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Scientific motivations

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Main research activity of the group focuses on both the neutrino properties and the measurement of very feeble radioactive decays. One of the most significant facts is that this activity blends perfectly into the NEMO3 and SuperNEMO international collaborations.

While the neutrino as itself is the most abundant particle of matter to be found in the Universe, its properties still remain a mystery. The foremost goal is the quest for its mass and nature, two ingredients of the uttermost necessity for particle physics and cosmology. Whether the neutrino is its own anti-particle or not and if the CP symmetry (for charge and parity) happens to be broken, then the neutrino could well be the source for matter in the Universe and give the clue to the desequilibrium matter/antimatter in favor of the former. The most sensitive way to uncover the nature and mass of the neutrino relies on the neutrino-less double-beta decay ββ(0ν), with two electrons emitted simulaneously. If we put in evidence such a process, then we will have the proof that the neutrino is indeed its own anti-particle. Morevover, this will open a gate to its mass and the correlated half-life T1/2 (ββ(0ν)) showing a non-conservation of the lepton number in the radioactive decay. Talking about leptons, finding such a decay will also lift a veil on the CP symmetry violation.

ββ(0ν) radioactivity can be experimentally put in evidence thanks to the two electrons emited in coincidence, the total energy of which is equal to the decay transition energy (≈3MeV). Nowadays limit on the hal-life of the ββ(0ν) decay is of the order of 1024 years. This implies to get rid of the background noise stemming from natural radioactivity. Two different apporaches are possible: either fine-tuning the energy measurement or having a tracking detector to retrieve the two electrons. Apart from the total energy of both electrons, the latter method gives way to the nature of the mechanism involved during the ββ(0ν) decay via both the angular distribution and the the energy of each electron.

This is what was chosen for NEMO3 and SuperNEMO. A thin source foil (of 80 μm for NEMO3 and 40 μm for SuperNEMO), made of double-beta emitting isotopes is placed in the center of a tracking chamber (an array of gas cells working as Geiger counters), itself surrounded by a calorimeter composed of plastic scintillators coupled to photo-multipliers (PM), so to measure energy and time-of-flight of the particles. This setup has the advantage to permit simultaneously the measurement of many double-beta emitters.

The NEMO3 detector has collected data from the year 2003 till 2011. It was to be found at the Laboratoire Souterrain de Modane (LSM), and its cylindrical shpae with a diameter of 4 m and a total height of 3 m contained mainly 7 kg of 100Mo and 1 kg of 82Se.

The obtained sensitivity on the half-life T1/2 of the 100Mo decay is 1.0x1024 ans (90% C.L.), which means an effective mass limit between 0.3 and 0.8 eV, depending on the elements of the nuclear matrix taken into account. This value is rather the same as the ones obtained in other double-beta experiments, of the pure calorimetry type. The NEMO3 experiement has allowed us to measure precisely the ββ(2ν) decay and to improve the limit on ββ(0ν) decay for several other isotopes. Starting in 2006, the SuperNEMO collaboration is a R&B program aiming at the construction of 100-fold better sensitivity apparatus for the ββ(0ν)decay, this time with 100 kg of double-beta decay isotopes as well as improving the energy resolution and the radio-purity of the detector. A demonstrator at 1/20 scale is being assembled since 2012, and the deadline for data acquisition has been fixed at 2015, in the LSM NEMO3-likewise. It is of rectangular shape (6 x 4 x 2 m3), and can contain up to 7 kg of 82Se isotope. This dectector is a proof-of-concept, and will help control tha radiopurity of the setup in the acquisition modus. Its sensitivity is expected to be 6.5x1024 years (90% C.L.) fro the half-life T1/2 of the ββ(0ν) decay of 82Se. Such a value will shade light on the positive result of the Heidelberg-Moscow experiment. The SuperNEMO detector will reach a sensitivity of 1026 ans (90% C.L.) on the half-life T1/2 of the ββ(0ν) decay of 82Se, which means a limit for the effective neutrino mass between 0.05 et 0.1 eV (still depending on which elements of the nuclear matrix are considered).