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Weinstein Physics Paper Reports Advance in Study of Free Neutrons

Lawrence Weinstein

Lawrence Weinstein, University Professor of physics at Old Dominion University and a researcher at the Thomas Jefferson National Accelerator Facility in Newport News, is lead author of a new research paper reporting the first ever experiment-based evidence of quark structure and behavior inside free neutrons.

The paper, "Short Range Correlations and the EMC Effect," was published Feb. 4 in Physical Review Letters.

The major hurdle for scientists who study the internal structure of the neutron is that most neutrons are bound up to protons inside the nucleus of atoms. In nature, a free neutron lasts for only a few minutes, while in the nucleus, neutrons are always encumbered by the ubiquitous proton.

To get a description of a free neutron, Weinstein and several collaborators compared data collected at Jefferson Lab and the SLAC National Accelerator Laboratory in California that detail a correlation between two very different effects on the bound protons and neutrons in the nucleus of the atom.

"The work allowed us to extract information about the free neutron's quark structure experimentally, without reliance upon a theoretical model," Weinstein explained.

Nucleons - another name for both protons and neutrons - appear to differ when they are tightly bound in heavier nuclei versus when they are loosely bound in light nuclei, the researchers reported. In the first effect, experiments have shown that nucleons tightly bound in a heavy nucleus pair up more often than those loosely bound in a light nucleus.

"The first thing was the probability of finding two nucleons close together in the nucleus, what we call a short-range correlation," Weinstein said. "And the probability that the two nucleons are in a short-range correlation increases as the nucleus gets heavier."

Other experiments have shown a clear difference in how the proton's building blocks - quarks - are distributed in heavy nuclei versus light nuclei. This difference is called the EMC Effect.

"People were measuring and discussing the EMC Effect. And people were discussing things about the short-range correlations effect. Nobody bothered to look to see if there's any connection between them," said Eliezer Piasetzky, a professor at Tel Aviv University in Israel and a Jefferson Lab researcher who is a co-author of the paper.

When the research team combined the data from a half-dozen experiments regarding these two different effects on one graph, they found that the two effects were correlated.

"Take a quantity that tells you how strong the EMC Effect is. And then take another quantity that tells you how many short-range correlations you have," said Doug Higinbotham, a Jefferson Lab staff scientist and also a co-author. "And you see that when one is big, the other one is big. When one is small, the other one is small."

Added Weinstein, "We found that paired nucleons not only behave differently from single nucleons, the quarks inside them also behave differently."

The scientists say that it's unlikely that one effect causes the other. Rather, the data shows that there is a common cause for both.

The group says the common cause may have remained a mystery for so long, because while the two effects they are studying are obviously related when laid out on a graph, the connection was previously obscured by the different, yet related ways in which the two effects are studied.

"What's very new here is that we have linked two fields that were completely disconnected. So now you can start asking questions about what that connection can help us learn," Higinbotham said.

The scientists say the next step is to further compare the data from all of the source experiments that they used in their analysis to see if data for one effect may now be used to learn something new about the other. Then, they'd like to use the knowledge that the two effects are connected to design new experiments for shining a light on other secrets buried in the nucleus of the atom.

This work was supported in part by the U.S. Department of Energy, the National Science Foundation, the Israel Science Foundation, and the U.S.-Israeli Bi-National Science Foundation.

This article was posted on: February 8, 2011

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