Trapping antimatter: antiprotons on hold.

Trapping antimatter: Antiprotons on hold

The trouble with trying to study antimatter is that, in our part of the universe at least, it is made only in high-energy activities of ordinary matter. The antimatter therefore comes out with a great deal of energy and a high velocity. To study antimatter precisely, physicists would like to slow it down, even Perhaps to stop it. One experiment aimed at doing that at the CERN laboratory in Geneva, Switzerland, has managed to capture antiprotons in a device called a Penning trap and hold them for periods of up to 10 minutes.

"People are used to seeing antiprotons whizzing by at the speed of light," says Gerald Gabrielse of the University of Washington at Seattle, one of the experimenters. "Now we have captured and held them in a container a few centimeters long." The report appears in the Nov. 17 PHYSICAL REVIEW LETTERS.

This achievement could make it possible, among other things, to precisely measure the mass of antiproton. The scientists in the group are working on an apparatus to do that. The group members, who include Xiang Fei, Kristian Helmerson, Steven L. Rolston, Robert Tjoelker and Thomas A. Trainor of the University of Washington, Hartmut Kalinowsky and Johannes Haas of the University of Mainz, West Germany, and William P. Kells of Fermi National Accelerator Laboratory in Batavia, Ill., intend to return to CERN with the apparatus late in 1987.

For the last 50 years, acceleration has been a large part of the history of nuclear physics and particle physics. Physicists have built ever more powerful accelerators to endow particles (protons, electrons or ions) with ever higher energies to study finer and finer details of the workings of matter. Now, for antiprotons, the word is deceleration. Only in recent years have proton accelerators been powerful enough to produce such large numbers of antiprotons that deceleration of the antiprotons seemed like a useful idea. CERN has therefore built an apparatus, the Low Energy Antiproton Ring (LEAR), which takes antiprotons, as they are made, with several billion electronvolts energy and "cools" them to an energy of 21.3 million electron-volts.

The present experiment takes the antiprotons as they come out of LEAR and first puts them through a "degrader" made of beryllium, in which they lose energy by collisions with electrons. The antiprotons come out of the degrader with a wide spread of energies, and the thickness of the degrader is adjusted so that the average energy is zero. This means that half the antiprotons get lost in the degrader, but it also means that a sizable number will have energies just above zero. It is these near-zero-energy antiprotons that are employed in the next step.

The Penning trap itself is a series of three electrodes, which are evacuated cylinders and have a magnetic field running lengthwise through them. In the magnetic field the low-energy antiprotons follow helical paths that corkscrew around the field lines in the cylinders.

When the antiprotons enter the trap, the first electrode, known as the entrance-end cap, and the central one are both grounded. The third electrode, the exit-end cap, is connected to a -3,000-volts potential. Thus when antiprotons with less than 3,000 electron-volts energy reach the region of the exit-end cap, they bounce back along the magnetic field lines.

After 3,000 nanoseconds, before the antiprotons can get back to the other end, the entrance-end cap is dropped to -3,000 volts, and the antiprotons are caught in the trap, bouncing back and forth. After some trapping period, which has ranged from 1 millisecond to 10 minutes, the exit-end cap is grounded, and the trapped antiprotons exit to an instrument that counts them. The whole thing is done at a temperature of 11[deg.]K for the ultrahigh vacuum the low temperature helps provide.

Antimatter is supposed to be the exact mirror image of matter, except that for properties that have polarity, the polarity is reversed. Thus an antiproton should be just like a proton except for having negative electric charge. Particularly the mass of one should exactly equal that of the other, or, to put it another way, the ratio of the mass of the proton to that of the antiproton should be 1,000...to an infinity of zeroes.

The experiment Gabrielse and his co-workers are now preparing is intended to measure that ratio by alternately trapping protons and antiprotons in the same trap with the same fields and the same ambient conditions. The size of the helix that a particle makes in the magnetic field depends on its mass, so a comparison of the paths of protons and antiprotons should get the mass ratio directly.

In the past, measurements of the mass ratio have been done by introducing antiprotons into atoms in place of electrons and measuring how the substitution changes the energy-level structure of the atoms. Gabrielse expects that the new method will increase the accuracy of the measurement by a factor of 100 or so. Up to now, nobody has found anything that could be called a deviation of the mass ratio from unity, but who knows what further refinement might turn up?

Other experiments that might now be possible with trapped antiprotons, and that have been suggested from time to time by a number of physicists, include the making of antihydrogen by mixing positrons with trapped antiprotons. Is the structure of antihydrogen the precise mirror image of that of hydrogen?

Another possibility is the making of protonium, a system in which a proton and an antiproton are bound together and orbit each other. The force that holds them is mainly electric, but the strong interaction, the force that holds atomic nuclei together, should contribute a part of it. The strong interaction exerts a powerful attraction between protons and protons, between protons and neutrons, and between neutrons and neutrons. Is it equally strong between proton and antiproton, and is still attractive?