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Union University Department of Physics

The Science Guys



Science Guys > December 2003

December 2003

What is a neutrino?

We’ve heard of atoms, and that atoms are composed of protons, neutrons, and electrons. Today we believe that these particles are composed of smaller constituents called quarks. However, neutrinos are certainly not found in the average person’s vocabulary.

The universe is actually teeming with many different ’particles’. The neutrino is but one of a vast "subatomic zoo." The neutrino was postulated in order to save one of the most cherished laws in physics- the conservation of energy. In the 1920’s it was realized that when radioactive atoms beta decay (when a nucleus emits an electron) the observed products of the decay violated the principle of energy conservation. A physicist named Wolfgang Pauli postulated the existence of another product of the decay- a very tiny particle to carry away the missing energy.

Enrico Fermi worked out the details of beta decay involving such a tiny particle. The particle had to have incredibly small mass, interact very little with matter, and be electrically neutral. He named this particle the neutrino, Italian for "little neutral one." Although neutrinos were predicted by the mathematics, they were not discovered experimentally until 1956.

The universe appears to be filled with these ghost-like neutrinos, which interact so little with matter that they pass through the Earth as if it were tissue paper traveling near the speed of light. That is why it took 30 years to detect neutrinos after they were predicted. Indeed, while reading this article billions have passed effortlessly through your body!

Neutrinos are produced in nuclear reactions like those in our Sun. These nuclear reactions generate the heat and light that bring life to our planet, and they also produce copious amounts of neutrinos. It was once believed that neutrinos were massless particles, but this picture of neutrinos is currently under revision. Understanding the neutrino and its production is essential in refining theories of how our Sun produces its energy.

Determining the precise properties of neutrinos is the focus of several experiments. Key among them is Japan’s Super Kamiokande experiment and the Sudbury Neutrino Observatory in Canada. The results of these experiments imply that the neutrino does possess a very tiny mass and moves at slightly less than the speed of light. Even more interesting, the basic neutrino may "oscillate" or change periodically into other forms or "flavors" of neutrinos. This oscillation implies the neutrino has some mass and that there are several types of neutrinos. A new facility - the Main Injector Neutrino Oscillation Search (MINOS) in Minnesota recently began acquiring data and should answer many lingering questions about neutrino oscillations.

Determining the properties of these mysterious particles will give us insight into our own Sun and may even tell us more about the "missing" matter astrophysicists are hunting for- the "dark matter." So the next time you ponder the universe, consider that an untold number of mysterious neutrinos are zipping all around you each second!