W particle: possible clue to nature's unifying principle

By , Natural science editor of The Christian Science Monitor

In producing the so-called W particle recently, scientists at the European Center for Nuclear Research (CERN) in Geneva have demonstrated the underlying unity of two basic natural forces.

These are the electromagnetic forces between electrically charged particles, and the ''weak'' force involved in radioactive decay. The W particle is a weak force manifestation.

Physicists recognize two other basic forces - the ''strong'' force, which holds atomic nuclei together, and gravity, which binds the universe.

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Physicists suspect these four, seemingly distinct forces may be aspects of one great underlying principle. They see CERN's demonstration of ''electroweak'' unity as an important step in discovering that principle. ''Unity'' means that at sufficiently high energy levels, the effects of the two forces are indistinguishable from each other.

Crucial as the new evidence is, physicists still lack a proven theory to explain that unity. Thus CERN has said merely: ''We confirm the idea of the unification of the weak and electromagnetic forces.''

CERN carefully did not confirm the standard ''electroweak'' theory, which assumes the unity and predicts the mass of these particles, although the results agree with the theory. Other particles predicted by the theory must also be found before confirmation is granted.

This theory was developed primarily in the 1960s by Sheldon Lee Glashow and Steven Weinberg at Harvard University and Abdus Salam at Imperial College, London. It won them a 1979 Nobel prize.

The weak force is so named because it is a trillion times weaker than the strong nuclear force. It is a short-range force, acting only over distances 1, 000 times shorter than the radius of an atomic nucleus. It can operate when the strong force or the electromagnetic force is inhibited by the rules of particle interaction.

Physicists think of particle interactions as being mediated by the exchange of other special force-carrying particles. Photons, particles of light, play this role in electromagnetism. An electron, for example, is bound to an atomic nucleus by electrical attraction. Physicists say they think this attraction arises from a stream of photons exchanged between the nucleus and the electron.

The weak force has similar force-carrying particles. Physicists call them intermediate vector bosons - ''intermediate'' because they mediate the weak force interaction, ''vector'' because of the way they are represented mathematically, ''boson'' because they are named after the late Indian physicist Satyendra Nath Bose. Albert Einstein and Bose showed how energy is distributed among bosons.

This is where the W particle comes in.

The Glashow-Salam-Weinberg ''standard'' theory groups weak force bosons with the photon as a single family. Together they represent the weak and electromagnetic forces. The theory correctly predicts that the photon has no mass. It also predicts the masses of three weak force bosons: the positively charged and negatively charged W particles, recently found at CERN, and a so-called Z particle that has no charge at all.

To predict these masses, the theory incorporates a fourth boson. It is named after Peter Higgs, whose work at the University of Edinburg in 1964 showed how to assign the W and Z masses.

The theory gained support in 1973 when CERN experimenters found a new class of weak force interactions predicted by the theory. They are mediated by the Z particle. There have been other hints of Z particle action in experiments at several research centers more recently.

But proof depends on showing that the W and Z actually exist with the masses predicted. The CERN experiment has begun to do this. It was carried out by a team of researchers from Austria, Britain, France, Italy, Switzerland, and the United States.

Out of a billion particle collisions in a CERN accelerator, the researchers have identified what Carlo Rubbia of Harvard University, a team leader, has called five ''clean'' events. They represent the presence of both positively and negatively charged W particles. The particles have masses of around 81 billion electron volts, as predicted by the standard theory. That's 86 times the mass of the proton. There is no sign yet of the Z particle.

Experimenters have only glimpsed the weak force bosons. This is enough evidence for Dr. Rubbia to call it ''a major step forward in contemporary physics.'' But it doesn't yet prove the theory.

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