Laboratory measurements and limits for neutrino properties

Upper limits for neutrino masses

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Limits from cosmology

Sum of neutrino masses: Σ m < 1.0 eV (Hannestad)
from WMAP and 2dF data.

Sum of neutrino masses: Σ m < 0.7 eV (Sperger et al.)
from WMAP, 2dF, CBI, ACBAR and Lyman-α forest data.

Also other analysis exists.

Measurements for electron neutrino mass

The 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.

Experiment measured mass squared formal limit C.L. Year
Mainz -1.6 ± 2.5 ± 2.1 2.2 95 % 2000
Troitsk -1.0 ± 3.0 ± 2.1 (**) 2.5 95 % 2000
Zürich -24 ± 48 ± 61 11.7 95 % 1992
Tokyo INS - 65 ± 85 ± 65 13.1 95% 1991
Los Alamos - 147 ± 68 ± 41 9.3 95% 1991
Livermore - 130 ± 20 ± 15 7.0 95% 1995
China - 31 ± 75 ± 48 12.4 95% 1995
Average of PDG (98) -27 ± 20 15 95 % 1998
Masses in units of eV.

(**)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

Experiment mass sensitivity/eV Status Year
KATRIN 0.3 proposed
2006?

The KATRIN experiment can push the limit for electron neutrino mass down for an order of magnitude.

Limits for electron neutrino Majorana mass

The 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,
< m > = Uei2 mi, i summed over all mass eigenstates.
For sufficiently small masses, the measured value is close enough to the diagonal component of the mass matrix connected to electron weak eigenstate,
< m > = mee
Some models, like the Zee model, predict very low values for neutrinoless double beta decay, still allowing the physical masses of all neutrinos to be orders of magnitudes larger than the observed limit of effective Majorana mass.

Experiment nucleid half life/a mass limit/eV majoron coupling Year
Heidelberg-Moscow 76-Ge 5.7 1025 0.2 (90%)   1999
IGEX 76-Ge 1.57 1025 0.33   1999
NEMO 116-Cd
82-Se
96-Zr
100-Mo		 5*1021
9.5*1021
1.3*1021
6.4*1021 		9.8
.
.
6-18 (90%)		 1.2*10-4
*
*
(2-6)*10-4		 1998
Elegants 76-Ge
100-Mo
116-Cd
48-Ca
4.5 * 1022
6.4 1021
5.2 * 1022


3 (90%)		   1995
Gothard tunnel 136-Xe 4.4*1023 (90%) 1.8-2.8 (90%) 1.5* 10-4 1993-
Milano-Gran Sasso 130-Te
128-Te		 1.4 * 1023 (90 %)
8.6 * 1022 (90 %)		 2.6 6.7 10-4 1995,1998
Solotvina 116-Cd 7.0 * 1022 (90 %) 2.6 (90 %) 1.2 * 10 -4 (90 %) 2000
  • The life time is given in second, the column corresponds to the lower limit for neutrinoless double beta decay.
  • The limit for the Majorana mass depends on the nuclear matrix elements.
  • Majoron column is the upper limit for the neutrino-majoron coupling, deduced from the non-observation of majoron emitting double beta decay.
So far, the only experiment claiming to have measured the Majorana mass, is the Heidelberg-Moscow -group. The best value given in (ref) is 0.39 eV. The data analysis has been critised ( ref).

Experiment nucleid half-life/a mass/eV Year
Heidelberg-Moscow 76-Ge (0.8-18.3)*1025 0.11-0.56 eV 2001

Future double beta decay experiments and proposals

Experiment nucleid detector sensitivity/eV Year
GENIUS 100-1000 kg 76-Ge Ge 0.01 200?
CUORE 225 kg 130-Te TeO2    
NEMO 3 several, 10 kg drift chamber   2002
MOON Mo      
The start times are only suggestive, and not all proposals have decisive funding. Other proposals exist.

neutrino magnetic moments

New projects:
  • MUNU- sensitivity up to (2-3) 10-11 µ B.

Number of neutrino types

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Last modified 11.4.2005 (webmaster)