The NEMO-3 Experiment

R. B. Pahlka on behalf of the NEMO-3 Collaboration

Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

Abstract

The NEMO-3 (Neutrino Ettore Majorana Observatory) experiment, located in the Modane Underground Labora-tory, is currently searching for neutrinoless double beta decay (0νββ). The experiment has been taking data since 2003with seven double beta isotopes and is scheduled to complete data acquisition in late 2010. Two neutrino double betadecay (2νββ) results for the main isotopes (7 kg of 100Mo and 1 kg of 82Se), new results for 150Nd, as well as results for96Zr, 48Ca, 130Te and 116Cd are presented. NEMO-3 uses a unique technique that allows for the in situ measurementof background contamination. No evidence for neutrinoless double beta decay has been found to date. The data arealso interpreted in terms of alternative models such as weak right-handed currents and Majoron emission.

Keywords:NEMO-3, neutrinoless double beta decay, neutrino mass hierarchy, Majoron, Majorana neutrino

Double beta decay is a rare nuclear process in whichtwo neutrons in the same nucleus are spontaneouslyconverted into two protons. This process occurs whensingle beta decay is either forbidden or highly sup-pressed and may happen in at least two different ways:by the emission of two electrons and two electron anti-neutrinos (2νββ decay, already observed in many nu-clei), or possibly by the emission of two electrons, butno neutrinos (0νββ). Neutrinoless double-beta decaycan only occur if the neutrino is a massive Majoranaparticle, i.e. the neutrino and anti-neutrino are the sameparticle.

The goal of the NEMO-3 experiment is to search for0νββ decay with a half-life sensitivity of 1025 years,which can probe the effective Majorana neutrino mass〈mν〉 down to the level of 0.1 eV. The NEMO-3 de-tector employs a tracking chamber and calorimeter andallows for measurements of many double beta decayobservables and measurements of the intrinsic back-ground contamination. The detector houses seven dif-ferent double-beta decay isotopes in the form of thinfoils. The detector is capable of identifying electrons,positrons, alphas and gammas. The observables of the

final state include the energy and trajectory of each elec-tron, the time coincidence, the angle between the twoelectrons, and the curvature of tracks due to the mag-netic field for charge identification.

The poster highlights the extensive and unprecen-dented background measurements obtained using sev-eral different final state topologies. The backgroundsare classified by contaminants inside the source foil (in-ternal) and outside the source foil (external). Severaldifferent processes can produce events that mimic dou-ble beta decay and these processes are carefully stud-ied using the NEMO-3 detector in situ. The poster alsohighlights many of the latest measurements of the 2νββhalf-lives, and lower limits on the 0νββ half-lives for theisotopes installed in the detector. No evidence of neutri-noless double beta decay has been found to date.

Using the NEMO-3 detector, we also investigate al-ternative models that predict emission of Goldstonebosons (Majorons) that couple to the neutrino as wellas neutrino coupling through right-handed currents(V+A). Additionally, due to the calorimeter segmen-tation and track identification, we are able to measure2νββ- and probe 0νββ-decay to excited states.

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 491

0920-5632/$ – see front matter, Published by Elsevier B.V.

www.elsevier.com/locate/npbps

doi:10.1016/j.nuclphysbps.2012.09.128