In the continuing quest to understand the fundamental structure of matter and the nature of the universe, physicists have designed a project to explore the ghostly subatomic particles called neutrinos. The MINOS (Main Injector Neutrino Oscillation Search) experiment will try to solve a mystery that has intrigued scientists for many years by answering the question whether neutrinos have mass. According to the current model that gives us our best explanation of the behavior of fundamental particles and forces, neutrinos are massless. But if experiments show that neutrinos do have mass, however tiny, that discovery will profoundly change our view of the universe.
The Mystery of the Vanishing Energy
In 1930, the Austrian-born physicist Wolfgang Pauli postulated the particle we call the neutrino. He invented the neutrino in response to a dilemma: in the aftermath of a reaction in which a neutron transforms into a proton and electron, some energy and some angular momentum seemed to vanish. The sum of energy and momentum after the decay event did not add up to the initial total energy and momentum. Pauli was confronted with a mystery. To solve it, he proposed a new particle, the neutrino. If he factored the neutrino into the picture, it would carry the missing energy and momentum.
A careful accounting of the energy and momentum before and after the decay showed that if a particle was indeed slipping away undetected, it must be uncharged, or neutral, and must have practically no mass and almost no interactions with matter. In other words, Pauli had invented a particle that would be almost impossible to observe. He himself thought that experimenters might never find proof of the existence of neutrinos.
Pauli was almost rightbut not quite. As often happens, developing technology led indirectly to a breakthrough in basic science. Calculations showed that newly developed nuclear reactors should produce huge numbers of neutrinosa large "neutrino flux"as a byproduct. Inspired by the challenge of finding a particle that was considered impossible to detect, Frederick Reines and his colleague Clyde Cowan set about trying to detect neutrinos from the nuclear reactor at Savannah River, South Carolina. In 1953, with an experiment that would eventually earn the 1995 Nobel Prize in physics, Reines and Cowan found direct evidence for Pauli's neutrino.
Today, neutrinos are an integral part of the theory of the fundamental particles and forces of nature. The first neutrino experiments at a particle accelerator demonstrated that there are at least two "flavors" of neutrinos: the electron neutrino, which appears only in connection with electrons; and the muon neutrino, which comes from the decay of pion and kaon particles. Physicist Martin Perl's discovery of the tau lepton, also recognized by the 1995 Nobel Prize, led scientists to believe that there must be a third flavor of neutrino associated with the tau. The tau neutrino would complete the third pair of the triad: electron and electron neutrino, muon and muon neutrino, tau and tau neutrino. Many experiments have verified that the tau neutrino does exist, but so far no one has observed it directly.
The Mystery of the Vanishing Neutrinos
Neutrinos permeate the world around us. Every cubic centimeter of space contains more than a hundred neutrinos. Yet we know very little of these elusive particles beyond their angular momentum and the fact that their mass, if any, must be very small. We do not know if they have magnetic moment, or what the lifetime of neutrinos may be. In contrast, we know the properties of other leptons, such as the electron, very precisely.
About 30 years ago, as improving technology made possible better and better detection of neutrinos, physicists came upon another neutrino mystery. Deep in the Homestake mine in South Dakota, they discovered the first of several "smoking guns" in the mystery of the vanishing neutrinos. Calculations of solar nuclear activity had predicted how many electron neutrinos should arrive on earth from the sun. But when experimenters in the Homestake Mine counted the electron neutrinos that actually showed up, they found only about half the number predicted. More recently, other experiments also observed a deficit of muon neutrinos from the interactions of cosmic rays with atoms in the upper atmospherea second smoking gun. Where were the missing neutrinos? Finally came the great puzzle of the "dark matter" that makes up 90 percent of the universe, but which we cannot see. Could dark matter be yet a third smoking gun in the neutrino mystery?
All of these clues seem to hint at a possible solution to the mystery. If experiments can show that neutrinos do indeed have mass, then some of them could change from one type into another, accounting for the missing neutrinos arriving on earth: they may simply have changed into another flavor. And if neutrinos turn out to have mass, they may account for some fraction of the dark matter of the universe.
In 1995, a group of scientists working at Los Alamos National Laboratory reported results that provide another hint of the existence of neutrino oscillations. In a study of neutrino interactions in an essentially pure beam of muon neutrinos, they observed events whose most likely explanation would be interactions of the electron neutrino. If this interpretation is correct, a possible explanation would be the oscillation of muon neutrinos into electron neutrinos.
Three years later, the Super-Kamiokande experiment in Japan turned in significant new results. The international group of scientists confirmed previous results showing that the number of atmospheric neutrinos is far less than the laws of physics would predict. But the scientists also found an asymmetry between the number of neutrinos that entered their detector from overhead and the number that entered from underneath, having traveled an extra 13,000 kilometers through the earth. These results highly suggest that neutrinos do indeed oscillate.
Solving the mystery of the vanishing energy in 1930 gave us the neutrino. As we solve the new mysteries surrounding these elusive particles, we will learn more about the fundamental nature of matter and the universe.
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