GERDA, a GERmanium Detector Array to Unravel the Nature of the Neutrino

Neutrinos play a fundamental role in our understanding of matter and the universe. They were postulated as elementary particles by Pauli in the 1930ies in order to explain the kinematics of radioactive decays. However, as their interaction with matter is so weak that they are likely to pass through light-years of lead, they were only observed directly in 1956 (Nobel prize 1995). Since then they are subject of an ever growing field of experimental and theoretical studies. With increasing knowledge it emerged that three kinds of neutrinos must exist. Nobel prizes were awarded for the discovery of the second (1988) and third (1995) kind of neutrino.

Neutrinos were originally assumed to be massless. The recent observations of neutrino oscillations (Nobel prize 2002) show that they are not. Neutrinos exist in different varieties, called `flavors'. A given neutrino cannot change its flavor if it is massless. But the neutrinos emitted from the sun do just that before they reach the earth. This implies that the flavors involved have different masses and the mass difference can be deduced from the observations. The absolute value of the masses involved can, however, not.

The determination of the absolute mass scale for neutrinos is one of the great challenges of experimental physics today. Of equal importance is the determination of the nature of neutrinos. Each fundamental building block of matter as we know it has a partner in the world of antimatter. Observations which are consistent with the interactions of the antimatter partners of neutrinos, anti-neutrinos, have been studied. For all other fundamental particles the observations allow to make a clear distinction between particle and anti-particle. In the case of the neutrinos the existing data cannot prove such a distinction. Neutrinos actually could be their own anti-particles and thus hold a special position in nature. If they hold this position, they could be the key ingredient for explaining the creation of the matter content of the universe.

The GERDA project addresses both issues. It searches for a special form of radioactive decay of the 76 Germanium isotope which is only possible if the neutrino is its own anti-particle. The observation of such a decay would determine the nature of the neutrino and measure the absolute mass scale. Several experiments performed searches in the past, but their sensitivity was not sufficient for conclusive results. Several new experiments are in the planning stage, but the only approved experiment featuring enriched 76 Germanium is GERDA. Germanium is particularly well suited for the quest, as the material is simultaneously the source of the decays and the basis for the detector. The development of specially designed Germanium detectors using crystals pulled exclusively from enriched material allows the full exploitation of the potential of Germanium. The new detectors, together with a new, innovative scheme to remove the disturbing effects of natural radioactivity, ensure that GERDA will be a leading edge experiment closely watched by the international community. It holds the potential of a major discovery and is one of the key experiments to further the understanding of matter and the universe.