August 29, 2003

ODU AND JEFFERSON LAB MEASURE PAIRED PROTONS


Old Dominion University

Just as people behave differently as couples than as individuals, protons
and neutrons (also known as nucleons) inside the nucleus of the atom behave
differently in pairs. Scientists at Old Dominion University and the Thomas
Jefferson National Accelerator Facility (Jefferson Lab), led by ODU physics
professor Larry Weinstein, have just completed one of the first clear
measurements of nucleon pairs in nuclei. Their findings are reported in a
paper submitted to the journal Physical Review Letters, "Two-Nucleon
Momentum Distributions Measured in 3He(e,e'pp)n."

The recent Old Dominion/Jefferson Lab experiment aimed a beam of two
billion-electron-volt electrons at a target of Helium-3 using the
Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance
Spectrometer to detect the electrons that bounce off and the protons that
are knocked out. The scientists then reconstructed what was happening prior
to the subatomic collision.

"Nuclear Physicists have spent the last 30 years measuring the behavior of
single protons in nuclei," said Weinstein. "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."

While most people 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 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 Rustam Niyazov, a graduate student in the Nuclear and
Particle Physics Group 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 violent coupling between two nucleons.

CEBAF at Jefferson Lab emits high-energy beams of electrons that are used
to study the nucleus of the atom. In this recent experiment, particles from
the collision were detected in the CEBAF Large Acceptance Spectrometer
(CLAS). 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. Old Dominion, in partnership with
Jefferson Lab, built one of the six CLAS detector layers.

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 is a
state-assisted university located in Norfolk, Va. The university's nuclear
physics program includes eleven tenured internationally known faculty.
Nuclear physics research at Old Dominion is supported in part by grants
from the Department of Energy.

For more information, contact Weinstein at (757) 683-5803 or
weinstei@physics.odu.edu. You can see an animation of a collision at
http://www.physics.odu.edu/~weinstei/eventloop.gif.

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