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Measuring Polarizabilities:
How the Experiments Will Work

By Eric Gorton, JMU Public Affairs

In particle experiments at Duke, gamma rays will be aimed at plastic targets containing hydrogen atoms that have been polarized. When the gamma rays, which are also polarized, interact with the nuclei of the hydrogen atoms, they cause a rearrangement of the quarks inside the neutrons and protons. The internal response of a nucleon to the electric and magnetic fields of the photon at low energies is characterized with six numbers called polarizabilities. These polarizabilities affect how the gamma rays scatter. Thus, when the scattering of gamma rays is measured, researchers can obtain information from which the polarizabilities can be extracted.

“There have been a variety of measurements stretching back almost 20 years, looking at these properties of the proton and the neutron," said Steve Whisnant, head of the JMU physics department. "These things are called polarizabilities and that’s because they describe the response of the nucleon to the applied electromagnetic field of the photon. A nucleon is sitting here being happy and a photon comes in. A photon is just a chunk of an electromagnetic field. It has an electric and magnetic field and when they pass over the neutron or the proton, (the quarks inside the neutron and proton) respond to this. Those quarks have charges. Charges move in an electric field. Since the charges themselves are already moving, they make little currents, they circulate around. So, if I have some plus charges and some minus charges inside a proton and a neutron, the electric field (of the photon— the gamma ray) will make them separate a little bit and how far apart you can pull them within a given field is called a polarizability.”

With the enhanced detectors at the Duke laboratory, Whisnant expects to get better information about the polarizabilities than what has been achieved before.

"When we have polarized light (the gamma ray) and a polarized target, there are details of the scattering process that are washed out, when everything is just averaged over and you just measure some big average value. Those are well known. What we are trying to find are the things that get averaged over. All the details, because certain parts of those details now depend on interesting properties of the nucleon, of the neutron-proton. And that's what we're trying to measure, are those interesting properties. We believe we can measure more of these properties and we can measure them with greater precision than what has been done before," Whisnant said.

The new annular sodium iodide detectors make detection of scattered gamma rays possible. The detectors will enable researchers to correct for photons that scatter out and improve resolution of the particles they want to see.

"I think this is really cool. This is going to be good stuff. I think that we're going to be able to get some really dandy results with this. We have to just make it work," Whisnant said.

The experiments at Jefferson Lab will be similar to those at Duke, but will use much higher energy and will be able to produce new particles.

"One of the things that's going to be really cool about this is that one of the particles that comes out, that we'll be able to produce with this, is called Lambda. It's one of the zoo of particles that happens to decay in a particular way. We never see it directly. It doesn't live very long, but we can see its decay products and we can infer its mass from detecting the things that it decays into. ... So we have the opportunity now to do something that nobody's ever done before. ... we will have an experiment in which everything that could be measured will be measured. All the polarizations, all the momentum, all the angles.”

Published February 2008