An experiment being built at the Gran Sasso laboratory in Italy could provide valuable insights into neutrino physics and the nature of dark matter. Ettore Fiorini of Milan University and colleagues have calculated that their tellurium detector – named CUORE – should be able to observe an extremely rare kind of beta decay for the first time. Such an observation would yield a value for the mass of the neutrino, thought to account for some of the universe’s dark matter (xxx.lanl.gov/abs/hep-ex/0201038).
Last year, physicists at the Sudbury Neutrino Observatory in Canada confirmed that neutrinos can ‘oscillate’ from one flavour (electron, muon or tau) to another, and that they therefore have mass. It is not possible to deduce the flavour masses directly from such oscillations, only the difference between them, but the results suggest that the masses lie somewhere between 0.01 and 0.05 eV.
In contrast, Fiorini and co-workers will study a phenomenon known as double-beta decay. This extremely rare form of beta decay, which does not involve the emission of antineutrinos, is only possible if neutrinos have mass and they are their own antiparticles. Although it is extremely unlikely to happen in any one nucleus, such a decay should be seen over the lifetime of the CUORE experiment – which will consist of 1000 cubes of tellurium oxide, each weighing 750 grams. The effective mass of the electron neutrino can then be deduced from the associated decay lifetime.
Current double beta-decay experiments are not large enough to make a direct observation of this rare decay, but they can set a lower limit to the decay half-life and a corresponding upper limit on the neutrino mass. Combining these data with the results from the SuperKamiokande laboratory in Japan, the neutrino mass is tentatively predicted to lie between 0.01 and 1 eV. Results from an experiment involving just 20 tellurium crystals indicate that CUORE should be able to probe down to about 0.02 eV, which would put it in sight of neutrino-less decay.
In the unlikely event that CUORE did not make such an observation, it would still be able to set a much more precise upper limit on the neutrino mass. If this is as low as the results from the oscillation experiments suggest then neutrinos can be ruled out as a significant source of dark matter – to account for all the dark matter in the universe, the neutrino would need to have a mass of between 10 and 50 eV.
CUORE has its rivals, however, including two proposals that would use the material found in the current generation of detectors, germanium-76. But according to the Milan group, these experiments would be more expensive since germanium-76 – unlike tellurium-130 – is a relatively rare isotope that requires enrichment. Fiorini and colleagues point out that in contrast to CUORE these detectors would also need significant R&D. But they add that a sighting of neutrino-less decay made by CUORE would need to be confirmed by a second detector of about the same size built from a different material.
