Proton Decay: Window to Future Particle Physics?

For decades, physicists have struggled with the problem of how to unify the four known forces of the universe. One Harvard professor says he hopes new experimental data will lead to significant advances in solving this problem.

Professor of Physics Howard M. Georgi '68, chair of the physics department, began work in the mid-1970s on the SU(5) theory. This theory attempts to bring together and to refine two previous theories on the relationship between three of these four forces--weak force, strong force, and electromagnetic force. The fourth force, gravity, is believed to be a derivation of the weak and strong forces.

"Particle physics is, or at least was before SU(5), a very homogeneous community of interests," he says.

"The question is always, 'How small?"'

The search for smaller and smaller particles is a natural outgrowth of quantum mechanical theory, Georgi says.

Quantum mechanics is the branch of physics which attempts to explain the apparent discrepancy between light's motion as a wave and as a particle. It reconciles evidence that light consists of small bundles of energy, called photons, with the classical belief that light travels in the form of waves.

Georgi's theory unites the SU(2) and SU(3) theories, both proposed by Higgins Professor of Physics and Mellon Professor of the Sciences Sheldon L. Glasgow. Glashow was awarded the 1972 Nobel Prize in Physics for his work in the field.

SU(2) theory implies a symmetry in which particles are organized into doublets, Georgi says. Strong force, associated with three equivalent particles known as quarks, is the basis of SU(3) theory, named for three-fold symmetry.

However, the two theories did not provide a complete explanation for the relationship between the weak and electromagnetic forces, says Georgi.

A particle, known as the W particle, has proven to be a key to the formulation of a more unified theory. Physicists have long searched for such particles, which are considered the fundamental causes of physical phenomena such as forces.

The W particle has assisted scientists in creating a more unified theory because it consists of a photon-like particle and of an unknown entity, known as the Goldstone Boson.

These sub-units were discovered by Glashow and Loeb Visiting Professor of Physics Steven Weinberg. Their existence was confirmed independently by researcher Abdus Salaam of the Imperial College of Science and Technology in London.

The W particle's involvement in creating forces is, therefore, similar to the involvement of photons in electromagnetic forces. Photons convey the force between charged particles, enabling the particles to act on each other at a distance.

But this discovery led only to a partial unification.

"By the middle of 1972, we had a preliminary picture of the SU(2) and SU(3) interactions," says Georgi. "Any idiot, you might think, would realize that two plus three equals five, and try to unify the weak and strong interactions into an SU(5) theory."

Georgi and Glashow then went beyond the third dimension to come up with the SU(5) theory. After overcoming some conceptual problems, Georgi was able to use his theory to better describe the unification of forces.

According to the physicist, to understand his SU(5) theory, it is important to realize that electric charge is quantized and that this charge is conserved in any reaction.

But an immediate consequence of this theory, as Georgi points out, is that the decay of protons is inevitable. Physicists have spent many years hoping to observe such an event, he says, but to date, they have not been able to detect any evidence of proton decay.

Despite the excitement generated at first by his theory, Georgi says that within ten years, it became clear that protons do not decay as fast as SU(5) theory predicts. By then, scientists had reached a consensus that the phenomenon of proton decay was a credible hypothesis, though difficult to confirm.

But Georgi says he remains hopeful that proton decay will be detected.

"If proton decay is not much slower than the current bounds, it may still be seen, but it could be that we will never see it," he says. "That would close the one tiny experimental window on the world at these distances."

Colleagues Praise 'Pioneering Work'

Fellow physicists, while not necessarily agreeing with Georgi's theory, say that his work has been important in making strides towards unification.

Professor of Physics Cumrun Vafa says that though the exact form of the SU(5) theory is under debate, "Georgi's and Glashow's work is pioneering in the field and has greatly influenced the thinking of physicists on unification."

Georgi says that though there are limitations to the sizes of particles that can be studied, the work of particle physics is hardly complete.

The W particle sub-units, for instance, may lead to important new discoveries. The Goldstone Boson, Georgi says, is an "entirely new form of matter, because it is unlike anything else that we have seen."

He says he is hopeful that the Superconducting Supercollider (SSC), currently being built by the University of Texas, will provide a clue. The SSC will smash the W particle against another, neutral, particle, known as the Z particle.

"This is an unparalled challenge to particle physics explorers," he says.

Georgi says he hopes the research done at the SSC will be revolutionary. "[Physicists] will struggle to find landmarks in a new and completely uncharted region of our map," he says.

But Georgi is not sure where the impending revolution will lead. "That is like asking what happens after discovering America," he says.