Search for neutrino-less double beta decay with EXO

Abstract

Neutrino oscillation experiments have shown that neutrinos have very small but non vanishing masses. These experiments however are not able to determine neither the absolute mass scale of neutrinos nor whether they are two-component Majorana particles, i.e. their own antiparticles. Neutrino-less double beta decay can only occur if the neutrinos are Majorana particles, a preferred scenario in most possible schemes leading to finite masses. Among several viable candidate isotopes, EXO has chosen Xe-136 to search for this decay. Its main advantage is that the final state, i.e. the barium ion, can be tagged using optical spectroscopy. The detection of the double beta decay daughter nucleus can be the key to a background free measurement of such a rare process. An intermediate size detector (EXO-200) of 200 kg enriched xenon (80% Xe-136) is about to take data at the WIPP underground site in New Mexico. A ton-scale experiment is being designed with Ba ion tagging capability. EXO-full will detect, in addition to the two electrons, the coincident appearance of a barium ion. This improved event signature is expected to provide total elimination of the background from radioactive impurities.