In the case of a deuteron — a single proton and neutron "walking lockstep" within the simplest atomic nucleus known — for physicists the basic question is at which point does a nucleon end and its constituent parts, the quarks, begin?
"Everybody agrees that at some distance scale you have to revert to a quark-gluon picture," says physicist Kees de Jager, a hall leader at the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab). "Most suspected this structure would be present at a much smaller scale than the few tenths-diameter of the proton at which we measured in some of our experiments. But it does seem — and the evidence is circumstantial — that at that distance scale a deuteron behaves more like six quarks than two nucleons bound together."
A paper that outlines the results of investigations to determine deutronic structure conducted in the Laboratory's Hall a will be presented during the American Physical Society's Centennial meeting in Atlanta, Georgia, from March 20 through March 26.
Separate but related experiments were conducted in the Lab's Hall C from May through August 1997, and in Hall A from September 1997 through December 1997. The Hall C investigation focused on distance scale of 0.5 fermis (one fermi is 10-15 meters) roughly half the size of a proton. No evidence of finer structure emerged.
"The objective of the Hall C research was to see if there were any deviations from the standard model at this scale," de Jager explains. "In general, there were no surprises. It validated the theory."
By contrast, the story was different in Hall A. There, the resolution was finer, on the order of 0.2 fermis. At that scale, the action of quarks became clearer. Although the evidence is far from conclusive, de Jager says initial findings are persuasive.
"There is some evidence that this is the onset of the quark substructure," he contends. "In the Hall A experiment, the effects of quarks show up. At that distance scale, a deuteron behaves more like six quarks than two nucleons bound together."
More experiments are planned. Additional studies should operate at even smaller distance scale levels.
Submitted: Monday, March 22, 1999 - 1:00pm