Improving The Picture Of Nucleon Structure

In a series of experiments at the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab), researchers are moving toward a substantially improved picture of nucleon structure and behavior.

"There are clearly new things we see in the data," says Volker Burkert, a senior staff scientist in the Lab's Physics Division. "We see preliminary evidence for additional excited states we haven't seen before. We believe our contribution will provide a much better understanding of the three-quark system."

Ten papers that deal with the results of nucleon-resonance excitation investigations conducted in Hall B will be presented during the American Physical Society's Centennial Meeting in Atlanta, Georgia from March 20 through March 26. Since Hall B began its experimental program in 1997, more than four dozen experiments have been approved, involving researcher teams from 30-plus universities and institutions in the Unites States, Armenia, France, Italy, Russia and South Korea.

Hall B experiments focus on three of the six known flavors of quarks: the up, down and strange. Scientists are attempting to better comprehend the interactions between those quarks and other particles, known as gluons, that hold quarks together in the form of protons and neutrons.

A Key component in Hall B operation is a unique particle detector, one of the most complex pieces of equipment ever designed and built at Jefferson Lab. The CLAS, or CEBAF Large Acceptance Spectrometer, was constructed over seven years at a cost of $60 million.

The detector's components include time-of-flight counters, energy-measuring calorimeters and particle-tracking drift chambers. CLAS records, on average, 2,200 particle interactions per second on 40,000 data channels. Software developed by physicists working closely with the CLAS research teams have produced data analysis and reconstruction programs for Hall B that interpret experimental results, adjusting for CLAS system changed and weeding out those events that are incomplete or degraded by background or equipment noise.

"The detector is working very well", says Lab scientist Burkert. "We understand its features and are able to calibrate it closely. Soon we'll be increasing out capacity to obtain an order of magnitude more data". As a matter of fact, the first few months of CLAS operation doubled the world's data on electroexcitation of the nucleon.

Equipment and software upgrades should boost Hall B's rates of data acquisition by almost 50 percent, from 11 megabytes to as many as 16 megabytes per second. At such speed, Hall B can produce a trillion bytes of raw information per day, so a week or two of data has as many bytes as the total number of characters contained within the 17 million books currently catalogued in the Library of Congress.

Other pending Hall B experiments will study the behavior and effects of nucleons within the atomic nucleus. These are slated to begin in April and run through the end of calendar year 2000.

The laboratory, in Newport News, VA, is managed and operated for the U.S. Department of Energy By Southeastern Universities Research Association, Inc., a consortium of 44 universities in the southeast.