A 40-Year-Old Science Puzzle Finally Solved! EMC Effect Explained скачать в хорошем качестве

A 40-Year-Old Science Puzzle Finally Solved! EMC Effect Explained

The EMC effect is the surprising observation that the cross-section for deep inelastic scattering (DIS) from an atomic nucleus differs from that of an equivalent number of free protons and neutrons (collectively referred to as nucleons). This observation suggests that the momentum distributions of quarks within nucleons, when they are bound inside nuclei, differ from those in free nucleons. This effect was first observed in 1983 at CERN by the European Muon Collaboration, which is why it is named the "EMC effect." This finding was unexpected, as the average binding energy of protons and neutrons inside nuclei is small compared to the energy transferred during deep inelastic scattering, which probes quark distributions. The EMC effect is surprising given the difference in energy scales between nuclear binding and deep inelastic scattering. Typical binding energies for nucleons in nuclei are around 10 megaelectron volts (MeV), while the energy transfers in DIS are typically several gigaelectron volts (GeV). Therefore, researchers previously believed that nuclear binding effects would not significantly impact quark distributions. Several hypotheses have been proposed to explain the EMC effect. While older theories, such as Fermi motion and nuclear pions, have been ruled out by studies involving electron scattering or Drell-Yan data, modern hypotheses generally fall into two main categories: mean-field modification and short-range correlated pairs. "As quarks are placed in a nucleus, they begin to move slower, which is very strange," said Or Hen, a physicist at the Massachusetts Institute of Technology and co-author of the study. This is puzzling because the strong interactions between quarks primarily determine their speed, while the forces that bind the nucleus, and also act on quarks within it are believed to be relatively weak, Hen explained. Moreover, there is no other known force that should be affecting the behavior of quarks in a nucleus to such an extent. Yet, the EMC effect persists. Until recently, scientists were unsure of its cause. In a nucleus, two particles are typically pulled together by a force of about 8 MeV, which is a measure of energy in particle interactions. In contrast, quarks within a proton or neutron are bound together by approximately 1,000 MeV. This raises the question: how can the relatively mild interactions among nucleons have such a dramatic impact on the strong interactions among quarks? As Hen noted, "What is eight next to one thousand?" Usually, protons and neutrons in a nucleus do not overlap; they maintain their boundaries, even though they are essentially systems of bound quarks. However, there are instances when nucleons briefly become linked and start to physically overlap, forming what scientists call "correlated pairs." At any given moment, about 20 percent of nucleons in a nucleus are in such an overlapping state. When this occurs, a significant amount of energy flows among the quarks, fundamentally altering their bound structure and behavior—a phenomenon driven by the strong force. In a paper published in the journal Nature (paper's link below), the researchers argued that this energy flow is precisely what accounts for the EMC effect. Sources: 1. https://www.nature.com/articles/s4158... 2. https://www.nature.com/articles/d4158... 3. https://eprints.gla.ac.uk/176072/7/17... 4. https://www.space.com/quarks-emc-effe... Thanks for watching. Make sure you subscribe to our channel for such interesting videos :)