Stanford physicists hope to catch more Zs

High-energy physicists at Stanford University are getting it down to an exact science - and they mean exact. Sometime during the next two months the physicists will flip the final switch on an experimental new machine - sending a tiny beam of electrons and a tiny beam of positrons hurtling toward one another almost at the speed of light.

If the scientists can manage to get electrons and positrons to collide with regularity, they'll know they have been exact enough.

And with that, they will be able to cast new light into the darkest recesses of the atom - perhaps revealing startling information about the forces that hold the universe together.

But if the physicists fail, it's back to the drawing board.

Precision is the key to success at the Stanford Linear Collider (SLC), an ``atom-smashing'' machine that makes a daring departure in design from the world's other colliders. Although even the project scientists are not 100 percent certain it will work, they say they are encouraged by tests they have run in recent weeks while fine-tuning SLC.

Electron-positron collisions are expected to help physicists in their efforts to identify the smallest particles that exist - the building blocks of matter.

``High-energy physics is basically the search for the smallest thing,'' says physicist Burton Richter, director of the Stanford Linear Accelerator Center and the driving force behind the new collider.

The only way to get ``inside'' a subatomic particle is to shatter it and watch it decay, a process that requires ever higher energy levels as the particles get smaller and smaller. By colliding an electron and a positron, physicists expect to produce a new particle known as Z degrees.

Discovered in 1983 at the CERN high-energy physics lab near Geneva, the Z degrees has proven to be so elusive that only a few dozen have ever been captured.

Scientists want SLC to become, in effect, a Z factory, producing at least 10,000 during the first two years of operation.

Z particles play an important part in scientific theory about the forces of nature.

In the past 20 years, physicists have developed a theory that the four forces at work in the universe were unified in one grand force at the time of the Big Bang.

``This search for unification has become our Holy Grail,'' says Sidney Drell,the center's deputy director. ``It is the religion of the scientist that there is a simple unified explanation of the universe, sometimes called the Theory of Everything.''

Discovery of the Z degrees confirmed the unification of two of the forces - electromagnetism and the so-called weak force, which allows atoms to break apart, releasing radioactivity. The other two forces are gravity and the strong force, which holds the center of atoms together.

``To show progress now, we've got to make the Z degrees, control it, and study it,'' Dr. Drell says. Any further advances in high-energy physics will be dependent on ``the machine people'' who are striving to build better colliders.

At the official unveiling of SLC last week, scientists said they have stretched modern technology to its limits in constructing the collider.

Although it is not yet operational, SLC already has achieved a number of firsts, Dr. Richter says. It is the first linear collider in the world. It is also the world's highest-energy electron accelerator, producing a beam with the energy equivalent of a stack of flashlight batteries 1 million miles high (53 billion electron volts). Finally, the electron and positron beams met for the first time in the collider hall on March 27.

Although no particles actually collided because the beams were relatively too wide, the feat that the tiny beams actually crossed boosted the physicists' confidence that the machine will work.

Until now, particle colliders have been circular, like race tracks.

Particles beams, guided by an electric field, continuously circle the ring at nearly the speed of light. Because the particles to be collided are traveling in opposite directions, they have thousands of opportunities each second to slam together.

The race track works well for most subatomic particles, but it is not efficient for electrons because an electron beam loses energy when it is forced to curve. Higher-energy collisions, therefore, require gentler curves - and wider and more expensive ``race tracks.''

Richter's vision was to build two straight accelerators, pointed at each other like rifle barrels. The US Department of Energy already had one linear accelerator in place at Stanford, but Richter was unable to drum up the money for another.

Instead, for a relatively small cost of $115 million, he added SLC to the old accelerator - and the final design emerged looking something like a tennis racket. The problem now is to shrink the beams enough to produce regular collisions - and lots of Zs - when the electrons and positrons reach each other at the head of the ``racket.''

Shrinking the beams to 1/25 the diameter of a human hair requires a precision that will push modern technology to its limits, perhaps beyond, the scientists say.

For that reason, they add, the linear design cannot make an immediate leap to the ultra high-energy superconducting supercollider, which the United States announced it will build at a cost of about $6 billion. Plans for the supercollider, which will collide protons in the search for even tinier particles, call for the race-track design, although Department of Energy officials are closely monitoring the progress at Stanford.

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