Neutrinos: Mysterious Subatomic Particles

T2K’s detector. (Photo: Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo) Credit:

Wolfgang Pauli first postulated the existence of the neutrino in 1930. At that time, a problem arose because it seemed that both energy and angular momentum were not conserved in beta-decay. But Pauli pointed out that if a non-interacting, neutral particle-a neutrino (the name neutrino was coined by Enrico Fermi as a word play on neutrone, the Italian name of the neutron)-were emitted, one could recover the conservation laws. The first detection of neutrinos did not occur until 1955, when Clyde Cowan and Frederick Reines recorded anti-neutrinos emitted by a nuclear reactor.  Neutrinos are among the lightest of the two dozen or so known subatomic particles and they come from all directions: from the Big Bang that began the universe, from exploding stars and, most of all, from the sun. It is appropriate to say that we are soaked in neutrinos.

They come straight through the earth at nearly the speed of light, all the time, day and night, in enormous numbers. About 100 trillion neutrinos pass through our bodies every second. Physicists study neutrinos in part because neutrinos are such odd characters: they seem to break the rules that describe nature at its most fundamental. And if physicists are ever going to fulfill their hopes of developing a coherent theory of reality that explains the basics of nature without exception, they are going to have to account for the behavior of neutrinos. The problem for physicists is that neutrinos are impossible to see and difficult to detect. Any instrument designed to do so may feel solid to the touch, but to neutrinos, even stainless steel is mostly empty space, as wide open as a solar system is to a comet. What’s more, neutrinos, unlike most subatomic particles, have no electric charge-they’re neutral, hence the name-so scientists can’t use electric or magnetic forces to capture them. Physicists call them “ghost particles.”

A neutrino is a subatomic particle that is very similar to an electron, but has no electrical charge and a very small mass, which might even be zero. Neutrinos are one of the most abundant particles in the universe. Because they have very little interaction with matter, however, they are incredibly difficult to detect. Nuclear forces treat electrons and neutrinos identically; neither participate in the strong nuclear force, but both participate equally in the weak nuclear force. Particles with this property are termed leptons. In addition to the electron (and it's anti-particle, the positron), the charged leptons include the muon (with a mass 200 times greater than that of the electron), the tau (with mass 3,500 times greater than that of the electron) and their anti-particles. Both the muon and the tau, like the electron, have accompanying neutrinos, which are called the muon-neutrino and tau-neutrino. The three neutrino types appear to be distinct: For instance, when muon-neutrinos interact with a target, they will always produce muons, and never taus or electrons. In particle interactions, although electrons and electron-neutrinos can be created and destroyed, the sum of the number of electrons and electron-neutrinos is conserved. This fact leads to dividing the leptons into three families, each with a charged lepton and its accompanying neutrino.

History of development

From what we know today, a majority of the neutrinos floating around were born around 15 billions years ago, soon after the birth of the universe. Since this time, the universe has continuously expanded and cooled, and neutrinos have just kept on going. Theoretically, there are now so many neutrinos that they constitute a cosmic background radiation whose temperature is 1.9 degree Kelvin (-271.2 degree Celsius). Other neutrinos are constantly being produced from nuclear power stations, particle accelerators, nuclear bombs, general atmospheric phenomenae, and during the births, collisions, and deaths of stars, particularly the explosions of supernovae.

1931-Pauli presents hypothetical "neutron" particle

1934-Fermi develops theory of weak interaction and baptizes the neutrino

1956-First discovery of the neutrino by an experiment by Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire were rewarded with the 1995 Nobel Prize.

1962-Discovery of another type of neutrino at Brookhaven National Lab

1968-First experiment to detect electron neutrinos produced by the sun

1978-Discovery of the tau lepton at Stanford Linear Accelerator Center, existence of tau neutrino theorized

1983-Kamiokande becomes operational

1985-The IMB experiment & Russian team reports measurement of non-zero neutrino mass

1987-Kamiokande and IMB detect simultaneous burst of neutrinos from Supernova

1989-Kamiokande becomes second experiment detecting neutrinos from the sun and confirms anomaly of finding only 1/3 expected rate

1990-IMB confirms deficit of muon neutrino interactions

1991-LEP experiments show that there are only three light neutrinos

1994-First proclamation of possible neutrinos oscillations seen by LSND experiment

1995-Missing solar neutrinos confirmed by GALLEX

1996-AMANDA neutrino telescope observes neutrinos at the south pole

1998-Super-Kamiokande collaboration announces evidence of non-zero neutrino mass

2000-First detection of actual tau neutrino interactions was announced by the DONUT collaboration at Fermilab, making it the latest particle of the Standard Model to have been directly observed.

Efforts for detection

To detect neutrinos, very large and very sensitive detectors are required. Typically, a low-energy neutrino will travel through many light-years of normal matter before interacting with anything. Consequently, all terrestrial neutrino experiments rely on measuring the tiny fraction of neutrinos that interact in reasonably sized detectors. For example, in the Sudbury Neutrino Observatory, a 1000 ton heavy water solar-neutrino detector picks up about 1012neutrinos each second. About 30 neutrinos per day are detected. To capture these elusive entities, physicists have conducted some extraordinarily ambitious experiments. Detectors are installed deep underground. Enormous ones have been placed in gold and nickel mines, in tunnels beneath mountains, in the ocean and in Antarctic ice. These strangely beautiful devices are monuments to humankind’s resolve to learn about the universe. Experimentalists began looking for it anyway. At a nuclear weapons laboratory in South Carolina in the mid-1950s, they stationed two large water tanks outside a nuclear reactor that, according to their equations, should have been making ten trillion neutrinos a second. The detector was tiny by today’s standards, but it still managed to spot neutrinos—three an hour. The scientists had established that the proposed neutrino was in fact real; study of the elusive particle accelerated.

A decade later, the field scaled up when another group of physicists installed a detector in the Homestake gold mine, in Lead, South Dakota, 4,850 feet underground. In this experiment the scientists set out to observe neutrinos by monitoring what happens on the rare occasion when a neutrino collides with a chlorine atom and creates radioactive argon, which is readily detectable. At the core of the experiment was a tank filled with 600 tons of a chlorine-rich liquid, perchloroethylene, a fluid used in dry-cleaning. Every few months, the scientists would flush the tank and extract about 15 argon atoms, evidence of 15 neutrinos. The monitoring continued for more than 30 years. Hoping to detect neutrinos in larger numbers, scientists in Japan led an experiment 3,300 feet underground in a zinc mine. Super-Kamiokande, or Super-K as it is known, began operating in 1996. The detector consists of 50,000 tons of water in a domed tank whose walls are covered with 13,000 light sensors. The sensors detect the occasional blue flash (too faint for our eyes to see) made when a neutrino collides with an atom in the water and creates an electron. And by tracing the exact path the electron traveled in the water, physicists could infer the source, in space, of the colliding neutrino. Most, they found, came from the sun. The measurements were sufficiently sensitive that Super-K could track the sun’s path across the sky and, from nearly a mile below the surface of the earth, watch day turn into night.

But the Homestake and Super-K experiments didn’t detect as many neutrinos as physicists expected. Research at the Sudbury Neutrino Observatory (SNO) determined why. Installed in a 6,800-foot-deep nickel mine in Ontario, SNO contains 1,100 tons of “heavy water,” which has an unusual form of hydrogen that reacts relatively easily with neutrinos. The fluid is in a tank suspended inside a huge acrylic ball that is itself held inside a geodesic superstructure, which absorbs vibrations and on which are hung 9,456 light sensors—the whole thing looking like a 30-foot-tall Christmas tree ornament. Scientists working at SNO discovered in 2001 that a neutrino can spontaneously switch among three different identities—or as physicists say, it oscillates among three flavors. The discovery had startling implications. For one thing, it showed that previous experiments had detected far fewer neutrinos than predicted because the instruments were tuned to just one neutrino flavor—the kind that creates an electron—and were missing the ones that switched. For another, the finding toppled physicists’ belief that a neutrino, like a photon, has no mass.  

A long-distance neutrino experiment is taking place under several Midwestern states. A high-energy accelerator, which generates subatomic particles, shoots beams of neutrinos and related particles as much as six miles deep, beneath northern Illinois, across Wisconsin and into Minnesota. The particles start at Fermilab, as part of an experiment called the Main Injector Neutrino Oscillation Search (MINOS). In less than three-thousandths of a second, they hit a detector in the Soudan iron mine, 450 miles away. The data the scientists have gathered complicates their picture of this infinitesimal world: it now appears that exotic forms of neutrinos, so-called anti-neutrinos, may not follow the same rules of oscillation as other neutrinos.

Growing dimensions of research

The fields related to neutrino particles and astrophysics are rich, diverse and developing rapidly. So it is impossible to try to summarize all of the activities in the field in a short note. That said, current questions attracting a large amount of experimental and theoretical effort include the following: What are the masses of the various neutrinos? How do they affect Big Bang cosmology? Do neutrinos oscillate? Or can neutrinos of one type change into another type as they travel through matter and space? Are neutrinos fundamentally distinct from their anti-particles? How do stars collapse and form supernovae? What is the role of the neutrino in cosmology? One long-standing issue of particular interest is the so-called solar neutrino problem. This name refers to the fact that several terrestrial experiments, spanning the past three decades, have consistently observed fewer solar neutrinos than would be necessary to produce the energy emitted from the sun. One possible solution is that neutrinos oscillate--that is, the electron neutrinos created in the sun change into muon- or tau-neutrinos as they travel to the earth. Because it is much more difficult to measure low-energy muon- or tau-neutrinos, this sort of conversion would explain why we have not observed the correct number of neutrinos on Earth.

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