Beams of electrons and positrons traveling near the speed of light collided in northern California on Sunday in the first test of the nation's largest colliding beam machine.
"We were jubilant a week ago when we stored electrons for the first time," said Donald Getz, assistant director of the $78 million Positron-Electron Project (PEP) at the Stanford Linear Accelerator Center at Stanford, Calif. "All I can do is use the same word again to describe how we felt at the time of collision."
Colliding beam machines are the newest atom smashers being used by the world's high energy physicists to probe the basic nature of matter. PEP is the first of three colliding beam machines planned for the United States in the next 10 years. The others are being built at Fermilab outside Chicago and at the Brookhaven National Laboratory on Long Island.
The colliding beam machine is a new concept in particle accelerators. Instead of firing a beam of electrons at a fixed target such as deuterium gas, the colliding beam machines propel particles at high speeds into each other.
The moving particles pick up weight as they accelerate. For example, protons are 400 times heavier at the end of their trip than when they started out. At collision time, electrons weigh 50,000 times more than when they started out.
"That has made for collisions that are like Mack trucks running into Volkswagens when the truck runs right over the Volkswagen and you lose a lot of energy," said Dr. Sidney Drell, deputy director of the Stanford center. "What you want is to run two Mack trucks into each other so that when they hit they splatter themselves all over instead of one pushing over the top of the other."
Sunday's collision of electron and positron beams produced 16 billion electron volts of energy, the most every generated in a colliding beam machine in the United States. Collisions producing 36 billion electron volts are planned this summer.
When beams of electrons and positrons (which are anit-electrons) collide, they annihilate each other. The action triggers tiny fireballs of gamma rays that are hotter and more densely concentrated than burning hydrogen and helium in the interior of the sun.
The collisions also produce a slew of subatomic particles so small they are thousands of times smaller than the atom. The goal of the new PEP machine at Stanford is to produce particles so small they have never been seen.