MIT 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?
Quarks, along with gluons, are the fundamental building blocks of the universe. These subatomic particles — the smallest particles we know of — are far smaller, and operate at much higher energy levels, than the protons and neutrons in which they are found. Physicists have therefore assumed that a quark should be blithely indifferent to the characteristics of the protons and neutrons, and the overall atom, in which it resides.
But in 1983, physicists at CERN, as part of the European Muon Collaboration (EMC), observed for the first time what would become known as 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. Since then, physicists have found more evidence that the larger an atom’s nucleus, the slower the quarks that move within.
“People have been wracking their brains for 35 years, trying to explain why this effect happens,” says Or Hen, assistant professor of physics at MIT.
Now Hen, Barak Schmookler, and Axel Schmidt, a graduate student and postdoc in MIT’s Laboratory for Nuclear Science, have led an international team of physicists in identifying an explanation for the EMC effect. 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.
Schmidt says an atom’s protons and neutrons can pair up constantly, but only momentarily, before splitting apart and going their separate ways. During this brief, high-energy interaction, he believes that quarks in their respective particles may have a “larger space to play.”
“In quantum mechanics, anytime you increase the volume over which an object is confined, it slows down,” Schmidt says. “If you tighten up the space, it speeds up. That’s a known fact.”
As 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.
In 2011, Hen and collaborators, who has focused much of their research on SRC pairs, wondered whether this ephemeral coupling had anything to do with the EMC effect and the speed of quarks in atomic nuclei.
They gathered data from various particle accelerator experiments, some of which measured the behavior of quarks in certain atomic nuclei, while others detected SRC pairs in other nuclei. When they plotted the data on a graph a clear trend appeared: The larger an atom’s nucleus, the more SRC pairs there were, and the slower the quarks that were measured. The largest nucleus in the data — gold — contained quarks that moved 20 percent more slowly than those in the smallest measured nucleus, helium.