Neutrinos and Neutrino Oscillations

In the 1930's, scientists exploring the nascent field of nuclear physics were confronted with a troubling mystery: one of their most cherished laws appeared to be no longer valid.

Natural scientists had postulated "conservation laws" to assist in their understanding of the physical world. Isaac Newton established the constancy of momentum, a quantity specific to a moving object’s state of motion. In the nineteenth century physicists and chemists such as Joule, Kelvin, and Carnot developed the science of thermodynamics, outlining the rules of heat and its exchange, a science of great importance to the Industrial Revolution that would soon follow. Rudolf Clausius first put forth the principle of conservation of energy--also called the first law of thermodynamics--stating that there is a constant amount of energy in the universe. In any change we observe, energy can be transformed from one form into another--from heat into motion, say--but it cannot be created or destroyed.

By the early part of the twentieth century Albert Einstein had formulated his famous equation stating that energy and mass were also interchangeable. In a revolution just as significant as that of the earlier century, physicists had begun to unravel the mysteries of the atom and its individual particles, a science they called quantum mechanics.

Throughout these scientific milestones, the concept of energy remained central, and its conservation well established. No matter how a closed system of particles, machine parts, or planets moved and interacted, they did so in a way that left the total energy of the system constant. That energy could be energy of motion, or heat, or radiation, but in the accounting book of the universe, the amount of energy in a system stayed constant. It seemed a fundamental principle of the universe.

Is energy really conserved?
Given what seemed the universality of this principle, physicists were quite surprised to discover a situation where energy appeared not to be conserved: a reaction called beta decay. The nuclei of some atoms are unstable and spontaneously emit an electron (a “beta particle”), and in the process change into a stable nucleus of a different element. (For example, a carbon nucleus with six protons and eight neutrons can eject an electron and change into a nitrogen nucleus with seven protons and seven neutrons.) But when the scientists measured the masses and energies of the particles visible before and after the beta decay, they discovered to their consternation that neither energy nor momentum appeared to be conserved. The nucleus appeared to break into two pieces--but the pieces didn't fit together. The Danish physicist Niels Bohr even dared to suggest that perhaps energy was not conserved after all.

The hallowed conservation laws were saved, though, when the German physicist Wolfgang Pauli postulated in 1931 that in fact a third particle was emitted in beta decay. (This apparent salvation came from a genius who once complained that physics was too hard and he wished he had become a movie comedian!) This particle, he said, would have no mass and no electric charge, and the fact that it had a particular value of a third conserved quantity called “spin” meant it could not be the already discovered quantum of light, the photon. Enrico Fermi subsequently christened Pauli's newcomer the neutrino, which means “little neutral one” in Italian.

The neutrino remained immune from direct detection until 1956, when Frederick Reines, Clyde Cowan, Jr., and collaborators mounted an experiment at the Savannah River nuclear reactor in South Carolina. They monitored nuclear reactions in which the electron’s opposite twin, an antiparticle called a “positron,” was emitted. ( Antiparticles have the same mass and spin as their particle partners, but opposite values of electric charge. Every known particle has such an associated antiparticle, although the universe predominantly consists of matter, not antimatter.) This first sighting of the antineutrino won Reines the 1995 Physics Nobel Prize.

Neutrinos and antineutrinos were unlike any particle seen before. Most characteristic was their shyness--neutrinos hardly interacted with anything. In fact, about 100 trillion neutrinos pass straight through your body each second. Neutrinos interact with other matter so weakly that they can travel through trillions of miles of lead without interacting.

A second type of neutrino
In 1962 three Columbia University physicists discovered a second type of neutrino, a discovery that also won a Nobel Prize. Working at Brookhaven National Laboratory on Long Island, Leon Lederman, Melvin Schwartz, and Jack Steinberger observed the decay of a nuclear particle called the pion. Decaying pions produce muons, particles similar to electrons but about 207 times more massive, and presumably neutrinos. The fast-moving muons were filtered out by a wall built of 5,000 tons of old battleship plating, but the neutrinos passed right through.

This new neutrino was called the “muon neutrino,” because it was found in particle reactions involving the muon. The earlier type of neutrino was dubbed the “electron neutrino.” Neutrinos were said to come in two “flavors,” the electron kind, and the muon kind. Neither flavor had an electric charge nor (at the time) a discernible mass.

In their work to elucidate the internal structure and energy source of our sun, nuclear physicists found that the sun emits enormous numbers of neutrinos. To observe such neutrinos physicists have had to go underground--deep underground. Because the earth is bombarded not just with neutrinos but with a host of charged particles called cosmic rays, physicists trying to catch the former needed to screen out the latter. They did this by placing their detectors in underground mine chambers, such as the Homestake gold mine in South Dakota. The 4,850 feet (1,480 meters) of rock above the chamber stopped the cosmic rays, but as always the neutrinos passed through like ghosts. Inside the mine chamber was a large tank of chlorinated cleaning fluid; very infrequently one of the neutrinos would collide with a chlorine nucleus in the fluid and change it into an argon nucleus. The argon produced in this process is radioactive; by measuring the amount of radioactive argon, the number of neutrinos from the sun could be inferred.

Where are the sun's neutrinos?
Physicists had been studying such solar neutrinos for nearly thirty years, and they had once again been confronted with a mystery--they only observed about half the electron neutrinos that the sun was expected to emit, based on the energy it was known to produce.

Once again physicists had to let go of their old ideas and try to imagine the creativity of Nature. One solution to the “solar neutrino problem” would be if neutrinos could oscillate--if, in the course of their travels through space, an electron neutrino could spontaneously change into a muon (or other kind of) neutrino. The Homestake detector would not respond to muon neutrinos, so as far as the cleaning fluid was concerned, some fraction of the sun’s electron neutrinos had disappeared.

Quantum mechanics, however, implied something more: neutrino oscillations, if they existed, require neutrinos to have mass. It could be a very small mass, but it could not be zero. (A tight neutrino burst observed from supernova 1987A meant that oscillating neutrinos would have very little mass indeed.)  In addition, because the universe is flooded with neutrinos, any neutrino mass at all could be a significant fraction of the mass of the universe. In fact, astrophysicists that have been measuring the mass of the universe, collected mostly in stars, have not seen as much mass as they expected, based on the rotating motion of galaxies and other considerations. Could neutrinos be the "missing mass" of the universe?

A 1995 experiment at Los Alamos National Laboratory stunned the physics community when it reported instances in which a neutrino of one flavor transformed itself into a neutrino of another flavor. It offered a tantalizing hint that neutrino oscillations might be real.

In 2000, scientists at Fermi National Accelerator Laboratory (“Fermilab”) made the first direct observation of a third flavor of neutrino, the tau neutrino. It was associated with the tau lepton, a quite massive cousin of the electron and the muon, discovered in 1975.

The Standard Model
The three flavors of leptons (electron, muon, and tau), their antiparticles, and their associated neutrinos and antineutrinos have taken their place in the “Standard Model” of particle physics. Somewhat like the chemists' periodic table of the elements, the Standard Model lays out all the particles, their properties, and their specific interactions that comprise everything seen in the universe. Though there is still some debate about some of its details--such as neutrino properties and the way that gravity fits into it--the Standard Model has shown itself to be quite successful as a theory of the workings of the subatomic world.

Physicists have continued to make progress on understanding neutrino oscillations and the question of neutrino mass. In 1998 the Super-Kamiokande Experiment in Japan found stronger evidence for a nonzero neutrino mass, indicating that neutrinos are oscillating. In 2001 the Sudbury Neutrino Observatory (SNO) in Canada discovered the strongest evidence yet that neutrinos are oscillating into one another. An underground detector similar to Homestake and Super-Kamiokande, SNO is a 10-story-tall apparatus located 1.2 miles (2 kilometers) underground, containing about 1,000 tons (almost one million kilograms) of heavy water. "Heavy water" is chemically the same as regular water, but its hydrogen nuclei all have a proton and a neutron (instead of normal hydrogen's solo proton).) Electrons produced when electron neutrinos collide with a heavy water nucleus emit a characteristic light when they travel rapidly through the water, which SNO detects via thousands of detection tubes that line the inside of its tank.

In 2002 SNO strengthened its results, with scientists saying they were now 99.999% certain that solar neutrinos oscillate one into the other, and that “the results were in excellent agreement with calculations of the nuclear reactions powering the sun.” SNO has not, however, been able to make any conclusions about neutrino types that might exist in addition to the three already known today (e.g., the so-called "sterile" neutrinos), or whether these three "normal-flavor" neutrinos might be oscillating back and forth into sterile ones.

The further solidifying of the evidence for neutrino oscillation moves us farther along a voyage physicists embarked upon more than 70 years ago. It demonstrates once again that, as scientist J.B.S. Haldane noted, the universe is not only stranger than we do suppose, but stranger than we can suppose. Neutrino scientists are actively striving to understand the details of neutrino oscillations, to someday measure the masses of the three neutrino flavors, and to completely understand their role in the evolution of the universe. The wispy neutrino is a particle whose time has most definitely arrived.

--David Appell