JK Sedgbeer

SuperNEMO is an experiment to search for neutrinoless double beta decay (NDBD). Direct searches for NDBD are the only way to probe the Majorana versus Dirac nature of the neutrino in a model-independent way; they also yield information on the absolute neutrino mass scale. The experiment is being constructed by an international collaboration of scientists and engineers from many countries.

SuperNEMO is based on the tracker-calorimeter technique developed by the NEMO series of experiments. This design allows full topological event reconstruction giving excellent background suppression and the possibility of identifying the underlying physics mechanism from the event kinematics.

Schematic of the tracker calorimeter technique
Schematic of the tracker-calorimeter technique. Charged particle trajectories in the tracker and energy and time-of-flight (TOF) information from the calorimeter allow reconstruction of the event topology and particle identification. A magnetic field gives charge information. The detector allows precise separation of background and signal events.

The full SuperNEMO experiment is designed to have 20 modules each housing ~7 kg of isotope. The first module is currently being assembled in the Modane Underground Laboratory (LSM) and data-taking will begin at the end of 2018.

Diagram of one detector module
Diagram of one detector module. A source foil is sandwiched between tracking detectors with a calorimeter surrounding.

Installation of the detector in the LSM
Installation of the detector in the LSM

Double Beta Decay

The observation of neutrino oscillations has demonstrated that neutrinos have mass. A consequence of this new, beyond the Standard Model physics is renewed interest in DBD experiments which provide the only way to determine the fundamental nature of the neutrino (Dirac or Majorana). In addition, DBD experiments offer the possibility of determination of the absolute neutrino mass scale and verification of the mass hierarchy of neutrinos (electron-neutrino, muon-neutrino, tau-neutrino).

DBD is a second-order weak process in which two neutrons inside a nucleus spontaneously transform into two protons. To conserve charge two electrons must be emitted. If lepton number is also to be conserved two antineutrinos must be emitted as well. This lepton-number conserving process, 2neutrino DBD decay (fig.1a), has been observed in several nuclei (e.g. 76Ge → 76Se + 2e- + 2neutrinos with a measured half life of ~1021 years). However, lepton-number is not associated with a gauge symmetry and hence need not be universally conserved. If lepton-number is violated neutrinoless DBD (fig.1b) may occur. In this case the neutrino is reabsorbed by the intermediate nucleus. This reabsorption requires that the neutrino is its own antiparticle (a Majorana fermion) and is not in a pure helicity state (i.e. must have non-zero mass).

Double beta decay diagrams
Figure 1. (a) Lepton-number conserving DBD. (b) Neutrinoless DBD.

The Majorana hypothesis is currently favoured in GUT (Grand Unified Theories) and supersymmetric theories. Neutrinoless DBD can be recognized by its electron sum energy spectrum. The nuclear recoil energy is negligible and hence the sum of the electron energies is just the decay Q value smeared by the detector resolution (see fig.2).

Double beta decay energy spectrum.
Figure 2. Double beta decay energy spectrum.

If neutrinoless DBD exists it will be an extremely rare decay and so to observe it will require a large source mass (of enriched isotope), a highly efficient detector with good energy resolution and very low (nearly zero) backgrounds.

Further information