New probe reveals unfamiliar inner proton

Physicists have known since the turn of the past century that protons lie at the heart of all ordinary matter. But this elementary component of nature has kept from scientists some secrets about its inner workings.

Blocks of lead-enriched glass in tall rack (center) helped detect surprises about protons. Hall A Collaboration/Jefferson Lab

Researchers taking one of the closest looks yet into the intact proton have found an unexpectedly complex interior electromagnetic environment. The new findings come as a shock to nuclear and particle physicists who have considered this aspect of the proton to be well understood for more than a half century.

The results “have drastic consequences for the way we understand what the proton is made of,” says Charles F. Perdrisat of the College of William and Mary in Williamsburg, Va., one of the experiment’s leaders.

To probe inside the proton, Perdrisat and his 80 or so collaborators fired a powerful beam of electrons at a beer-can-size cylinder of frigid liquid hydrogen. By slamming the electrons into the single-proton nuclei of hydrogen atoms, the researchers have peered more deeply into the intact proton than ever before–as deeply as 90 percent of the way toward its center. The physicists carried out the work at the Thomas Jefferson National Accelerator Facility in Newport News, Va.

“It’s really beautiful work,” comments theorist T. William Donnelly of the Massachusetts Institute of Technology.

Protons are remarkably minuscule. If a 1-meter length were enlarged to span the distance from Earth to the sun, the radius of a proton would still measure no more than the width of a human hair.

Yet scientists have long known that protons contain still smaller objects known as quarks and gluons. Researchers have verified the presence of those constituents by using different kinds of experiments that shatter the protons.

Perdrisat and his colleagues’ experiments, instead, examine protons that are unbroken. By carefully measuring how the accelerated electrons scatter from the proton’s quarks and gluons, the scientists inferred new details about the proton’s electromagnetic structure.

Over the past 50 years, researchers have used electron-scattering patterns to measure the distribution of local magnetic field strength, or magnetization, and electric fields in the proton. These two fields reflect the presence of quarks and gluons.

Quarks have electric charge and therefore also can induce magnetic fields when they move, but gluons are uncharged. Both particles also have a quantum mechanical property known as spin, like the twirling of a top, which contributes to magnetic fields.

Previous measurements have shown that both magnetization and electric charge fall off sharply with distance from the proton’s center. To date, the distribution of magnetization has been more clearly discernable than that of the electric charge.

In the new electron-beam studies, which began in the late 1990s, researchers have made use of electrons that are polarized, which means their spins all align in the same direction. Collisions with the electrons make the protons polarized as well. That provides the information for a crisper picture of the proton’s electric-charge distribution, Donnelly says. Only recently have polarized beams of electrons become feasible.

Last week in Washington, D.C., at a meeting of the American Physical Society, Perdisat’s colleague Olivier Gayou, also of William and Mary, presented preliminary findings from the team’s most recent experiment. In it, the electrons imparted to the protons more than 65 percent their energy, the highest transfer so far in such experiments.

The data from this experiment suggest that, while magnetization may still rise smoothly from the proton’s edges to its center, the proton’s electric charge has a more complicated distribution. For example, says Kees de Jager of the Jefferson lab, charge may actually increase with distance from the center at certain radiuses.

Theorist Ulf G. Meissner of the National Research Center in Jülich, Germany, admits to being stumped by the new results and wonders if they will hold up. He says he knows of no model of the proton that would lead to such bizarre distributions of the electric field. Says Meissner, “For me, it’s a real puzzle.”

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