Science: Window on Mystery

As atom smashers have grown larger and more powerful, the subatomic particles that scientists have been able to find have grown stranger and more elusive. Still, it hardly seemed probable that anyone would ever discover another bit of matter quite so peculiar as the neutrino, first detected near a nuclear reactor in 1956. So light that it weighs nothing at all, the neutrino is free of electric charge and can pass through the heaviest materials as if it were hurtling through empty space. But last week, a team of Columbia University physicists did the improbable: using 5,000 tons of battleship armor along with the most powerful atom cracker yet built, they found another variety of neutrino. Around the world, great laboratories are already planning experiments to exploit the tiny new window opening on the unknown.

Guilty Particles. Hardly had the neutrino become established as a real particle when physicists noticed that pi mesons (middleweight particles, also called pi-ons, that are created by powerful atom smashers) disintegrate into slightly lighter mu mesons (muons) while an unseen particle carries away part of their energy. At first the physicists assumed that ordinary neutrinos were the guilty particles. Then they began to have their doubts. Maybe another kind of neutrino was stealing the pion’s energy. But it had been hard enough to trap regular neutrinos; how were scientists to locate and study an even more evasive particle?

They found their answer in the enormous alternating gradient synchrotron at Brookhaven National Laboratory on Long Island. That mighty machine can spin protons up to the energy of 33 billion electron-volts, bounce them off targets and produce all sorts of atomic debris—including neutrinos. Physicists figured that any new type neutrinos created by this monstrous slingshot should have as much as i billion volts of energy. They would not be nearly so numerous as the neutrinos flooding out of a nuclear reactor, but their high energy should allow them many more ways of interacting with matter; as a result they would be more easily detectable.

The team that laid out the momentous experiment was led by Columbia Professors Leon Lederman, Melvin Schwartz and Jack Steinberger, and helped by Brookhaven scientists in charge of the synchrotron. First step was to shoot the machine’s high-energy protons at a beryllium target and produce an intense beam of pions—which decay rapidly into muons, neutrinos (perhaps the new type), and other nuclear odds and ends. After shooting across some 70 ft., this beam of mixed particles hit a shield of battleship armor 42 ft. thick that stopped everything but the neutrinos, which sailed on unheeding.

On the other side of the shield the neutrinos entered a 10-ton spark chamber made of inch-thick plates of aluminum separated by half-inch gaps filled with neon gas. When particles carrying an electric charge pass through the chamber, they show their tracks as vivid pink lines drawn by electricity jumping from plate to plate through the neon. Neutrinos would have no charge and could not be expected to leave tracks, but there was a good chance that a few of them would crash into nuclei of aluminum atoms and create particles with track-making electric charges. If some of the neutrinos created high-energy electrons, this would prove them to be the older type, which always associates with electrons. But if they created muons only, they would have to be a new type associated with muons.

Cosmic Alarms. After setting up their massive apparatus, the neutrino hunters waited anxiously. They knew that the spark chamber was protected against false alarms caused by cosmic rays striking down from space, and they knew that a dense beam of neutrinos was passing through it. What they did not know was whether the neutrinos were a new kind, or even if they were, whether they would interact with the aluminum and make telltale tracks.

The apparatus worked almost exactly as far-out theory predicted. Several times a day, an automatic camera changed its film after recording an “event” inside the chamber. After 600 hours of intermittent operation, during which 100 trillion neutrinos passed through the chamber, more than 50 events were photographed. And 29 of the films showed the long, straight tracks of muons created by invisible neutrinos. None showed the diverging “shower” track of a high-energy electron. To the scientists, this was conclusive proof that the neutrinos entering the chamber were not the ordinary, electron-frequenting type. They were a brand-new variety that associates only with muons.

The big spark chamber is now being dismantled for storage, and the Brookhaven synchrotron will soon turn to other work. But the scientific interest in neutrinos is only beginning. Experiments now being planned in the U.S. and Europe will try to discover details about the new neutrinos—besides the bare fact that they exist—and will watch out for other neutrinos still undiscovered. The new particles, which have not yet been officially named, promise to point the way to important discoveries. Their study may reveal the basic forces that make energy, the equivalent of matter, shape itself into the 30-odd different particles that comprise the material universe.