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Using generalized particle distributions: Research collaboration seeks 3-D image of the proton

Carlos Muņoz Camacho, a user from the Department of Energy's Los Alamos National Lab, is a co-spokesperson for two new experiments approved for Hall A -- an experiment at 6 GeV in preparation for the 12 GeV Upgrade and a GPD (Generalized Parton Distributions) experiment to run once the upgrade is in place. Visit JLab's website for more scientific information about deeply virtual Compton scattering research conducted at JLab.

A computed tomography - CT - scan can help physicians pinpoint minute cancer tumors, diagnose tiny broken bones and spot the early signs of osteoporosis. Now physicists are using the principles behind the procedure to peer at the inner workings of the proton. In a recent experiment, members of Jefferson Lab's Deeply Virtual Compton Scattering (DVCS) collaboration found that it will be possible to construct three-dimensional images of the building blocks of the proton.

In 1895, German Physicist Wilhelm Röntgen was working in his laboratory when he noticed that a nearby screen was glowing. The glowing screen was Röntgen's first glimpse of the effects of x-rays, a discovery that later won him the inaugural Nobel Prize in Physics in 1901. Seventy years later, researchers tested the first prototype computed tomography, or CT, system. CT systems acquire two-dimensional x-ray images and combine them into three-dimensional pictures, allowing doctors to see cancer tumors, bone fractures and osteoporosis.

Now, physicists are benefiting from the radical idea that transformed flat x-ray films into 3-D pictures of the inside of the human body. They're testing whether it's possible to measure a series of 2-D images of the interior of a proton and then combine them into a single, 3-D image. The research is made possible by a relatively new concept in nuclear physics called Generalized Parton Distributions (GPDs). GPDs are mathematical functions that may one day allow physicists to map out the location and momentum of the building blocks of the proton - quarks and gluons - to provide an essentially holographic picture of the proton's inner structure.

The first dedicated experiment to explore whether nuclear experiments could provide enough information to plug into GPDs for obtaining that 3-D picture of the proton was performed in Jefferson Lab's Experimental Hall A in late 2004. Physicists sent a beam of electrons into a liquid hydrogen target. The researchers were interested in those collisions where a high-energy electron strikes an individual quark inside the proton, giving the quark an extremely large amount of extra energy. This quark then quickly gets rid of the excess energy by emitting a high-energy photon, or particle of light. The whole process takes place without breaking the proton apart. This effect is called deeply virtual Compton scattering (DVCS).

To obtain the new result, researchers needed to measure the energy and angle of scattering of the electron that bounced off the quark and the energy and emission angle of the photon given off by the quark. The researchers used one of Hall A's High Resolution Electron Spectrometers to measure the electron, and the photon was measured by a unique detector built specifically for this experiment, the DVCS calorimeter.

The researchers found that this type of experiment is, indeed, capable of collecting enough information in the future to be used in GPDs for generating a 3-D map of the internal structure of the proton. "This is the most important result of the experiment, because it means that we can use the information from the photon we detect in order to gather information from the quark that emitted that photon. Strictly speaking, it means that we can access Generalized Parton Distributions (GPDs) from DVCS," says Carlos Muņoz Camacho, a User from the Department of Energy's Los Alamos National Lab and lead author on the recent paper in Physical Review Letters.

Thanks to the positive result, Muņoz Camacho sees a future for DVCS research at Jefferson Lab. "The GPD program is at the heart of the scientific motivation for the Jefferson Lab upgrade. A new experiment at 6 GeV in preparation for the 12 GeV Upgrade was approved with the highest scientific rating (A), and a GPD 12-GeV experimental program in Hall A was approved last year," he notes. Muņoz Camacho is a co-spokesperson for both experiments.


As with any major experiment, a number of institutions contributed the people, expertise, funding and equipment that made this experiment possible, including Jefferson Lab, French CEA/DSM/DAPNIA & CNRS/IN2P3, Old Dominion University and Rutgers University. The co-spokespersons of the Hall A proton DVCS experiment are Pierre Bertin (Clermont-Ferrand and JLab), Charles Hyde-Wright (Old Dominion University), Ron Ransome (Rutgers University), and Franck Sabatié (CEA-Saclay). The experimental effort also benefited from a close relationship with JLab theorists Anatoly Radyushkin (ODU) and Marc Vanderhaeghen (William and Mary).


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