Taking a smash peek deep inside the proton.

In the modern picture of the constituents of matter, the proton's composition appears straightforward: two "up" quarks and one "down" quark, all tied together by gluons. But there's nothing simple about how a proton is put together--or how it falls apart. Hit a proton hard enough, and it can fragment in surprising ways.

Researchers using the Hadron Electron Ring Accelerator (HERA) in Hamburg, Germany, have discovered that a proton exhibits a peculiar sort of lumpiness that apparently doesn't correspond to individual quarks or gluons. The results of high-energy collisions between electrons and protons suggest that an electron sometimes penetrates deeply enough to encounter a new kind of object buried within the proton.

"We weren't looking for this," says Allen Caldwell of Columbia University, a member of the team running the ZEUS detector at HERA. Caldwell reported the team's findings at an American Physical Society meeting this week in Arlington, Va.

Researchers have long used electron beams as probes to study the structure of the proton and other subatomic particles. Typically, when an electron transfers a lot of momentum to a proton, the collisions forces the ejection of a quark. But the ejected quark can'[ exist by itself. It interacts with other quarks and antiquarks created out of the vacuum as it separates from what's left of the proton.

These quarks combine in various ways to create a jet of assorted subatomic particles that sprays out in the same direction as the ejected quark. The proton remnant also interacts with extra quarks and antiquarks to produce another jet of subatomic particles.

Researchers normally detect the deflected electron and two distinctive jets of subatomic particles. However, the ZEUS team found that nearly 10 percent of the time, only one jet appears. "This was a very surprising result, given that the proton was getting such a large kick," Caldwell says.

The most likely explanation is that the electron is deflected not by a quark, but by some other, unknown object--perhaps a particular combination of quarks and gluons--which then breaks up to form the observed jet and leaves behind a proton remnant that somehow stays intact.

"Different possibilities have been put forward," Caldwell remarks. "But the complete solution remains largely a mystery."