NEUTRINO PHYSICS

Advances in Physics: Evidence for Masses

NEUTRINOS, (pronounced nu), are a class of subatomic particles that are "colourless" (see Lattice Gauge Theory & Quantum Chromodynamics article) and electrically neutral. For more than twenty years after the proposal of their existence in 1930 by the Austrian physicist Wolfgang Pauli, these weakly interacting particles that could pass through the Earth without resistance had escaped detection. Today, three types, or flavours, of neutrinos are known to exist, one for each electrically charged lepton --- the electron e, muon mu and tauon tau. Conventionally, the neutrino emitted with an electron in ordinary beta-decay is dubbed the "electron neutrino". The remaining two flavours are then labelled accordingly.

The Standard Model of Particle Physics predicts neutrinos to be exactly massless. There are, however, several experimental indications that neutrinos do have mass. Though tiny are the masses, confirmation of these results would not only be an exciting discovery of new Particle Physics, but would also impact on aspects of Cosmology and Astrophysics.

A sensitive way to search for small neutrino masses is to look for neutrino oscillations. Neutrino oscillations are a Quantum Mechanical effect whereby propagating neutrinos of one type convert back and forth periodically to another. Neutrino masses are necessary for such a conversion. To date, perhaps the two strongest pieces of evidence for oscillations are the atmospheric and solar neutrino anomalies.

Atmospheric neutrino anomaly Cosmic rays interact with the Earth's atmosphere to produce electron and muon neutrinos, which are detected in experiments around the world. Interestingly, there appears to be a deficit of muon neutrinos. Furthermore, the deficit shows directional dependence --- there are roughly twice as many muon neutrinos travelling directly downwards as there are coming up through the Earth, having covered an extra 1 000 to 10 000 km in the course. This fits in snugly with the oscillation picture --- large amplitude muon neutrino oscillations with an oscillation length of 100 to 1 000 km. From this, the inferred muon neutrino mass is 0.03 to 0.1 eV (if the muon neutrino oscillates with a neutrino of negligible mass). One electron volt (eV) corresponds to $1.8\; X\; 10-36$ kg. The neutrino is ten million times lighter than the electron!

Solar neutrino anomaly As a natural nuclear reactor, the Sun is a steady source of electron neutrinos, which have been detected on Earth since the 1960s. However, the number of electron neutrinos measured also appears to be less than the expected number by roughly a factor of two. Again, neutrino oscillations seem to offer the simplest explanation.

On the Cosmological and Astrophysical side, massive neutrinos may well be an important constituent of dark matter in interstellar and/or intergalactic space, which, in turn, is crucial in the study of galactic evolution and in determining the curvature of the universe and thus its ultimate fate. The success of a supernova explosion at the end of stellar evolution also depends greatly on neutrinos. In particular, supernova sites are speculated to be heavy nuclei (mass number > 70) factories. A detailed knowledge of neutrino properties will allow a deeper understanding of the observed elemental abundances in the universe today.

More information on neutrino research around the world may be obtained from The Ultimate Neutrino Page and The Neutrino Oscillation Industry.

See also:
The Mathematics of Neutrino Oscillations
Nine Thousand Eyes...