In 1983, it was discovered that the internal structure of a nucleon — a proton or a neutron — depends on its environment. That is, the structure of a nucleon in empty space is different from its structure when it is embedded inside an atomic nucleus. However, despite vigorous theoretical and experimental work, the cause of this modification has remained unknown. In a paper in Nature, the CLAS Collaboration presents evidence that sheds light on this long-standing issue.
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