Tardy antiprotons trapped in liquid helium.

Antiprotons--the negatively charged, antimatter counterparts of protons -- normally last just a few nanoseconds in the presence of ordinary matter. But researchers have now discovered that some of the antiprotons fired into a vat of liquid helium survive as long as 15 microseconds.

"This is a thousand times longer than the normal lifetime of an antiproton in a system like this," says particle physicist Peter Kitching, who works at the TRIUMF cyclotron at the University of British Columbia in Vancouver. The finding suggests that a small number of antiprotons get captured by helium nuclei and settle into atomic arrangements that delay proton-antiproton annihilation.

T. Yamazaki of the University of Tokyo, Kitching and their collaborators describe the new results in the Sept. 2 PHYSICAL REWIEW LETTERS.

The researchers fired a beam of antiprotons into a liquid-helium target, then used a sodium iodide detector to measure the energy--in the form of unstable particles known as pions--coming out of the target. From these data, they deduced the proton-antiproton annihilation rate.

"You see a big peak at the very short times corresponding to the fact that most of the antiprotons annihilate almost immediately," Kitching says. But the plotted results also show a long tail, indicating that about 3.6 percent of the antiprotons somehow delay annihilation. Most of these antiprotons appear to last an average of 3 microseconds, although a few survive longer.

The tardy antiprotons apparently get trapped in relatively long-lived atomic states, in which an antiproton temporarily replaces one of the two electrons bound to a helium nucleus. For a brief period, the orbiting antiprotons stays far enough away from the protons in the nucleus to forestall immediate annihilation.

"We saw it in liquid helium, where it was predicted to occur, but when we put the antiprotons into other things like liquid nitrogen, there was no long-lived state apparent," Kitching says.

A possible next step in the experimental work, he suggests, would be to determine whether the lifetimes of these trapped-antiproton states depend on the temperature or physical state of helium. "It would be very exciting to see a state that was in some sense stable," Kitching notes. "That would offer a means of storing antiprotons. You could put some into liquid helium and carry them around."

"We haven't found that," he adds, "but we have seen long-lived states that are much longer than the normal lifetimes of antiprotons."