May 29, 2000 Page 2F UNIVERSAL PARTICLE New experiments show evidence of neutrino's cosmic significance by Alexandra Witze Poetry is supposed to be eternal. Poets, therefore, should be careful when writing about science. Take John Updike, who in 1960 published a light-hearted look at the subatomic particles called neutrinos. "Neutrinos, they are very small/They have no charge and have no mass/And do not interact at all," reads his poem "Cosmic Gall." But the poem was already out of date when Updike wrote it. Neutrinos do, in fact, interact with other matter. That's how scientists first detected the neutrino, four years before "Cosmic Gall" appeared. And in the past two years, scientists have found that neutrinos have mass after all. Far from being a mere literal postscript to a humorous poem, this discovery is reshaping scientific understanding of the fundamental nature of matter and the fate of the universe. Taken by itself, a solitary neutrino is an insignificant thing, an everyman, a Rabbit Angstrom - so trivial that nobody notices as billions of them pass through your body every second. But their sheer numbers give them cosmic meaning. Put enough neutrinos together and they take on enormous power, just as a dull character becomes bigger than life in the hands of a fiction genius. If each neutrino carries just the tiniest bit of mass, as scientists now believe, that could be enough to account for a significant chunk of the entire universe. Neutrinos may play a role in sculpting the intricate architecture of galaxies and in seeding space with chemical elements spit out by exploding stars. To learn the secrets of neutrinos, scientists are building new observatories, ranging from a giant sphere of heavy water buried in Canada to strings of equipment lowered into the Antarctic ice cap. Next month, researchers will gather in Sudbury, Ontario, to unveil their latest insights from these ventures. The new work could help resolve some long-standing mysteries, such as why the sun doesn't seem to shine as it should. And all these insights could come from an itty-bitty particle, whose inventor groaned, "I have done a terrible thing. I have postulated a particle that cannot be detected." Austrian physicist Wolfgang Pauli didn't know any better in 1930, when he was struggling with equations to describe radioactive decay. To get the math to work, without violating the law of conservation of energy, he was forced to "invent" a new, invisible particle. Pauli called his particle a neutron, since it had no electrical charge, and added that it was a "desperate remedy." To his colleagues, he wrote that "one should have seen those neutrons very earlier if they really exist.... Thus, dear radioactive people, look and judge." Judgment came out on Pauli's side. But "neutrons" became the name for the heavy particles that rest in an atom's nucleus along with protons. Italian-American physicist Enrico Fermi renamed Pauli's invention the "neutrino," or "little neutral one." Like a neutron, a neutrino would be electrically neutral, but it would be very lightweight. In fact, it might have no mass at all. Not until 1956 did scientists actually detect the slippery neutrino. Clyde Cowan and Frederick Reines, of the Los Alamos National Laboratory, set up equipment near a South Carolina nuclear reactor, hoping to catch the telltale signs left by a neutrino interaction. In fact, they managed to capture the track of an antineutrino. But if there's an antineutrino, there must also be a neutrino. To celebrate, Cowan and Reines handed out carefully wrapped empty boxes, each containing a card that read "This box is guaranteed to contain at least 100 neutrinos." Reines won the 1995 Nobel Prize in physics for the discovery. (Cowan died in 1974.) More questions But rather than solving Pauli's problems, the neutrino's discovery only opened the door to even greater mysteries, says John Bahcall of the Institute for Advanced Study in Princeton, N.J. The biggest mystery, which has endured for more than three decades, is the question of where all the solar neutrinos have gone. Theorists believe that the sun should be spitting out trillions of neutrinos each second as it burns its nuclear fuel. Measuring how many solar neutrinos arrive at Earth could directly test ideas about how the sun shines. In the 1960s, Raymond Davis of Brookhaven National Laboratory tested this theory by trying to trap solar neutrinos with 100,000 gallons of dry-cleaning fluid in South Dakota's Homestake mine. "We thought we were just going to use neutrinos to look at the center of the sun and find out how the sun shines," says Dr. Bahcall, who collaborated with Dr. Davis. Instead, they opened a Pandora's box of neutrino problems. The Homestake experiment found far fewer solar neutrinos than expected. Scientists were mystified. The solar neutrino census appeared to have a terrible return rate. The "missing" solar neutrinos have become "one of the most fascinating and longest running mysteries in particle physics," says physicist Chang Kee Jung of the State University of New York in Stony Brook. Today, most scientists think the solar neutrinos aren't really missing - just hiding. It turns out that neutrinos can experience a sort of identity crisis. They can transform themselves from one form to another as they fly through space or matter. Scientists call this changeling behavior "oscillating." Neutrinos come in three forms - electron, muon and tau neutrinos - that can mutate back and forth at will. An electron neutrino born in the sun's heart might change into a muon neutrino as it flies outward. So any experiment set up to detect just electron neutrinos - as the original Homestake experiment was - would miss the muon neutrinos they'd changed into. Most scientists now think this oscillating behavior can explain the "missing" solar neutrinos: The neutrinos are only missing if an experiment looks for the wrong neutrino form. But it wasn't until 1998 that scientists learned that oscillations really do happen. At the Super-Kamiokande neutrino detector, buried beneath the Japanese Alps, physicists measured neutrino oscillations for the first time. "Super-K," as it's called, can detect neutrinos from many different places: from inside the sun, from nearby exploding stars, or from the decay of high-energy particles from space - cosmic rays - hitting Earth's atmosphere. Just as solar neutrinos oscillate on their way out from the sun, atmospheric neutrinos oscillate as they pass through Earth. Some neutrinos eventually make it to the detector, where they interact with 50,000 tons of pure water. The interactions create tiny flashes of radiation, picked up by light- sensitive sensors lining the water tank. By analyzing these faint flashes, scientists can learn more about the types of neutrinos that passed through Super-K. Most important, the Super-K researchers have measured the difference in mass between two neutrino types. The physicists determined that a muon neutrino oscillated into a tau neutrino and found there was a small - but not zero - difference in mass. So, they concluded, neutrinos must have mass. The team announced its headline-making findings at the last major neutrino conference, held in 1998 near the Super-K site. In contrast, this year's neutrino conference will be held at the most promising new entry in the neutrino-hunting game - the Sudbury Neutrino Observatory, now taking its first data beneath Sudbury, Ontario. Like Super-K, the Sudbury detector is a big ball of water buried underground. "It really is fascinating that you can go 2 kilometers underground in order to study the sun," says observatory director Art McDonald. "You really get a new approach by doing so." In place of regular water, the Sudbury sphere contains 1,000 tons of heavy water, in which atoms of hydrogen are replaced by their heavier version, deuterium. Neutrinos zipping through the sphere react with the deuterium atoms in different ways, depending on what type of neutrino it is. Scientists will be looking for two specific reactions: one triggered just by electron neutrinos, the second by any type of neutrino. This way, the Sudbury observatory can directly address which neutrinos are being transformed into what, says Dr. McDonald, a physicist at Queen's University. That, in turn, could help scientists narrow down the mass of each individual type. And knowing the mass of neutrinos could have universal importance - literally. Big chunk of the cosmos By itself, a neutrino might have a tiny mass, something like a billionth the mass of a proton. But the sheer number of neutrinos in the universe mean that, collectively, they have a tremendous impact. In fact, neutrinos could make up anywhere from less than 1 percent to perhaps 20 percent of the mass of the whole universe, says cosmologist Max Tegmark of the University of Pennsylvania. It may be years before more experiments can narrow that estimate down. But the important thing about Super-K, Dr. Tegmark says, is that it made people realize the universe is complicated. Neutrinos could play a significant role in making up its mass. "There could be lots, or there could be very little still," he says. "The fact that it's not zero makes people take this much more seriously than before." If neutrinos make up a big fraction of the universe's mass, it could pose problems for cosmologists trying to explain why the universe looks the way it does, Dr. Tegmark says. That's because, in the early universe, neutrinos acted like cosmic erasers. As they zipped through the universe, they literally erased cosmic structure, so that galaxies formed in certain regions and not in others. Dr. Tegmark is currently trying to calculate exactly how much neutrinos could weigh and still give rise to the structure in the universe visible today, he says. Theorists are also working on different ways to reconcile oscillating neutrinos with the Sudbury and Super-K results. Some scientists have been forced to introduce a fourth type of neutrino - a "sterile" neutrino, which by definition doesn't interact with anything. A sterile neutrino could not be seen in an observatory, but some researchers think it has to exist in order to explain the experimental results. Neutrino scientists are most puzzled by an experiment that took place at Los Alamos beginning in 1993. The Liquid Scintillator Neutrino Detector experiment seemed to suggest that neutrinos were oscillating much more often than expected. Muon neutrinos seemed to be popping into existence within an electron-neutrino beam. Few theorists can reconcile this data with other studies, and many now believe that something could be wrong with the Los Alamos results. "It may not be evidence in the end, but it requires further experiment to decide," says Peter Rosen, a neutrino expert at the Department of Energy and former dean of science at the University of Texas at Arlington. Still looking Other experiments may also help provide more details about exactly how neutrinos oscillate and how much each kind might weigh. One popular approach is to shoot neutrinos through the Earth. By making a neutrino beam, rather than waiting for solar or cosmic-ray neutrinos, researchers can get a much better idea of what the particles are doing. For instance, Super-K researchers are now zapping a neutrino beam into the detector from 150 miles away. The experiment began last June - "the first time a manmade neutrino was actually shot through the Earth and detected," says Dr. Jung of Stony Brook. Because scientists can create a specific kind of neutrino for the beam, then watch what arrives at the detector, they can calculate whether oscillations are actually occurring. Other neutrino-beam experiments will also be coming online in the next few years. At the Fermi National Accelerator Laboratory in Batavia, Ill., scientists plan to fire a beam of neutrinos into a mine in northern Minnesota, some 450 miles away. That experiment will be able to determine exactly which neutrino is oscillating into which type, not just that oscillations exist, says Stan Wojcicki, the project spokesman. Meanwhile, European researchers are planning to fire a neutrino beam across the Swiss Alps, from Geneva to Italy, in hopes of seeing tau neutrinos pop into view as other neutrinos oscillate into them. Another European experiment, called BOREXINO, may reveal even more than the much-touted Sudbury observatory, says Dr. Bahcall. The Sudbury experiment is designed to detect relatively high- energy neutrinos. But most of the universe's neutrinos possess lower energy. Only the BOREXINO experiment will be able to detect some of those low-energy neutrinos. Other experiments, still on the drawing board, will look at even lower energies - the kind of neutrinos that account for more than 90 percent of the sun's neutrino output, says Dr. Bahcall. "We'll never really be sure of what's happening until we look at the really low energies and test the theories there," he says. Meanwhile, other observatories are looking for even farther-out neutrinos. At the South Pole, a novel telescope is waiting for neutrinos to flood in from astronomical sources - exploding stars or other cosmic objects - that might send neutrinos zooming many light- years across the universe. The telescope's 10 cables, peppered with glass detectors like beads on a string, hang more than a mile into the ice cap. The detectors watch for neutrinos to interact with the ice. The scientists plan to expand this telescope, called AMANDA, beyond its recent upgrade in January. Eventually, they hope, it will grow 20 times bigger. With enough detectors, the telescope could do a neutrino survey, investigating all three kinds of neutrinos, says Steven Barwick, a physicist at the University of California, Irvine. "As big as AMANDA is, it's still small compared to what we need to get to, to do this science," he said earlier this year in Washington, D.C., at a meeting of the American Association for the Advancement of Science. With all the new neutrino observatories coming online, and still more planned for the future, researchers can expect plenty of data to play with, says Dr. Tegmark. The neutrino bonanza may finally resolve the decades-old puzzle of the missing solar neutrinos, as well as answering other particle puzzles. "In the end," he says, "I predict there will be closure. Illustrations/Photos: (Institute for Cosmic Ray Research; University of Tokyo) ,Lawrence Berkeley Laboratory; Sudbury Neutrino Observatory) This giant water tank at Super-Kamiokande has yielded the best hint that subatomic particles called neutrinos have mass. In 1996, workers on a raft polished light-collecting tubes. 2. This 40-foot-wide acrylic sphere, filled with 1,000 tons of heavy water, is the heart of the new underground Sudbury Neutrino Observatory in Ontario, Canada. Neutrinos passing through the tank sometimes interact with atoms in the heavy water, leaving behind telltale traces of the characteristics of the neutrinos. 3. (The Dallas Morning News: Layne Smith) ILLUSTRATION(S)/CHART(S): An illustration of A NEUTRINO TRAVEL LOG and A Look AT THE SUPER-KAMIOKANDE DETECTOR. 4. (The Dallas Morning News) STANDARD MODEL.