While Congress is reluctant to fund a super atom smasher for particle physicists, the two major existing United States accelerator laboratories are ready to probe the basic structure of matter more deeply than ever before. New facilities at the Fermi National Accelerator Laboratory (Fermilab) near Chicago and at the Stanford Linear Accelerator Center in Stanford, Calif., provide what amounts to virtually new machines. Physicists and engineers at both sites are fine tuning their equipment. They expect to be doing their first experiments through the summer, according to official announcements from the laboratories.
Physicists talk of the probing power of their accelerators in terms of energy, which they measure in units called electron volts. One electron volt (eV) is the energy an electron gains when it is accelerated by a voltage difference of one volt. The higher the energy to which an accelerator can boost its particle beams, the finer is the detail of the particle interactions the beams can explore.
Also, the more energetic the collisions between beam and target, the more massive are the new particles those collisions produce.
Accelerator physicists now favor head-on colliding of matter and antimatter particles. There is much more energy available for making new particles and probing particle interactions in these head-on collisions than in the old practice of having the beam hit a fixed target.
For one thing, when a particle and its anti-particle meet, they annihilate each other in a burst of pure energy.
More important, there is the law of momentum conservation to reckon with. The net momentum of debris produced in a particle smash-up has to equal the net momentum of the original particles. When a speeding beam particle hits a target particle that is at rest, much of the beam energy has to go into giving the debris the needed momentum. Relatively little of the original beam energy is available for probing particle interactions or creating new particles. But when two particles coming from opposite directions with equal speed collide, their net momentum is zero. Virtually all their energy is available.
The US Department of Energy had asked Congress for $363 million for research and initial construction of a $4.4 billion supercollider that would smash protons together with 20 trillion electron volts (TeV) in each beam. However, on Wednesday both the House and Senate voted to grant only $100 million to keep the project alive until the next administration can decide whether or not to proceed with construction.
With only about one TeV per beam, the new facilities that now allow Fermilab to begin collider experiments can't match the proposed supercollider. But Fermilab still has the most powerful proton-anti-proton collider in the world.
Likewise, the new electron-positron (anti-electron) collider at the Stanford center is the most powerful machine of its type, even though it will ultimately provide only about 70 billion electron volts (GeV) per beam.
Although these machines can't probe as deeply as the supercollider may one day explore, they do ``have the potential for making more discoveries in high-energy physics than at any other time in history,'' Fermilab director Leon Lederman has noted.
There was a revolution in basic physics during the 1970s. Theorists and experimenters established the structure of protons and neutrons in which these building blocks of atomic nuclei are themselves made of components called quarks. Physicists developed the theory that explains the strong force that binds the quarks and holds nuclei together. They showed the essential unity of electromagnetism and the weak force involved in some forms of radioactive decay.
Both Fermilab and the Stanford center now have machines that probably can explore the limits of these theories. They may even find phenomena that the theories do not encompass.
Also, the Stanford machine is unique among electron/positron colliders. Other designs accumulate their particles in large rings. However, electrons circulating in such rings rapidly lose energy through radiation.
At Stanford, the particles are accelerated mainly through a two-mile-long linear machine. They don't radiate when traveling straight. Only at the end are electrons and positrons diverted in relatively small arcs to meet head-on.
The European Center for Particle Physics at Geneva is building a billion-dollar electron/positron collider with a storage ring some 23 miles in diameter. Stanford's machine not only avoids the radiative energy loss, it also costs only $120 million to build.