Seemingly reasonable assumptions define the human view of the world. But ruled of thumb can mislead — or be altogether incorrect. Fortunately, in physics, most assumptions can be tested. Those that don't measure up are amended.
An experiment at the Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) has determined that the quark-carried charge distribution on a proton is larger than its magnetic distribution. Currently, the reason for the disparity is not known or understood.
A paper detailing the outcomes of the Lab's nucleon-form-factors related investigations will be presented during the American Physical Society's Centennial Meeting in Atlanta, Georgia, from March 20 through March 26.
"Everybody assumed that both the charge distribution and the magnetic distribution were the same," says physicist Kees de Jager, a Jefferson Lab hall leader. "So the theorists didn't pay much attentions. Now that we know both are different, we have to find an explanation. Our work has definitely generated a lot of interest."
While the findings don't appear to challenge the basic tenets of current quantum chromodynamics theory (QCD) — theory nevertheless can't yet account for whatever underlying mechanism is responsible for the size difference in charge and magnetic distribution. In any case, the Lab's findings are generating both attention and enthusiasm among theoreticians and experimentalists. Researchers at the Lab and elsewhere will have their hands full trying to solve the mystery.
"Measuring what a theoretical model predicts is exciting," de Jager says. "Finding something no one expects is very exciting."
Additional form-factor experiments have been planned which will probe the proton charge distribution within even smaller spatial resolution. 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.