IF WE look deep into the universe, we see stars and galaxies of all shapes and sizes. What we do not see, however, is that the universe is filled with particles called neutrinos. These particles have no charge and have little or no mass created less than one second after the Big Bang, and large numbers of these primordial low-energy neutrinos remain in the universe today because they interact very weakly with matter. Indeed, every cubic centimetre of space contains about 300 of these uncharged relics.
|Ground-based telescopes, like the Anglo-Australian Observatory, saw the light from supernova 1987A several hours after the Kamiokande and IMB experiments had already detected the neutrinos that were emitted.|
Trillions of neutrinos pass through our bodies every second almost all of these are produced in fusion reactions in the Sun's core. However, neutrino production is not just confined to our galaxy. When massive stars die, most of their energy is released as neutrinos in violent supernova explosions. Even though supernovas can appear as bright as galaxies when viewed with optical telescopes, this light represents only a small fraction of the energy released (see figure).
Physicists detected the first neutrinos from a supernova in 1987 when a star collapsed some 150 000 light-years away in the Large Magellanic Cloud, the galaxy nearest to the Milky Way. Two huge underground experiments the Kamiokande detector in Japan and the IMB experiment near Cleveland in Ohio, USA detected neutrinos from supernova 1987A a full three hours before light from the explosion reached Earth.
The event marked the birth of neutrino astronomy. New neutrino telescopes were built soon after, including the AMANDA experiment in Antarctica, and plans are under way to build an even larger experiment called ICECUBE to detect neutrinos from gamma-ray bursters billions of lightyears away.
However, neutrinos are still the least understood of the fundamental particles. For half a century physicists thought that neutrinos, like photons, had no mass. But recent data from the SuperKamiokande experiment in Japan overturned this view and confirmed that the Standard Model of particle physics is incomplete. To extend the Standard Model so that it incorporates massive neutrinos in a natural way will require far-reaching changes. For example, some theorists argue that extra spatial dimensions are needed to explain neutrino mass, while others argue that the hitherto sacred distinction between matter and antimatter will have to be abandoned. The mass of the neutrino may even explain our existence.
Read the rest of the story what we know about neutrinos and what we are learning about them right now.
This homepage is based on Feature Article "Origin of Neutrino mass" in Physics World, May 2002, by Hitoshi Murayama. The whole article can be download as a PDF file.