New experimental data, which show that neutrinos have mass,
are forcing theorists to revise the Standard Model of particle physics

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.

1. Introduction
   Neutrinos are everywhere. Trillions of them are passing through your body every second,but they are so shy and we do not see or feel them. They are the least understood elementary particle we know that exist.
2. Birth of Neutrinos
   Existing of neutrinos was suggested as a "desperate remedy" to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei.
3. The Standard Model
   The Standard Model of particle physics can describe everything we know about elementary particles. It says that neutrinos do not have mass. Neutrinos do not have mass because they are all "left-handed" and do not bump on the mysterious "Higgs boson" that fills our entire Universe.
4. Evidence for neutrino mass
   In 1998, a convincing evidence was reported that neutrinos have mass. The Standard Model has fallen after decades of invicibility. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines.
5. Implications of neutrino mass
   Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lighest elementary particle: electron. How do we need to change the Standard Model to explain the neutrino mass? Some argue that our spacetime has unseen spatial dimensions, and we are stuck on three-dimensional "sheets". Other argue that we need to abandon the sacred distinction between matter and anti-matter.
6. Why do we exist?
   When Universe started with the "Big Bang", there were almost equal amount of matter and anti-matter. Most of matter was annihilated by anti-matter when Universe cooled. We are leftover of one part in ten billions. Why was there a small excess matter over anti-matter so that we can exist? Once we abandon the sacred distinction between matter and anti-matter, it provides a key to understand why we exist.
7. Outlook The mysteries about neutrinos are now being unraveled dramatically. We will learn much more in the coming years.

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.