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Study of quark speeds finds a solution for a 35-year-old physics mystery

Physicists now have an answer to a question in nuclear physics that has puzzled scientists for three decades: Why do quarks move more slowly inside larger atoms? An international team of physicists has identified an explanation for the EMC effect: In the nucleus of an iron atom containing many protons and neutrons, quarks move significantly more slowly than quarks in deuterium, which contains a single proton and neutron. They have found that a quark’s speed depends on the number of protons and neutrons forming short-ranged correlated pairs in an atom’s nucleus. The more such pairs there are in a nucleus, the more slowly the quarks move within the atom’s protons and neutrons. Since atoms with larger nuclei intrinsically have more protons and neutrons, they also are more likely to have a higher number of proton-neutron pairs, also known as "short-range correlated," or SRC, pairs. Therefore, the team concludes that the larger the atom, the more pairs it is likely to contain, resulting in slower-moving quarks in that particular atom.

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