Burst of Particles Detected : Supernova Watched for Clues to Cosmic Evolution

Subatomic particles called neutrinos, which are so small that trillions are constantly passing through the Earth at nearly the speed of light without touching anything, have been detected by an experiment buried deep inside the French and Italian Alps, leading astronomers said Friday.

The elusive particles, which have almost no mass, probably had come from the supernova, or dying star, that was discovered last month in the Large Magellanic Cloud, the galaxy nearest to our own Milky Way, they said.

The finding, if confirmed, would represent the first direct evidence that key theories about the evolution of the universe are correct.

Among them is that supernovae release neutrinos and other elements that eventually form other stars and even organic matter--such as people. That is why nearly every textbook on astronomy points out that its readers are made of stardust.

Stars emit neutrinos in a continuous stream as part of the nuclear reaction that keeps them alive. Current theories hold that when a star nears death, its core temperature becomes enormously high, and particles collide at very great velocities, leading to much higher levels of production of neutrinos.

“The time comes when the temperature rises to such an enormous level that the star cannot hold itself together, and it blows up,” said Yervant Terzian, chairman of the department of astronomy at Cornell University. “During that phase there are countless nuclear reactions which form heavy elements. These nuclear processes also produce enormous quantities of the subatomic particles we call neutrinos.”

The report of the neutrinos is the latest development in the story of the newly discovered supernova, and scientists around the world are “waiting with bated breath” to learn if the neutrinos captured at Mont Blanc came from the supernova, according to one astronomer.

“Clearly the supernova event itself is the most important (astronomical) event of this century, and if the detection of neutrinos is confirmed, it rivals the importance of the event itself,” Terzian said.

Scientists believe that a supernova should send out a giant burst of neutrinos so numerous that they would stand out from the regular emissions of the sun. The existence of such a heavy flow of neutrinos would confirm that a supernova is a star that is collapsing on itself.

But while the theory itself fits nicely with mathematical models of the events that should take place when a giant star dies, it has so far been just that.

“This will change the field from an intellectual game to a hard science,” said John Bahcall of the Institute for Advanced Study at Princeton, a leading theorist on neutrinos.

The latest report may also mark a major turning point in one of the most frustrating areas of modern science, the quest for the elusive neutrino.

Neutrinos are so small that they zip freely through matter, passing through the relatively large voids that separate the subatomic particles of matter, like comets whipping unmolested through the solar system. Since they have no electrical charge, they also are not inclined to interact with other particles.

Scientists Go Underground

Thus the effort to catch them has literally driven scientists underground, where they have carved out darkened caverns and filled them with lakes of water and chemicals in the hope of catching a few passing neutrinos. The experiments so far have not been marked by great success, although an occasional neutrino has struck an electron, thus producing a positron and a burst of light that is captured photographically.

Those neutrinos, however, have not been concentrated in bursts that would indicate that they had come from anyplace but the sun.

On Feb. 23, however, scientists at the neutrino detection facility at Mont Blanc in the Alps, which is operated by Italy and the Soviet Union, detected a sudden burst of five neutrinos within a seven-second period.

Although there have been some false alarms in the past, the scientists there concluded that the burst probably came from a supernova.

The next night, half a world away, a young astronomer with the University of Toronto routinely guided the telescope at the Las Campanas Observatory in Chile toward the Large Magellantic Cloud for his continued studies of our nearest neighboring galaxy. What he saw, however, turned the world of astronomy on its head.

Sights Bright Star

Ian Shelton saw a bright star where only a faint one had been seen before.

He immediately reported his finding to the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory in Cambridge, Mass. Word went out that a supernova--now named after its discoverer, Supernova Shelton 1987--had been found, and that it is the closest supernova seen in nearly 400 years.

The star actually exploded about 150,000 years ago, but it has taken that long for the light to reach Earth.

Back at Mont Blanc, the timing could not have been better. Carlo Castagnoli of Italy and G. T. Zatsepin of the Soviet Union sent in their own report to the Cambridge bureau, stating in the arcane language of astronomical codes that they had detected “five pulses . . . in agreement with collapsing iron core models” for supernovae.

Many astronomers, while cautiously waiting further confirmation, are optimistic that the bursts recorded at Mont Blanc had indeed come from the supernova.

“These are very competent people,” said Hans Bethe, the grand old man of astrophysics, in a telephone interview.

Scientists like Princeton’s Bahcall found it difficult to overemphasize the importance of the discovery, if confirmed.

Game May Become Real

“Up until now, the supernova theory has been a dream and a game played very intelligently, but we had no idea if we were on the right track,” Bahcall said. “If confirmed, then we will know that what was a game is in fact real.”

Bahcall noted that some of the assumptions about neutrinos will die quickly if the discovery is confirmed.

Because of frustration in detecting neutrinos, many scientists were beginning to conclude that neutrinos decayed relatively quickly, so that only those from the sun survived long enough to reach the Earth. However, if those detected at Mont Blanc did in fact come from the supernova, they have been around for at least 150,000 years, because that is how long it took them to reach the Earth traveling at nearly the speed of light, Bahcall noted.

Furthermore, since they reached the Earth about as quickly as the light from the explosion of the star, they must have been traveling at very close to the speed of light. And Albert Einstein’s theory of relativity shows that to be traveling that fast, their mass would have to be very close to zero.

That finding is particularly important to scientists who are wrestling with the nagging problem of the universe’s “missing mass.” Mathematical models of the universe, based on the movement of celestial bodies and the distribution and composition of matter, have troubled scientists for years because they show there should be far more mass in the universe than the strongest telescopes can find.

Something Undiscovered

That has led many scientists to conclude that something else is out there--something as yet undiscovered--that must account for most of the universe’s mass.

Some have postulated that the missing mass might be nothing more than the sum total of the universe’s neutrinos.

But Bahcall said that if the neutrinos detected at Mont Blanc are from the supernova, “it would show they are not important in cosmological mass” because they could not travel that close to the speed of light if they had enough mass to be significant, even when all of them are added together.

Meanwhile, scientists from around the world continued their pilgrimages toward the Southern Hemisphere, where the supernova can be seen with the naked eye. Some leading astronomers were reported frantically scraping up equipment to head south for experiments.

The importance of it all was summed up by Hans Bethe:

“Supernova are important because they make all the stuff that we are made of.”

THE NEUTRINO

The neutrino is a subatomic particle that has no electrical charge and no measurable mass. Some of its properties:

Speed. Neutrinos travel at or near the speed of light.

Detection of neutrinos is very difficult, since they lack mass or charge. They hardly interact at all with matter. Only one of trillions of trillions of neutrinos passing through the center of the earth will encounter another particle.

Origin. Neutrinos are produced when subatomic particles decay and disintegrate. Scientists have theorized that neutrinos carry off much of the energy released by exploding stars, called supernovae .

Spin. Neutrinos are characterized by their spin, among other properties. Neutrinos have counterparts known as antineutrinos, which also have no charge or measurable mass.

Usefulness. The ability of neutrinos to penetrate matter makes them useful in the study of nuclear particles. Physicists have used tracings of rare collisions between neutrinos and atomic nuclei to understand the makeup of subatomic particles.

Collisions between neutrinos and other particles can result in transformed matter.

Discovery. Existence of the neutrino was hypothesized in the 1930s, but direct evidence was first established in 1956 by American physicists Frederick Reines and Clyde L. Cowan Jr., who created antineutrinos in a nuclear reactor and traced the tracks of their collisions with other particles.