Nucleons Go Two-by-Two

Scientists using the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) have just completed one of the first clear measurements of nucleon pairs in nuclei. Their findings are reported in a paper recently accepted by the journal Physical Review Letters, "Two-Nucleon Momentum Distributions Measured in 3He(e,e'pp)n."

Physicists directed Jefferson Lab's continuous electron beam at an energy of 2.2 GeV (billion electron volts) toward a target of helium-3 nuclei. Unlike ordinary helium nuclei, helium-3 is made up of three nucleons, two protons and a neutron. The scientists then reconstructed the subatomic collisions with the unique CEBAF Large Acceptance Spectrometer (CLAS) in Hall B, one of Jefferson Lab's three experimental halls.

"Nuclear Physicists have spent the last 30 years measuring the behavior of single protons in nuclei," explains Larry Weinstein, professor of Physics at Old Dominion University and Chair of the CLAS Collaboration. "Thanks to the capabilities of the CLAS spectrometer (and a bit of luck), we have now taken an important step toward measuring the behavior of protons in pairs."

To help visualize the particles' movements Weinstein compares them to people exhibiting human behavior. While most people tend to pair up, only about a quarter of nucleons exist in pairs at a time. Human relationships can endure for decades, but nucleon pairs last only a fraction of a second. However, like some people, at great distances nucleons seemingly ignore each other, at medium distances they attract each other and when they get too close, they violently repel each other.

Weinstein's experiment measured the behavior of very close nucleon pairs. He, along with ODU graduate student Rustam Niyazov and their collaborators, reconstructed billions of collisions to find the 3,000 events where one of the three nucleons of helium-3 was knocked out cleanly, leaving behind an almost undisturbed nucleon pair. Unpaired nucleons move relatively slowly, with a speed that rarely exceeds 20 percent of the speed of light. The paired nucleons were measured to have speeds up to 60 percent of the speed of light, providing clear experimental evidence of the strong interactions between two nucleons.

Rustam Niyazov worked on this experiment for his graduate thesis and is now a postdoctoral fellow at Jefferson Lab. He says there are two main reasons why nobody has measured this reaction before. Firstly, it's very hard to extract a clean signal; there are so many interactions between final-state particles that the signals of interest are hidden. And secondly, the CLAS detector allows scientists to measure the final states of many particles.

Weinstein echoes Niyazov's comment. "What makes CLAS unique for this experiment is that you're measuring everything," says Weinstein. "That means you can discover patterns in the data that you would never have gotten otherwise, and greatly increases the scope for pleasant surprises."

"The 'a-ha' moment came about three years ago, 10 years after first proposing the experiment and one year after running it," says Weinstein. But then, physicists have been waiting 30 years for measurements of paired nucleons.

CEBAF at Jefferson Lab emits high-energy beams of electrons that are used to study the nucleus of the atom. In this experiment, particles from the collision were detected in the CEBAF Large Acceptance Spectrometer. The CLAS is designed to detect almost all of the charged elementary particles (electrons, protons and pions, etc.) that emerge from an electron-nucleus collision. The spectrometer, a 30-foot diameter, multimillion-dollar particle detector, has six layers of detectors arranged around a toroidal superconducting magnet.

Jefferson Lab is a basic research, nuclear physics user facility managed by the U.S. Department of Energy and operated by the Southeastern Universities Research Association, a consortium of 61 universities. Old Dominion University is a state-assisted university located in Norfolk, Va. The university's nuclear physics program includes 11 tenured, internationally known faculty. Nuclear physics research at Old Dominion is supported in part by grants from the Department of Energy.