Physicists Are Hot on the Trail Of the Elusive 'Top Quark'

HENRY FRISCH is hot on the trail of an elusive form of matter.

Physicists generally are convinced that it exists. They need it to complete the set of elementary particles that current theory requires. Yet they have never seen it in spite of many years of searching.

Now the University of Chicago physicist says he is "willing to bet even odds on $1,000" that it will turn up in the data his group plans to collect next spring at the nearby Fermi National Accelerator Laboratory (Fermilab).

Dr. Frisch's confidence, such as it is, also is a measure of the progress physicists have made in understanding the basic structure of matter and the challenge they believe they now face.

Using their current best concept - the so-called Standard Model - particle physicists describe all the matter they know in terms of a limited set of particles and the forces acting between them. There are six particles of the type called quarks and six of the type called leptons.

The quarks are grouped into three families of two quarks each - the up and down quark family, the charm and strange quark family, and the top and bottom quark family.

Each of these families also contains two leptons - an electron-like particle and its associated neutrino. Neutrinos are shadowy particles with little, if any, mass. They interact so weakly with other particles that they zip through a solid mass like Earth as though it weren't there. They emerge from particle interactions mainly as entities that carry some of the associated energy.

Besides gravity, which the Standard Model scheme doesn't cover, these particles interact through three forces - the electromagnetic force, the weak force involved in some forms of radioactivity, and the strong force that binds quarks together.

For example, the strong force binds up and down quarks in various combinations to form protons and neutrons and some other particles. The protons and neutrons join to form atomic nuclei. The electromagnetic force, in turn, forms the chemical elements by binding different nuclei to ordinary electrons, which are part of the up and down quark family.

Other quark families form more exotic particles seen only fleetingly when particles collide in high-energy particle-accelerator experiments.

Special force-carrying particles mediate these interactions. Gluons carry the strong force. Photons (particles of light) carry the electromagnetic force. And particles known by the letters W and Z carry the weak force. Actually, physicists now know that these latter two forces are just different aspects of a single underlying force they call the electro-weak interaction.

This Standard Model scheme is so neat and tidy that physicists find it almost - but not quite - believable. John Ellis, who heads the theory division at the European Center for Particle Physics (CERN) at Geneva says "the Standard Model is called a model rather than a theory because no one regards it as the last word on elementary particles and their interactions."

Reviewing the particle physicists' challenge in New Scientist, he explained that many particle attributes - such as mass, charge, and spin - do not emerge naturally from the model. They are what he called "put in by hand," meaning they are specified when their values are found experientially. He notes further that, while experimenters have found five of the quarks, no one "has detected the top quark which is absolutely necessary for the Standard Model to remain intact."

This is the object of Henry Frisch's quest. He says the reason he is "pretty confident" his group can capture this elusive quarry is that their new equipment will be able to gather far more particle-interaction data at higher interaction energies than those of previous searches. They should get enough information to turn up the top quark if its mass is in the expected range.

That's a big "if." No one is sure what the top quark mass should be. In fact, no one is absolutely sure that it exists at all, even though the Standard Model requires it. That's why Frisch will bet only $1,000 on his experiment's outcome.

He explains: "The top quark mass is one of the parameters that determines all those other parameters that we measure [among particles]. If it comes out wrong, then we know there is something new."

The question of how particles acquire mass is a fundamental mystery of nature that tantalizes physicists.

Frisch's University of Chicago colleague, the theorist Yoichiro Nambu notes that, in spite of physicists' expectation of what the top quark mass should be and with the masses of five quarks already known from experiment, "one is still at a loss to predict the mass of the sixth quark."

This is the mystery physicists hope to solve with the next generation of particle accelerators: the Superconducting Supercollider being built in Texas and the LHC, a powerful new machine proposed for CERN, each of which will carry particle probing to energies 10 times or more higher than physicists now work with. This energy range may well bring the origin of mass itself to light.