Star of Annihilation

One of physics' most exciting recent discoveries is the antiproton. It resembles an ordinary proton (present in the nuclei of all atoms), except that its electric charge is negative instead of positive. There may be (but probably are not) places in the universe where antiprotons can exist permanently, but on earth they are short-lived. As soon as one of them touches the nucleus of an ordinary atom, it is annihilated. Both its own matter and the matter of a proton or neutron in the nucleus turn into a flash of energy.

Antiprotons were created last fall by the world's most powerful particle accelerator, the Bevatron, at Berkeley, Calif., but they could be detected only indirectly by a complicated electronic method (TIME, Oct. 31). Scientists wanted to "see" them by one of the more direct methods that they use to make subatomic particles visible. So the Berkeley scientists shot antiprotons from their great machine into a stack of photographic films. Their hope was that they would find microscopic tracks in the films that could be identified as the work of an antiproton.

At last week's New York meeting of the American Physical Society, Dr. Owen Chamberlain of the University of California showed a drawing of films exposed to antiprotons (see cut).

The wheel-like star tells what happens when one of the alien particles is annihilated. The antiproton (P- ) enters from the left. It is moving fast at first, but gradually slows down and merges with the nucleus of some unfortunate atom. There it combines with a proton or neutron, and both particles vanish, turning into 1,876 million electron volts of energy. The resulting explosion--extremely violent on the atomic scale--drives off fast-moving fragments that trace the lines of the star.

Some of the lines (1, 4 and 6) were traced by protons. The rest were made by pi mesons: short-lived particles that emerge when a nucleus is disrupted. There must have been neutrons too, but since they have no electric charges, they do not leave tracks in photographic film.

The energy carried away by each particle can be measured by examining closely the track that it made. In this star, the total energy of the visible particles alone adds up to 1,230 million electron volts. Since only 938 million electron volts can be released by turning a single particle into energy, more than one particle must have been annihilated. Physicists consider this an elegant proof that antiprotons really perform as theorists many years ago predicted that they would.

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