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The EMC Effect was first discovered just over 35 years ago by the European Muon Collaboration in data taken at CERN. The collaboration found that when they measured quarks inside a nucleus, they appeared different from those found in free protons and neutrons.
"There are currently two main models that describe this effect. One model is that all protons and neutrons in a nucleus [and thus their quarks] are modified and they are all modified the same way," says Douglas Higinbotham, a Jefferson Lab staff scientist.
Model - One - Paper - Protons - Neutrons
"The other model, which is the one that we focus on in this paper, is different. It says that many protons and neutrons are behaving as if they are free, while others are involved in short-range correlations and are highly modified," he explains.
Short-range correlations are fleeting partnerships formed between protons and neutrons inside the nucleus. When a proton and a neutron pair up in a correlation, their structures overlap briefly. The overlap lasts just moments before the particles part ways.
Modification - Function - Re-analysis - Data - Experiment
The universal modification function was developed from a careful re-analysis of data from an experiment conducted in 2004 using Jefferson Lab's Continuous Electron Beam Accelerator Facility, a DOE Office of Science User Facility. CEBAF produced a 5.01 GeV beam of electrons to probe nuclei of carbon, aluminum, iron and lead as compared to deuterium (an isotope of hydrogen containing a proton and neutron in its nucleus).
When the authors compared the data from each of these nuclei to deuterium, they saw the same pattern emerge. The nuclear physicists derived from this information a universal modification function for short-range correlations in nuclei. They then applied the function to the nuclei used in measurements of the EMC Effect, and they found that it was the same across all measured nuclei that they considered.
"Now we have this function, where we have...
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