Design study of the SuperNEMO experiment

In this proposal we ask for support to do research on a big new detector, SuperNEMO, which will search for neutrino-less double-beta decay. Even though the process is 'neutrino-less', the experiment actually helps us to answer fundamental questions about the nature of neutrinos. Neutrinos are very common elementary particles. Several trillion neutrinos pass through your finger every second. Still, we know very little about them. One thing we don't know is whether neutrinos have anti-particles, called anti-neutrinos, like the electron and the positron. Electrons and neutrinos are part of the families of leptons which together with the families of quarks make up the matter in the Universe. Very recently it was experimentally proven that neutrinos have a very small mass but we don't know how large these masses are. Why is this important ? For once because there are so many neutrinos around us (your finger!) that even a tiny mass would make a big difference for the amount of mass contained in the universe. The are many other unanswered questions: why are the neutrino masses so small in comparison to other particles ? Why is the number of leptons in all processes we observe conserved ? The NEMO experiment is trying to answer these questions by searching for a very rare process which has never been observed before: neutrino-less double-beta decay. Beta decay of radioactive nuclei is a very common phenomenon, it occurs in nuclear reactors or in rocks. During this decay a neutron in the nucleus is turned into a proton, while an electron and an anti-neutrino are emitted (one lepton and one anti-lepton, so the total number of leptons is zero). For some special nuclei this process is forbidden by the energy conservation law, but the decay of two neutrons simultaneously is allowed. Two electrons and two anti-neutrinos are emitted in this 'double beta decay'. The goal of the proposed experiment is to search for double beta decay without the emission of anti-neutrinos. In this process two neutrons decay into protons by emitting two electrons and no neutrinos. The neutrino is still there, but the neutrino emitted in the beta decay of one neutron is immediately absorbed in the beta decay of the other. This is only possible if the neutrino has mass and if it is its own anti-particle! The number of leptons is not conserved in this process. This has never been observed. The SuperNEMO experiment performs the search for neutrino-less double beta decay by putting foils consisting of material which could undergo neutrino-less double-beta decay in a big detector which looks for the emitted electrons. The experiment consists of two steps, first we look for two tracks produced by the two electrons in a large Helium volume surrounding the foils. The tracks are measured using a huge assembly of wires which work similar to a classical Geiger counter. The electrons are then absorbed in a calorimeter detector, this absorption process produces light which is used to measure the energy of the electrons. Since the double-beta decay process is very rare, the detectors must be large and backgrounds from other processes must be very well suppressed. These backgrounds are either internal from the radioactivity of the detector (ordinary beta decay) or come from outside source such as cosmic rays. The external backgrounds are reduced by putting the detector deep underground, for example in tunnels or mines, and by using very clean materials, containing no radioactivity, and by measuring for a long time, usually for years, because the process is so rare. But the reward will be very high: The proposed SuperNEMO experiment will address fundamental questions about nature by probing a mass range for neutrinos below 0.05 eV, which is 10,000,000 times less than the mass of the electron !