Sweating the small stuff Home

Protons Are Not Round
by Dan Vergano

The humble proton, an atomic particle with mysteries long thought solved, turns out to have a hidden secret, scientists report.

Experimental results released this year by the Department of Energy 's Jefferson Lab in Newport News, Va., have upturned the normally placid world of nuclear physics with the suggestion that protons, the positively charged particles found in the center of every atom, aren't round. Instead, they seem somewhat elliptical.

The round proton has been a staple of textbooks for 40 years, tied to the theory that protons and neutrons are built of three smaller particles called ''quarks'' slowly bubbling inside their interiors.

What difference does it make whether protons are round or elliptical? Plenty, physicists say. Adjustments in protons and neutrons could affect scientific understanding of the magnetic ''spin'' of atoms. Scientists hope to use ''spintronics'' in future computers and tiny ''nano-scale'' devices. Understanding the fundamental shape of particles will affect those application's success.

At a Jefferson Lab meeting in May, about 60 nuclear physicists met to debate the ''crisis,'' in the words of physicist John Ralston of the University of Kansas, over the odd shape of the proton.

During a 10-month experiment, a large team of physicists led by Charles Perdrisat of the College of William & Mary in Williamsburg, Va., gave protons a high-energy smacking-around. The effort involved watching billions of collisions between protons and electrons, which are much smaller negatively charged atomic particles, and recording how they scattered. From those reactions, they assembled a perplexing picture of the proton.

At the May meeting, a consensus emerged that Einstein's theory of special relativity, which explains how things moving near the speed of light have smooshed lengths and increased mass, seemed the likeliest explanation for the weird experiment results.

"The new thing we've figured out is that quarks are moving around inside the proton at relativistic (near speed of light) speeds," says physicist Gerald Miller of the University of Washington-Seattle. Quarks moving at those speeds simply elongate the particle's electromagnetic shape, Miller says. In a paper in the journal Physical Review C, he outlines how quarks moving at high speeds, about 90% of the speed of light, stretch out protons.

"The proton is the simplest thing around, and it is not spherical," says physicist Charles Glashausser of the Rutgers University campus in Piscataway, N.J. The neutron, the uncharged partner-particle to the proton in the nucleus of atoms, also is built of quarks, he notes.

A Jefferson Lab experiment now underway is looking for similar effects, harder to tease out, in the neutron. Neutrons contain three quarks, like protons, but they're organized in a different fashion.

"We've got to understand protons and neutrons. They are all the matter in the nearby universe," Miller says. "These are tremendously exciting new results."