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'Skinny' pions sail through nucleus

Skinny particles. An electron (bright green) has just scattered from a nucleus and created a pion (green-skinned particle). This pion's quarks are so tightly packed that they nearly cancel each other's color charge, allowing the pion to slip through the nucleus without interacting, an effect now seen at the lowest possible energies.

A recent Jefferson Lab experiment may have demonstrated the onset of color transparency for pions, a necessary ingredient for interpreting related experimental results in nuclear and particle physics. The experiment was performed in Jefferson Lab's Experimental Hall C.

In the experiment, scientists used JLabís electron beam to produce pions inside the nucleus of the atom.

"An electron hits a proton, creating a pion from it. And then this pion has to travel through the rest of the nucleus before it can come out and be detected," says Dipangkar Dutta, an assistant professor at Mississippi State University and a spokesperson for the experiment.

Like protons and neutrons, pions are built of smaller subatomic particles called quarks. These quarks are held together by the strong force, sometimes called the color force. Thus, each quark has a color charge and can feel color charges from other quarks; like color charges repel and different color charges attract.

Normally, pions traveling through the nucleus feel the color charges from the quarks inside the protons and neutrons they encounter. These pions are often absorbed back into a proton or neutron before they can exit the nucleus. Theory predicted that if pions were made in smaller and smaller sizes, eventually the pions would be so small, that the color charges of the two quarks inside would not feel the color charges of the other quarks outside, making the pion invisible to protons and neutrons. According to this theory, pions of different sizes are produced by adjusting the momentum exchanged in the electron-nucleus collision.

In this manner, scientists produced pions of different sizes and measured how many of each size escaped the nucleus. The result was published in the Dec. 14 issue of Physical Review Letters. The experimenters found that as the size of the pions decreased, the number that made it out of the nucleus increased.

Eventually, the pions produced should become small enough that all of them escape the nucleus.

"The pion doesn't see the protons and neutrons anymore. So the pion moves as if there are no longer any nucleons. This unique phenomenon is known as color transparency, and itís been used to describe high-energy scattering experiments. Witnessing the onset of this phenomenon has, however, remained elusive until now," Dutta says.

He says finding the onset of color transparency bridges the gap between traditional nuclear physics, which considers the nucleus as protons and neutrons, and traditional high-energy physics, where smaller particles such as quarks are primarily studied.

Jan Ryckebusch is a staff member in the Department of Subatomic and Radiation Physics of Ghent University in Belgium. He and his colleagues calculated the nuclear physics view of what would happen as pions were produced in the nucleus.

"We used a model that tries to describe traditional nuclear physics as well as we could and then see whether it describes the experiment," Ryckebusch says. "You do the best you can in traditional nuclear physics, and at some point you see that the reality deviates from your model."

This experiment found that deviation, the onset of color transparency, where the nucleus appears to be made up of simply quarks and gluons instead of quark-built protons and neutrons.

"Now for the very first time, by looking at smaller objects like the pion, they are starting to see deviations from the traditional nuclear physics view," Ryckebusch comments.

The experiment is described in Massachusetts Institute of Technology Ph.D. student Ben Clasie's doctoral dissertation.

Dutta says the scientists are now planning an experiment to determine the onset of color transparency for particles larger than pions, such as protons. "Now that we've seen it with pions, we expect to continue this program with protons with the 12 GeV Upgrade. And perhaps we'll see it turning on there."


Technical Paper: Measurement of Nuclear Transparency for the A(+) Reaction B. Clasie et al. (Link: http://link.aps.org/abstract/PRL/v99/e242502)

Also featured by Physical Review Focus: Transparent Nuclei, Dec. 21, 2007


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