Big Results From Small Detector


“Picture this in the jaws of a magnet,” says Wit Busza, a nuclear physicist at the Massachusetts Institute of Technology. “Look in here. This is where the particles went in.”

He pointed to the inside of a tiny tube covered in a tinier silicon grid, a crucial piece of the Phobos particle detector.

One of four detectors that made up Brookhaven’s Relativistic Heavy Ion Collider (RHIC), Phobos recorded the properties of a special substance known as quark-gluon plasma from 2000 to 2005. Physicists are only now sorting through the last of the small detector’s wealth of data.

Though particle physicists study the universe’s smallest inhabitants, their laboratories are often the realms of big machines, multiple-story detectors in seemingly endless underground tunnels. Phobos’ three main pieces, though, each can fit on a desk. Like model ships in bottles, they now sit in separate glass boxes on the fourth floor of MIT’s Building 24.

In 1989, Busza had imagined a much larger experiment. “It was called the Modular Array of RHIC Spectra, or MARS,” he says. “It got rejected. It was too expensive.” Back at the drawing board, he designed a smaller, cheaper detector: “A colleague of mine said that, since MARS was rejected, I should name this one after a moon of Mars.” In 1999, Brookhaven finished construction. Phobos was born.

Protons and neutrons may be the building blocks of every atomic nucleus, but they are also made of other particles. These particles are called quarks and are held together by force particles, appropriately named gluons. But unlike the particles you know—the ho-hum protons, neutrons, and electrons—quarks cannot exist by themselves, at least not at the energies of our everyday experiences. To roam free, even for an instant, quarks need energies that approach those of the Big Bang.

Brookhaven’s collider created those energies—corresponding to temperatures roughly a billion times hotter than the surface of the Sun—by accelerating gold nuclei to 99.995 percent of the speed of light, racing them 2 miles around an underground track, and then slamming them together to create a glob of intense heat and pressure.

Phobos watched—looking at the particles that formed as the plasma cooled—to tease out the properties of the tiny fireball. “The thirtieth of June 2000. It was one of those dates that, years later, you still remember exactly what you were doing,” says Gunther Roland, another MIT nuclear physicist and collaborator in the project. Before sunrise that summer morning, Phobos observed its first collisions.

Roland described the intense research that followed. “I ran into a friend from one of the other experiments,” he says. “He claimed to have already written the first paper . . . I thought, ‘let’s see about that.’”

“We were working on this fourteen hours a day, seven days a week,” continues Roland, “and we were first.”

These results, published only months after the first tests in 2000, and other findings during the five-year lifetime of the small detector, added to physicists’ understanding of the material properties of the plasma. Among other things, the high-energy substance, though theoretically believed to act like a gas, turned out to be less viscous than water, better able to flow than any known substance.

The experiment also provided new insights into the earliest moments after the Big Bang, when the entire universe was about the size of our solar system. “Things worked out rather well for us,” Busza says with a modest smile.

“It was a small experiment, an exploration,” adds Johann Rafelski, professor of physics at the University of Arizona.

Comparing Phobos to Brookhaven’s two largest detectors, STAR and PHENIX, he praised the relatively inexpensive detector, able to produce results quickly, for keeping everybody on their toes. “Especially for big experiments, there is an enormous opportunity to have errors,” notes Rafelski. “The presence of Phobos kept everybody honest—it was wonderful.”

Roland is now leading a group at CERN’s Large Hadron Collider and looks forward again to the intense research schedule. “I hope we have that much fun at the LHC,” he says from his MIT office.

Still, he hasn’t forgotten the tiny, retired detector downstairs: “I hope I get to take a piece of it home.”


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