Positrons, electrons form supernuclei.

Positrons, electrons from supernuclei

Naturally occurring atomic nuclei get as large as about atomic mass 250. By striking nuclei against each other, however, physicists can sometimes make them amalgamate for a fleeting moment into something like a supernucleus, with atomic weight around 500. One thing such a supernucleus has is an extremely strong electric field. And physicists were hoping that in this way they could make a field strong enough to produce positrons out of the vacuum--or, as some of them put it, to produce positrons by ionizing space-time itself.

Experiments at the Gesellschaft fur Schwerionenforschung (GSI) in Darmstadt, West Germany, are in fact finding positrons that come out of such heavy-nucleus collisions. However, these positrons seem to come not from the vacuum but from some other source, perhaps a new kind of subatomic particle. Such a particle, if it is real, would be something unexpected by current theories of subatomic particles.

At last week's meeting of the American Physical Society in Washington, D.C., Jack Greenberg of Yale University described the course of the experiments, which began about a decade ago and now include three international groups known as EPOS, ORANGE and TORI. Theorists had predicted that if a nucleus could be made with atomic number (that is, electric charge) greater than 173, it would produce an electric field strong enough to bring the energy of its innermost shell of electrons to the energy of the "Dirac sea.'

Decades ago, as part of his prediction of the existence of antimatter, the late P.A. M. Dirac postulated that the vacuum, which physicists regard as the zero energy level devoid of all matter or energy, actually contains a sea of virtual electron-positron pairs, which can be pulled into actual existence by the proper forces. As an electric force of a certain strength can ionize an atom, pulling positive and negative charges apart, so this procedure, in the words of D. Allan Bromley of Yale, "ionizes space-time itself,' producing a positive and a negative charge.

The electric field of the supernucleus should do this, if there is a vacancy among the electrons of the innermost shell into which the new electron can fall. The positron would then come out to be detected. The first experiments, which collided uranium against thorium to produce a supernucleus with charge number 188, brought forth positrons that seemed to be the right kind. Checking the result, the experimenters then tried thorium on thorium. Theory predicts that the energy of the positrons should increase as the 20th power of the nuclear charge, so with this combination they expected a fivefold increase in positron energy. Positrons came out with the same energy as before. Every combination they tried produced positrons with about the same energy.

By now suspecting they were seeing positrons from some unexpected source, the experimenters tried thorium against tantulum to produce a supernucleus with charge 163, well below the theoretical threshold for producing positrons from the vacuum. Again they found positrons, and again the positrons had more or less the same energy.

Then they decided to look for something else produced with the positrons, if anything. The positrons have a very sharply defined energy, and that means they have to come from a process that produces a positron and only one other particle. The two obvious candidates for the other object are a neutrino and an electron. Electrons are easier to detect, so the experimenters started with them. Most recently, Greenberg reports, they have started to find electrons of the proper energy.

The experimenters now suppose that some new subatomic particle, electrically neutral with a mass three to four times that of the electron, is produced in these nucleus-nucleus collisions, and it then decays to an electron-positron pair. So far its existence is only supposition needing much more experimental work for confirmation, but if it is confirmed, Greenberg says, "it would upset the usual wisdom' about subatomic particles.