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MIT physicists now have an answer to a question in nuclear physics that has intrigued scientists for 30 years: why are quarks moving more slowly inside bigger atoms?
Quarks, with gluons, are the fundamental building blocks of the universe. These subatomic particles – the smallest particles we know – are much smaller and operate at much higher energy levels than the protons and neutrons in which they are located. Physicists have therefore assumed that a quark should be blithely indifferent to the characteristics of protons and neutrons, as well as to the overall atom in which it resides.
But in 1983, physicists at CERN, as part of the European Muon Collaboration (CEM), observed for the first time what is now called the CEM effect: in the nucleus of an iron atom containing many quarks and neutrons, quarks move much more slowly than quarks in deuterium, which contains only one proton and neutron. Since then, physicists have found more evidence that the larger the nucleus of an atom, the more slowly moving quarks are.
"For 35 years people have been racking their brains trying to explain why this effect is occurring," says Or Hen, an assistant professor of physics at MIT.
Now Hen, Barak Schmookler and Axel Schmidt, a graduate student and postdoctoral fellow at the MIT Nuclear Science Laboratory, have led an international team of physicists to find an explanation for the effect of electromagnetic compatibility. They discovered that the speed of a quark depends on the number of protons and neutrons forming pairs correlated at short distances in the nucleus of an atom. The more such pairs exist in a nucleus, the more the quarks move slowly in the protons and neutrons of the atom.
Schmidt says that the protons and neutrons of an atom can mate permanently, but only momentarily, before separating and separating. During this brief high-energy interaction, he thinks the quarks of their respective particles may have "a larger space to play."
"In quantum mechanics, every time you increase the volume on which an object is confined, it slows down," says Schmidt. "If you tighten the space, it speeds up. This is a known fact.
Since larger nuclei have intrinsically more protons and neutrons, they are also more likely to have more proton-neutron pairs, also known as short-range correlated pairs (SRCs). Therefore, the team concludes that the larger the atom, the more likely it is to contain pairs, resulting in quarks moving more slowly in that particular atom.
Schmookler, Schmidt and Hen, members of the CLAS collaboration at the Thomas Jefferson National Acceleration Center, published their findings today in the newspaper Nature.
From a suggestion to a complete picture
In 2011, Hen and his collaborators, who focused much of their research on SRC pairs, wondered whether this ephemeral coupling had anything to do with the effect of CEM and quark velocity in atomic nuclei.
They collected data from various experiments on particle accelerators, some of which measured the behavior of quarks in some atomic nuclei, while others detected pairs of CRS in other nuclei. When they plotted the data on a graph, a clear trend emerged: the larger the nucleus of an atom, the more pairs of CRS and the more measured quarks were slow. The most important core in the data – gold – contained quarks moving 20% slower than those of the smallest measured core, helium.
"It was the first time this connection was concretely suggested," says Hen. "But we had to do a more detailed study to build a complete physical picture."
Then he and his colleagues analyzed data from an experiment comparing atoms of different sizes and measuring both the quark speed and the number of CRS pairs in the nucleus of each atom. The experiment was conducted using the CEBAF Wide Acceptance Spectrometer, or CLAS, a huge four-stage spherical particle accelerator from the Thomas Jefferson National Laboratory in Newport News, Virginia.
Within the detector, Hen describes the target configuration of the team as a "kind of Frankenstein trick", with mechanical arms, each holding a thin sheet made of a different material, such as carbon, aluminum, iron and lead, each made of atoms containing respectively 12, 27, 67 and 208 protons and neutrons. An adjacent vessel contained liquid deuterium, with atoms containing the smallest number of protons and neutrons in the group.
When they wanted to study a particular sheet, they sent an order to the concerned arm for it to lower the sheet of interest, following the deuterium cell and directly on the path of the beam of ## EQU1 ## 39 electrons of the detector. This beam sent electrons to the deuterium cell and the solid sheet at the rate of several billion electrons per second. While a large majority of electrons pass by targets, some strike protons or neutrons inside the nucleus, or much smaller quarks themselves. When they strike, the electrons disperse widely, and the angles and energies at which they disperse vary according to what they strike – information captured by the detector.
Electronic adjustment
The experiment lasted for several months and eventually raised billions of interactions between electrons and quarks. The researchers calculated the quark velocity in each interaction, based on the energy of the electron after its diffusion, and then compared the average quark velocity between the different atoms.
By examining much smaller dispersion angles, corresponding to pulse transfers of a different wavelength, the team was able to "zoom out" so that the electrons dispersed out of the protons and neutrons more big, rather than quarks. The SRC pairs are generally extremely energetic and would therefore scatter electrons at higher energies than the unpaired protons and neutrons, a distinction used by the researchers to detect CRS pairs in each studied material.
"We see that these high momentum couples are the reason for these slow-moving quarks," says Hen.
In particular, they found that leaf quarks with larger atomic nuclei (and more proton-neutron pairs) moved at most 20% slower than deuterium, the material with the least number of pairs.
"These pairs of protons and neutrons have this crazy interaction of high energy, very quickly, and then dissipate," says Schmidt. "At that time, the interaction is much stronger than normal and the nucleons have significant spatial overlap. So we think that quarks in this state slow down a lot. "
Their data show for the first time that slowing down the speed of a quark depends on the number of SRC pairs in an atomic nucleus. Lead quarks, for example, were much slower than those made of aluminum, themselves slower than iron, and so on.
The team is currently designing an experiment in which they hope to detect quark speeds, particularly in SRC pairs.
"We want to isolate and measure correlated pairs, and we hope this will produce this same universal function, in that the rate of change of quarks inside pairs is the same in carbon and lead and should be universal in nuclei, "says Schmidt.
Ultimately, the team's new explanation can help illuminate the subtle but important differences in quark behavior, the most fundamental building blocks of the visible world. Scientists have an incomplete understanding of how these tiny particles build protons and neutrons that then combine to form the individual atoms that make up all the material we see in the universe.
"Understanding how quarks interact is really the essence of understanding visible matter in the universe," says Hen. "This EMC effect, although 10-20%, is so fundamental that we want to understand it."
This research was funded in part by the US Department of Energy and the National Science Foundation.
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