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The Search For Neutrino Mass

Physicists have observed three different types of neutrinos, but know little about their masses.
The subatomic particles called neutrinos are among the most elusive in the particle kingdom. In scattered locations around the world, scientists have built underground laboratories to detect these ghostly particles that come from the sun, from supernovae and from many other celestial objects. In fact, neutrinos fill the whole universe, with about 10 million of them per cubic foot. Because they almost never interact with matter, only very sophisticated experiments can now and then catch the tell-tale sign of a neutrino. Most neutrinos just zip through the earth, and through particle detectors, without leaving a trace.

To learn more about neutrinos, physicists use powerful accelerators to produce neutrino beams consisting of billions of neutrinos, of which a tiny fraction can be detected by detectors placed in the beam line. At Fermilab, such an experiment led in 2000 to the discovery of the tau neutrino, the third of the three known types of neutrinos.

Now, Fermilab is preparing for two more experiments with neutrino beams. Both experiments will look for neutrino oscillations, the transformation of one type of neutrino into another type. If they find that some neutrinos have oscillated from one neutrino to another, then the experimenters will know that they must have mass. Because there are so many neutrinos in the universe, if they do have mass, neutrinos could account for up to five percent of the entire mass in the universe.

The new MiniBooNE experiment will look for oscillations of muon neutrinos into electron neutrinos. It uses a large tank filled with mineral oil to look for particles produced when a neutrino hits the nucleus of an atom. The signature of such an interaction is a cone of light that hits light-sensitive devices mounted inside the tank.

The second experiment, MINOS, will look for oscillations of muon neutrinos into tau neutrinos. Fermilab is now building the facilities needed to create a muon neutrino beam (the NuMI project). Physicists will then send the beam 450 miles through the earth to a detector in an old iron mine in Soudan, Minnesota, where a few neutrinos will leave traces and tell whether they arrived as muon or tau neutrinos.

Fermilab theorists are also active in many aspects of studying neutrinos, from making predictions for what experiments will measure to trying to understand what new experimental results tell us. One important area of research concerns the neutrino masses --- how can they be measured? why are their masses so much smaller than those of other particles? how do the masses of the different types of neutrinos compare? Another concerns using neutrinos to study astrophysical sources, for example to see into the interior of the sun or a supernova, or by their impact on cosmology, for example their role in revealing the particle-antiparticle asymmetry of the universe.

Neutrinos matter (PDF, 24 pages)
Neutrinos and their history
More details

*Material on this web page courtesy of Fermi National Accelerator Laboratory
FermiLab advances the understanding of the fundamental nature of matter and energy by providing leadership and resources for qualified researchers to conduct basic research at the frontiers of high energy physics and related disciplines.

URL: http://www.fnal.gov/pub/inquiring/physics/neutrino/index.html
Last updated October 14, 2003