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New Antimatter Beam Revved Up by ODU's Weinstein and Colleagues at Jefferson Lab

ODU nuclear physicist Lawrence Weinstein

Researchers at the Thomas Jefferson National Accelerator Facility in Newport News have devised an experiment that utilizes a new kind of antimatter beam to probe the fundamental building blocks of matter.

Lawrence Weinstein, University Professor of physics and part of the experimental nuclear physics group at Old Dominion University, leads an international team of scientists that plans to create the most intense high-energy beam of both matter and antimatter the world has ever known.

This beam will contain both negatively charged electrons and positively charged positrons, which are the anti-particle of electrons. The electrons and positrons in the mixed beam will not annihilate each other because they are spread over a distance of about two inches, Weinstein said.

Work begins this week on the project in experimental Hall B at Jefferson Lab.

At the core of the experiment is the 5.5 billion electron-Volt electron beam created by the facility's mile-long accelerator. High-energy electrons will pass through a thin metal foil, and about 1 percent of them will radiate a high-energy gamma ray, Weinstein said. Electrons will be separated from the gamma rays by a large magnet. When the gamma rays, which are virtually pure energy, pass through another thin metal foil, about 5 percent of the rays will transform into matter in the form of electron-positron pairs.

Another set of magnets will separate the matter and antimatter beams, block the gamma rays, and then recombine the matter and antimatter beams. Weinstein said the mixed beam will be used to study the structure of the proton.

This method of creating a mixed, matter-antimatter beam was the brainchild of Bernhard Mecking, a retired Jefferson Lab researcher who also formerly taught at ODU.

"So far as I know, every other positron beam has been made by smashing relatively low-energy electrons into material, collecting the positrons, and then accelerating them," Weinstein explained. "This is the only design where we are starting from high-energy electrons and ending up with a mixed identical beam of electrons and positrons.

"We tried this method in an engineering run in 2006. It was so successful that we were approved to do the experiment."

Jefferson Lab is perfect for the work, according to Weinstein, because of its high-energy initial electron beam and because Hall B has a place to dump electrons after the intermediate photon beam is created.

With the new mixed beam, physicists at Jefferson Lab hope to precisely measure the difference between how the high-energy negatively charged electrons and positively charged positrons collide with protons. This will reveal new information about how electric charge is distributed in the proton.

"Right now, there are two different measurements of how electric charge is distributed in the proton that disagree by a factor of three in places," Weinstein said. "As you might imagine, this difference is causing serious difficulties in our understanding of the proton."

These existing measurements, oddly enough, indicate that electrons and positrons interact with the proton in almost identical fashion. Although the electron is attracted to the proton and the positron is repelled by it, these measurements show both have an equal probability of scattering to the left or to the right.

"By using a mixed electron-positron beam and measuring their interactions simultaneously, we will be able to measure the difference between their interactions very precisely," Weinstein explained. "The dramatic difference in measurements we have now could be explained by a several percent difference between electron and positron scattering from the proton."

Physicists involved in the experiment got a workout helping to build the more than 20-foot-long apparatus, called a chicane, that will create the mixed beam and channel it toward the target protons. The apparatus includes two concrete and lead tunnels connecting three large magnets. "And, yes, physicists get to help stack all of the concrete and lead that is needed," Weinstein said.

This article was posted on: November 30, 2010

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