Neutrinoless double beta decay

Further information and contact:

Dr. Michael Marino

TU München - Excellence Cluster Universe
Boltzmannstr. 2/EXC
85748 Garching

Tel: +49 89 35831 7149
The Enriched Xenon Observatory EXO intends to detect the rare transition of the isotope 136Xe to 136Ba via double beta decay, possibly without the emission of a neutrino. The detection of the neutrinoless channel would aid in determining the mass of the neutrino and the neutrino-mass hierarchy. At the same time the discovery of this channel would indicate the Majorana nature of the neutrino, that the neutrino and anti-neutrino are the same particle. Such a feature would be theoretically very attractive, as it would provide a mechanism to construct baryogenesis via leptogenesis, an important early-universe process leading to the observed asymmetry between matter and antimatter. EXO-200, a prototype detector with 200 kg of enriched 136Xe built at Stanford University, is currently the largest-mass double-beta decay experiment in the world; it recently started operation (Fall 2010) at WIPP in Carlsbad, New Mexico. The detector is at the moment (Jan-Feb 2011) being refilled with liquid xenon to begin low-background data taking.

Example data event:

The general principal of the liquid Xe Time Projection Chamber (TPC) is outlined: a deposition of energy in the detector creates an initial burst of light – collected by the Avalanche Photodiodes (APDs) – and is followed by a subsequent signal on the wires (U are collection wires, V, induction wires). This example event shown above depicts a possible 214Bi→214Po→214Pb correlated set of decays in the detector. In particular, the two colored arrows (red and blue) help to interpret the results between the two readout channels, delineating which signal in the light (APD) channel yields the corresponding signal in the charge (wire) channel. In this event, two light signals are clear, separated by around 164 μs. However, only one signal in the wire channel is apparent. The initial signal (red arrow) is consistent with energy deposition from the beta particle from the initial 214Bi→214Po, which generates a more balanced set of signals in the charge and light channels. The second signal (blue arrow) is however more consistent with energy deposition from an alpha particle from the ensuing 214Po→214Pb decay, which generates much more scintillation light than charge.

Group activities:

We currently set up an R&D experiment for the development of the next phase of EXO, which should be a ton-scale detector with the additional online identification of the individual daughter nuclei, 136Ba++. This possibility basically removes all backgrounds. Our contribution focuses on an experiment that should provide crucial information on the future layout of this next phase: we intend to investigate the behavior of radioactive barium isotopes in different states of xenon quantitatively.

Radioactive barium ions in xenon ice experiment at the Argonne National Lab (Chicago)

Other current activities of the Fierlinger group include development of data analysis software and framework. Initial analysis effort is focused towards a deep understanding of the detector and will transition towards physics analyses as low-background data becomes available in the early to middle part of 2011. In particular, such activities include looking for events possibly associated with radioactive backgrounds (e.g. U and Th decay chains, 40K, etc.), deriving cuts for the data, and refining event reconstruction techniques. As the experiment is currently underway, many opportunities exist for interested students. Further information is available from Mike Marino.