Protons and antiprotons held in the balance.

Protons and antiprotons held in the balance

Created by slamming speeding protons into a metal target, antiprotons begin their lives with far too much energy to allow precise measurements of their properties. But by slowing down these elusive particles -- the negatively charged, antimatter counterparts of protons -- and holding them for extended periods of time in an electromagnetic trap, physicists can begin probing their characteristics.

A research team led by Gerald Gabrielse of Harvard University has now managed to cool antiprotons to a temperature of 4 kelvins and hold them in a trap for several months. The feat enabled the scientists to demonstrate that a proton and an antiproton have the same mass with a precision of four parts in 100 million -- a thousandfold improvement over previous measurements of the proton-antiproton mass ratio.

"That's a big, big increase in precision," Gabrielse says. "One doesn't get to do that very often, and we're hoping to go up by another factor of 10 or perhaps 100 in the future." The achievement also opens the possibility of measuring various other properties of antimatter with sufficient precision to test the validity of present-day theories of the structure of matter.

To make their unprecedented measurements, Gabrielse and his collaborators at the University of Washington in Seattle and the University of Mainz in West Germany overcame a number of significant obstacles, including the antiproton's quick annihilation in any encounter with ordinary matter.

Produced in a low-energy proton accelerator at the CERN laboratory in Geneva, Switzerland, bursts of antiprotons emerge with a kinetic energy of 6 million electron-volts, traveling at a significant fraction of the speed of light. Most of the antiprotons pass through an aluminum plate, which decelerates a fraction of the particles enough to allow their capture in a specially designed electromagnetic trap consisting of a stack of five cylindrical electrodes.

Awaiting the trapped antiprotons is a cloud of electrons already cooled to 4 kelvins. As the antiprotons repeatedly pass through this cloud, they gradually lose energy by interacting with the electrons. They eventually reach an average kinetic energy of less than 1 million-electron-volt, moving at the subatomic equivalent of a snail's pace.

It's like a car plowing through a tunnel full of table-tennis balls, Gabrielse says. "You need lots of collisions, but after a while you really do lose energy. And it turns out to be incredibly effective for cooling."

Indeed, the cooling is so effective and the trap so empty of stray gas molecules that antiprotons can stay trapped for at least 15 weeks. "Our results are consistent with no losses at all over several months," Gabrielse says.

To compare the proton and antiproton masses, the researchers measure the frequencies at which protons, and then antiprotons, orbit around magnetic field lines within the trap. The value of this socalled cyclotron frequency depends on a particle's charge-to-mass ratio. The resulting proton-antiproton mass measurement agrees with the theoretical prediction that particles and their antiparticles must have identical masses.

Gabrielse and his co-workers hope to achieve even greater precision in future measurements, perhaps by reducing the number of antiprotons stored in a trap or by moving the apparatus away from its electromagnetically noisy surroundings at CERN.

"We've concentrated all of our efforts in the last few years on slowing and cooling antiprotons by more than 10 orders of magnitude in energy, and that was a rather large challenge," Gabrielse says. "Now, we and others are beginning to think about how we can use it. We've opened up a new low-energy frontier, and we don't know exactly where it's all going to take us."