Results and Prospects of Neutrinoless Double Beta Decay Search with EXO
Recent observations of tiny neutrino masses in solar, atmospheric, and reactor neutrino data raised several intriguing questions. Why are neutrinos so much lighter than the other particles? What is the absolute scale of the neutrino mass spectrum? And is the neutrino its own anti-particle, i.e. a Majorana particle? These questions are best addressed by searching for neutrinoless double beta decay, an exotic nuclear process which can shed light on both the absolute scale of the neutrino mass spectrum, and on the underlying mechanism responsible for the tiny masses that we observe in nature.
The Enriched Xenon Observatory (EXO) is an experimental program designed to search for the neutrino-less double beta decay of Xe-136. The current phase of the experiment, EXO-200, uses 200 kg of liquid xenon with 80% enrichment in Xe-136, and also serves as a prototype for a future 1-10 ton scale EXO experiment. The detector has been taking low background physics data with enriched xenon at Waste Isolation Pilot Plant (WIPP) in New Mexico since early May 2011. The collaboration has previously reported the first observation of two-neutrino double beta decay in 136Xe. In our new data, no signal is observed for an exposure of 32.5 kg-yr, with a background of 1.5 × 10-3 kg-1 yr-1 keV-1 in the ±1 sigma region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay T1/2(136Xe) > 1.6 × 1025 yr (90% CL), corresponding to effective Majorana masses of less than 140-380 meV, depending on the matrix element calculation. Current R&D efforts towards a ton-scale experiment will also be discussed.