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Physicists at Old Dominion University are involved in an accelerator experiment that is shedding new light on the distribution of elementary quarks inside the proton, and the work is the subject of an article in the March issue of Physics Today.

The article, headlined "A step toward tomography of protons," reports on an experiment at the Thomas Jefferson National Accelerator Facility in Newport News. A paper about the work was published in December in Physical Review Letters.

ODU physicists working on the project include professors Charles Hyde-Wright, Lawrence Weinstein and Paul Ulmer, and associate professor Moskov Amarian. Other ODU co-authors of the paper in Physical Review Letters are Gagik Gavalian, a postdoctoral researcher, and two graduate students, David Hayes and Hassan Ibrahim.

Medical tomography-which is known as computed tomography and often referred to as a CAT scan-builds a three-dimensional image of an internal part of the body by imaging one slice of the part at a time and stacking the slices. With the Jefferson Lab atom-smashing accelerator, a particular type of imaging event called deeply virtual compton scattering (DVCS) can be detected when an intense beam of electrons hits a hydrogen target. Computed integration of the DVCS evidence can provide a three-dimensional model of the spatial position of quarks inside the proton.

In an interview last year, Hyde-Wright described in general terms how DVCS works. "We use one photon to excite the atomic nucleus, and then use the re-emitted photon to essentially take a picture of the nucleus." He predicted then that the project would provide the first ever three-dimensional images of the quark waves that comprise the proton. "Our larger goal," he added, "is to understand the origin of the mass of ordinary matter. Ninety-eight percent of the mass of ordinary matter comes from the quark-anti-quark and gluon waves in the atomic nucleus."

Hyde-Wright and Amarian, as well as nuclear theorist Anatoly Radyushkin, ODU professor of physics and eminent scholar, are known for their pioneering work with DVCS. Weinstein and Ulmer have been involved in a variety of experiments probing the properties and behavior of protons and neutrons.

Jefferson Lab's Program Advisory Committee (PAC) has allocated generous amounts of time for DVCS experiments over the near future, underscoring the interest among nuclear physicists in the results of "tomography of the proton" research. For a DVCS experiment expected to run in 2009, the PAC allotted 23 days of accelerator beam delivery in Jefferson Lab Hall A. ODU physicists will build new detector equipment for this experiment. DVCS experiments will also be one of the "flagships" of the proposed $300 million doubling of the Jefferson Lab electron beam energy.

This article was posted on: March 16, 2007

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