Probing matter's smallest particles to understand the Big Bang

THEY call it the Superconducting Supercollider (SSC) - a particle accelerator so powerful its probings could significantly narrow the gap between what physicists learn about the nature of matter in the laboratory and what astrophysicists infer from the Big Bang explosion of primordial energy, from which the universe emerged 10-20 billion years ago.

That's the next major goal on which US high-energy physicists have set their sights. It's superconducting because the magnets that control the particles are cooled to liquid-helium temperature, where their coils lose all resistance to electricity, thus reducing their need for power.

The mere fact that these physicists consider such a machine practical and that the Department of Energy is seriously considering funding it shows how far the investigation of the fundamental nature of matter has progressed. It has reached a level where physicists can envision using an SSC to approach - perhaps even to reach - a summit of understanding where the physics of the universe can be summed up in a single equation. It could then be written on a T-shirt that physicists would wear.

Secretary of Energy Donald P. Hodel has asked the physicists to detail what they have in mind. The rough draft of this study has now been delivered to the Department of Energy. It reflects a cooperative effort among US physicists, which is led by the University of California at Berkeley. After any revisions suggested by the department, the final report should be ready next month.

Details of the study have not yet been made public. It is known, however, that it describes a machine that would have something like 20 times the energy of the largest accelerator in the world. This is the so-called Tevatron, now being brought up to full power at the Fermi National Acelerator Laboratory (Fermilab) here in Batavia, Ill. The SSC would also cost several billion dollars.

As Fermilab director Leon Lederman points out, the study has delivered a reference design. It is not a specific proposal for a specific machine. That would come later, if this feasibility study leads to a design project. Meanwhile , it represents what Lederman calls a ''worst case'' from the economic viewpoint , showing what a feasible machine could cost today. Thus it sets up a target for expert critics who will try to find ways to beat that cost down.

The study also lays out the rationale for wanting such a machine as the next logical step in particle research.

This field is called high-energy physics, because the more deeply physicists probe the basic structure of matter, the more energetic the probing particles have to be.

These energies are specified in terms of electron volts (eV). One eV is the energy a particle gains when it is accelerated by a voltage of one volt. Physicists, today, work at energies up to several hundred times 1 billion electron volts, or one GeV. (It's ''G'' and not ''B'' because, in the metric system, billion is represented by the term giga.)

At these energies, physicists have probed deeply into the atomic nucleus and have discovered some of the inner structure of the protons and neutrons of which it is built. Protons, neutrons, and related particles are combinations of other particles called quarks, together with gluons, the particles that represent the force which binds quarks together. These energies, too, have revealed a basic unity between the electromagnetic force and what is called the weak force, which is involved in some forms of radioactive decay.

Taken together, the unifying electro-weak theory and the theory that quarks and gluons combine to build up composite particles constitute what physicists now call the standard theory, or standard model, of matter's underlying nature. It is highly successful. It accounts for virtually all of the high-energy physics data gathered so far.

But physicists know it isn't the last word. They want to test it thoroughly and to look for phenomena where the theory's ability to explain things and predict effects breaks down. For this, they need considerably higher accelerator energies than are available today.

At the European Center for Nuclear Research (CERN) at Geneva, beams of protons and antiprotons - each with an energy of 540 GeV - crash head on. That's a total energy of just over 1,000 GeV or one TeV (for tera, or trillion) electron volts. The Tevatron at Fermilab is designed to produce beams of 1 TeV each. That will give a total energy of 2 TeV when beams of protons and antiprotons are made to collide at Fermilab, in about two years.

With collisions at 2 TeV, physicists will begin to explore some of the boundaries of their standard theory. But for full understanding, they need collisions that are 10 to 20 times as energetic.

Chris Quigg, head of Fermilab's theoretical-physics department, explains that the standard model needs to be tested at energies of around 2 TeV for interactions between individual quarks and gluons.

A proton is really a ''bag'' of three quarks (which carry half the proton's energy) plus gluons (which carry the other half). Thus, to get individual quarks and gluons to interact with 2 TeV energy, you need beams of protons and antiprotons of about 20 TeV each, which then collide at a total energy of 40 TeV.

That is what the SSC concept is all about.

It would provide collisions on that energy scale. It would enable physicists to test their standard theory thoroughly and to see how they should extend or supersede it.

In doing this, physicists will not only be peering deeply into the submicroscopic world of particles, they will also look outward into the universe and into its early history. Immediately after the Big Bang - in the first trillionth of a second or less - events happened on energy scales ranging from 1 ,000 TeV down to about the 1 TeV achievable at CERN and Fermilab. By probing matter at 20 to 40 TeV, physicists may be able to begin to extend their understanding of matter as probed on Earth to link up with what they can infer about the conditions of the Big Bang.

''We have that great accelerator in the sky - the Big Bang,'' Lederman observes. ''That gives us the ultimate. We have data from that (in leftover debris). We have data from Fermilab. We have data from CERN. There's a big gap between those. . . . If you narrow the gap a bit, maybe that's enough to make the bridge.''

If it is, he adds, it may all be summed up in one equation, so ''you can write it on a T-shirt.''