R Beuselinck, JK Sedgbeer, YA Shitov

SuperNEMO is a proposed experiment to search for neutrinoless Double Beta Decay (see below) as evidence for Majorana neutrino masses down to a level below 0.05eV (equivalent to a half life of ~1026 years), the region suggested by the discovery of neutrino mass from neutrino oscillation experiments. It is a next generation Double Beta Decay (DBD) experiment and builds on expertise from the NEMO series of previous DBD experiments. The collaboration proposing SuperNEMO includes institutes from France, UK, Russia, Ukraine, USA, Slovakia and Japan. The collaboration has embarked on a design study with the main aims of producing a full technical design for the experiment and identifying the optimal location for the experiment. The imperial group’s interests are in the calorimeter design and site-specific background studies.

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

NEMO
Neutrinos in Particle Physics, Astrophysics and Cosmology
(61st Scottish Universities Summer School in Physics, 2006)