Laboratory measurements and limits for neutrino properties
Upper limits for neutrino masses
Limits from cosmologySum of neutrino masses: Σ m < 1.0 eV (Hannestad)
from WMAP and 2dF data.
Also other analysis exists.
Measurements for electron neutrino massThe mass of electron neutrinos is measured in tritium beta decay experiments. The decay results in a 3-helium, electron and an electron antineutrino. If neutrinos have non-zero mass, the spectrum of the electrons is deformed at the high energy part, i.e. the neutrino mass determines the maximum energy of emitted electrons.
To be excact, the experiments measure the neutrino mass squared. Curiously, when taken at the face value, all results point to a negative mass squared, particularly the oldest experiment. This is probably due to a systematic error, and actually two running experiments, Mainz and Troitsk, have been able to measure physically acceptable values.
(**)The electron endpoint spectrum of Troitsk experiments can be fitted by an ordinary decay spectrum with a massless neutrino and a monoenergetic line just beyond the endpoint. In the values quoted in the above table the monoenergetic line has been extracted, assuming it to be of an external source. Making a fit to full data leads to negative mass squared and weaker limits. The position of the line varies, with a period of 0.503 ± 0.003 a.
The Mainz experiment does not support the Troitsk anomaly.
Presently the choice for the best limit of the electron neutrino mass is ambiguous. One should be careful with the interpretations until the anomalies will be clarified.
Future tritium beta decay experiment
The KATRIN experiment can push the limit for electron neutrino mass down for an order of magnitude.
Limits for electron neutrino Majorana massThe Majorana mass is measured by double beta decay experiments. These experiments use a nucleid that is stable in normal beta decay (involving one weak interaction vertex) but it can decay by a double weak interaction process that changes the charge of the nucleus by two units. In such a decay two neutrinos are emitted. However, if neutrinos have Majorana mass, a vertex with no external neutrinos is possible. A neutrinoless double beta decay is an unambiguos signal of a Majorana mass. In practice the neutrinoless double beta decay is identified from the normal two neutrino double beta decay by electron spectra, which requires lots of data to be taken.
In case of neutrino mixing the Majorana mass
experiments measure a specific mixture
of neutrino mass eigenvalues,
Future double beta decay experiments and proposals
neutrino magnetic moments
Number of neutrino types
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Last modified 11.4.2005 (webmaster)