SuperNEMO demonstrator module construction.

Experiments over the last decade have confirmed that the elusive neutrino is not a strictly massless particle, like the photon, but does in fact possess a tiny non-zero mass. These measurements do not, however, tell us what that mass is. Physicists are therefore busy devising novel ways of pinning down the absolute mass of the neutrino. In a fascinating theoretical twist, it may be the case that the very small neutrino mass is naturally explained in terms of physics at a much higher energy scale known as the Grand Unification scale, which is far beyond the energies that can be directly accessed by experiments. Such a mechanism would require the neutrinos to have another bizarre property : they are their own anti-particles. In ordinary beta decay, an electron and a neutrino are emitted when a neutron in the nucleus converts into a proton. Very rarely and only in certain isotopes, two such decays can happen simultaneously, resulting in the emission of two electrons and two neutrinos. This process has been confirmed in a number of previous experiments. However, if neutrinos are their own anti-particles, it may be possible for the same decay to occur but with no neutrinos emitted - a process called neutrinoless double-beta decay. The SuperNEMO experiment is designed to search for neutrinoless double-beta decay with unprecedented sensitivity. SuperNEMO is, in essence, a giant Geiger counter. However, rather than a single 'click' indicating the presence of a radioactive decay, the detector will accurately reconstruct the trajectories of the emitted electrons and precisely measure their energies. This way, real double-beta decay events can be distinguished from 'background' or fake events. The precise measurement of the electron energies is crucial for the identification of neutrinoless double-beta decay, since in these events all the energy of the decay is carried by the two electrons. By contrast, double beta-decay in which part of the energy is carried away by the undetected neutrinos is characterised by a wide range of measured energies. At the centre of each SuperNEMO module sits a thin film containing the double beta-decay isotope, which to begin with will be Selenium-82. Surrounding the source is a Geiger tracking detector that is being developed and built in the UK. Electrons are then absorbed in blocks of plastic scintillator and the resulting light is recorded in photomultiplier tubes, giving an estimate of their energy. UK physicists have recently demonstrated a better measurement accuracy using this technique than has ever been achieved before. The UK will also be involved in developing the simulations that will be required to optimise the final detector design. The goal of this project is to build a demonstrator module, which is essentially a prototype of a final SuperNEMO module. This will enable us to prove all the steps involved in the construction of the final detector, and will enable us to demonstrate that the detector we have designed does have the required sensitivity. Approximately 90 physicists from several countries will be involved, but the largest contributions will come from France and the UK. The demonstrator module will initially be assembled and commissioned in the 'Nu-Lab', a new laboratory recently built at UCL's Mullard Space Science Laboratory. From there, it will be taken to the Laboratoire Souterrain de Modane, deep underneath the mountains on the French-Italian border, where we will be able to assess the performance of the detector under very low background conditions. The outcome of this project will be a working demonstrator module of the future SuperNEMO detector. We will be able to do exciting and competitive physics measurements using just this module, but more importantly we will know exactly how to build the much larger detector that we will need in order to discover neutrinoless double-beta decay.