a 2004 particle detector experiment in Jefferson's lab. Then the nuclei of carbon, aluminum, iron and lead, as well as deuterium (an isotope of hydrogen containing a proton and a neutron in the nucleus) were irradiated with electrons with an energy of 5.01 gigaelectronvolts. In this case, the detectors recorded both knocked-out particles and scattered electrons.
The results obtained confirmed the presence of short-range correlations between protons and neutrons, which last for several seconds. Moreover, their structures temporarily overlap. The researchers have derived a function for short-range correlations that describes the EMC effect - the mismatch between momenta in free protons and neutrons and nucleons in atomic nuclei. According to this model, correlations promote the redistribution of quarks and change their momenta.
The EMC effect was discovered in 1983 by CERN scientists as part of the European Muon Collaboration. Although more than a thousand scientific papers have been published on this topic, there is still no unambiguous explanation for this phenomenon, in connection with which it is called one of the unsolved problems of physics.