Proton not shaped like a basketball

THE EXPERIMENTAL results from the Thomas Jefferson National Accelerator Facility, has questioned some long-held notions about the proton, a primary building block of atoms.

The shape that Gerald A. Miller first saw was not in tune with the general notion. But in time, the findings proved to be right, and led physicists to the conclusion that the proton isn't always spherically shaped, like a basketball.

"Some physicists thought they did the experiment wrong," said Miller, a University of Washington physics professor. "Even I thought so initially. And then I remembered that it looked like something else I thought was wrong — our own conclusion in 1995."

In fact, by 1996 he and two colleagues were ready to publish a paper theorizing the angles at which protons would bounce off electrons after collisions in a nuclear accelerator. The paper described collisions that were quite different.

Miller was sure he had gotten it wrong somehow — until he saw the results of the actual experimental work at Jefferson, a national laboratory in Newport News, Virginia.

What he discovered from those results is that a proton at rest can be shaped like a ball — the expected shape and the only one described in physics textbooks. Or it can be shaped like a peanut, like a rugby ball or even something similar to a bagel.

He was able to use his model to predict the behaviour of quarks, and he discovered that different effects of the quarks could change the proton's shape.

The model showed that the highest-momentum quarks, those moving nearly at light speed inside the proton, produced the peanut shape.

If the quarks are moving more slowly, the surface indentations of the peanut shape fill in and the proton takes on a form something like a rugby ball, or a beehive.

The slowest quarks produce the spherical shape that physicists generally expected to see. Another shape a flattened round form like a bagel — is sort of a cousin to the peanut shape with the high-momentum quarks.

In the peanut shape, the quarks spin in the same direction as the proton, while in the bagel shape they spin in the opposite direction as the proton.

Miller presented his findings at an invited talk at the American Physical Society meeting in Philadelphia.