Physicists solve 35-year-old mystery about quarks

Quarks, the smallest particles in the universe, are far smaller and operate at much higher energy levels than the protons and neutrons in which they are found. 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.

Now physicists from Tel Aviv University, the Massachusetts Institute of Technology (MIT) and the Thomas Jefferson National Accelerator Facility know why quarks, the building blocks of the universe, move more slowly inside atomic nuclei.

"Researchers have been seeking an answer to this for 35 years," says Prof. Eli Piasetzky of TAU's Raymond and Beverly Sackler School of Physics & Astronomy. Prof. Piasetzky; Meytal Duer, also of TAU's School of Physics; and Prof. Or Hen, Dr. Barak Schmookler and Dr. Axel Schmidt of MIT have now led the international CLAS Collaboration at the Thomas Jefferson National Accelerator Facility to identify an explanation for the EMC effect. Their conclusions were published on February 20 in the journal Nature.

The researchers discovered that the speed of a quark 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 larger the number of slow-moving quarks within the atom's protons and neutrons.

Atoms with larger nuclei intrinsically have more protons and neutrons, so they are more likely to have a higher number of proton-neutron pairs. The team concluded that the larger the atom, the more pairs it is likely to contain. This results in slower-moving quarks in that particular atom.

"In short-range correlated or SRC pairs, an atom's protons and neutrons can pair up constantly, but only momentarily, before splitting apart and going their separate ways," Duer explains. "During this brief, high-energy interaction, quarks, in their respective particles, may have a larger space to play in."

The team's new explanation can help to illuminate subtle yet important differences in the behavior of quarks, the most basic building blocks of the visible world.

For the research, the scientists harnessed a Large Acceptance Spectrometer, or CLAS detector, a four-story spherical particle detector, in an experiment conducted over several months at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility. The experiment amassed billions of interactions between electrons and quarks, allowing the researchers to calculate the speed of the quark in each interaction based on the electron's energy after it scattered, and to compare the average quark speed among the various atoms.

"These high-momentum pairs are the reason for these slow-moving quarks," Prof. Hen explains. "How much a quark's speed is slowed depends on the number of SRC pairs in an atomic nucleus. Quarks in lead, for instance, were far slower than those in aluminum, which themselves were slower than iron, and so on."

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