The observation of neutrino oscillations has shown that neutrinos have mass. This is the first direct evidence of physics beyond the Standard Model. This discovery has renewed and strengthened interest in neutrinoless double beta decay experiments which provide the only practical way to determine whether neutrinos are Majorana or Dirac particles. Such experiments have also the potential to determine the absolute scale of the neutrino mass and help resolve the neutrino mass hierarchy question. The answers to these fundamental issues may be important to a better understanding of the fundamental forces of Nature and the evolution of the Universe.
The authors of this proposal wish to participate in the ongoing experiment NEMO-3, currently the most sensitive search for neutrinoless double beta decays, and in the next generation experiment Super-NEMO. The NEMO team currently operates the NEMO-3 apparatus, located in the Modane Underground Laboratory in the Frejus Tunnel under the French-Italian Alps. In Super-NEMO, just as in NEMO-3, and unlike in most other neutrinoless double beta decay experiments, the final state is reconstructed through several observables relating topology and electron energy. This is a technique common to particle and nuclear physics experiments and provides a powerful tool for measuring and rejecting background. This group has been invited to join the NEMO collaboration to work on both NEMO-3 and Super-NEMO and to bring their expertise with scintillator calorimeters. They are developing a simulation package based on GEANT-4 which is becoming the core optics package for Super-NEMO and an important R&D tool in several labs investigating various calorimeter options for Super-NEMO. They have both validated the results of simulations and constructed prototypes which could provide a valuable input to the final design of Super-NEMO.
The broader impact of this proposal stems from the fact that Super-NEMO is essentially an assembly of many small sub-detectors which can be prototyped and operated on a bench. This is ideally suited for training and educating undergraduate students.
The support for this NSF grant PHY-0702695 has enabled our group at the University of Texas at Austin to get involved in an experimental search for very rare nuclear transitions which, if discovered, would determine that neutrinos - the most copious known elementary particles in Nature - are the same as their antimatter counterparts - antineutrinos. Such finding would provide the best direct measurements or constraints for the mass of neutrinos. If neutrinos indeed exhibit such a unique property (i.e., if they are Majorana particles), they would help to significantly advance our knowledge of the theory of nuclear physics and elementary particles and their role in the early evolution of the Universe. Additionally, they would shed light on our better understanding of the matter-antimatter asymmetry in the presently observed Universe. A vigorous experimental and theoretical program is pursued worldwide. We are now a member of the NEMO Collaboration - an international group of scientists which has built and operated the NEMO-3 detector and is beginning construction of a larger experiment, named SuperNEMO, both located in the Modane Underground Laboratory in the Frejus Tunnel under the French-Italian Alps. To reach the desired experimental sensitivity, the collaboration has employed a powerful particle physics technique to identify and measure the two electrons in the final state. This technique allows to observe the trajectories and measure energies of electrons, and is capable of suppressing the omni-present background due to natural radioactivity of materials used to build the detector. NEMO-3 collected data for eight years and the analysis of the full data set is underway. With partial samples, the experiment has measured half-lives for seven isotopes with the best precision ever achieved. The values of these half-lives are about billion times longer than the age of the Universe and constitute some of the rarest phenomena in physics. No evidence for transition without neutrinos has been observed. This allows to set stringent bounds on the neutrino mass - an important property of particle physics and cosmology. Our research was primarily conducted by a team of young researchers, graduate and undergraduate students, and a postdoc. Our activities created many opportunities for undergraduates seeking research experience. As a result, we have worked with eight undergraduates who performed a spectrum of tasks in our everyday work. Their work ranged from hardware construction, through data acquisition to software data analysis. Just like our two graduate students and a postdoc, they have received an excellent experimental training in particle physics and acquired skills which will allow them to flourish professionally in their future careers.
