Physicists hope the superconducting supercollider will help them explain the very origin of matter itself and perhaps lead them to their holy grail: a single theory that unites all the forces of nature, from gravity spanning galaxies to the mysterious bonds that hold the proton together.

The first task of the collider may be a search for the Higgs particle, named after Peter Higgs of the University of Edinburgh in Scotland, the scientist who first postulated its existence.Scientists say this subnuclear particle should emerge from proton-proton collisions 20 times more powerful than anything possible today. Finding it would be a giant step forward in understanding nature.

The supercollider, a 53-mile ring, will be the first atom smasher able to concentrate in a small volume of space the energy density that must have existed in the theoretical "big bang" origin of the universe.

The current picture of nature, called the "standard model," is tantalizingly incorrect - but nobody is sure just where it goes wrong. At very high energies, the theory predicts, certain particle interactions will occur with greater than 100 percent probability - which is an impossibility.

Modifications to fix up the standard model almost always require extremely energetic collisions for their experimental tests.

All mass is equivalent to energy, shown in the "E=MC squared" relationship first developed by Albert Einsten. That is, the energy of any particle is equal to its mass times the speed of light squared.

It's convenient to speak of mass in terms of "electron volts," the energy acquired by an electron falling through a voltage. Electrons in the filament of a flashlight bulb are accelerated through 3 electron volts.

The mass of a proton at rest is a little less than a billion electron volts.

The total collision energy must be around 40 trillion electron volts to create a particle of 1 trillion electron volts. The world's most powerful accelerator, at Fermi Lab in Chicago, can produce 2 trillion electron volts, whereas the supercollider is designed to generate 40 trillion electron volts.

The Energy Department on Thursday selected a site near Dallas for the giant machine, even though Congress has not yet decided it should be built.

The Texas site chosen for the new superconducting supercollider is the only one given four ratings of "outstanding" by the Energy Department's Site Task Force. Here are some points raised about each site: