Search for double beta decay with the EXO-200 TPC and prospects for barium ion tagging in liquid xenon

Abstract

Neutrino oscillation experiments have shown that neutrinos have very small but non vanishing masses. These experiments are not able to determine neither the absolute mass scale of neutrinos nor whether they are Majorana particles. 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. The final state (i.e. the barium ion) can be tagged using optical spectroscopy. The efficient detection of the double beta decay daughter nucleus is a key step toward a background free measurement of such a rare process. An intermediate size detector (EXO-200) of 200 kg enriched xenon has been installed underground at WIPP (US). It is an ultra-low background detector with a design sensitivity of 6 × 1025 years for the half-life of neutrino-less double beta decay in Xe-136. A larger, ton-scale experiment is being designed with Ba ion tagging capability. We are presenting the status of the EXO-200 detector and review the R&D activities for a ton-scale EXO detector with barium ion tagging.