Referee report on the project
“Study of double beta decay processes in the NEMO experiment”
The double beta decay studies are being carried out in the JINR in a frame of the
international NEMO collaboration since 1993. The main objective of these studies is the
direct experimental search for the neutrinoless double beta decay (0. The interest to this
kind of research has seen a significant renewal after the observation of neutrino oscillations
in experiments with solar, atmospheric, reactor and accelerator neutrinos, proving that
neutrinos have non vanishing mass. Inspired by this spectacular discovery the physics
community worldwide is embarking the next challenging problem, finding whether neutrinos
are indeed Majorana particles as many particle physics models suggest. The neutrinoless
double beta decay is the most sensitive process to prove the Majorana nature of massive
neutrinos. An observation of the neutrinoless double beta decay would demonstrate that the
total lepton number is not conserved and would establish a non-zero neutrino mass. This
would be a discovery of major importance, with impact also on the determination of the
absolute neutrino mass scale, on the pattern of neutrino masses, and possibly on the problem
of CP violation in the lepton sector.
NEMO-3 is currently running experiment using a tracking-calorimeter technique to
detect . The thin source foils are located in the middle of a tracking chamber surrounded by
calorimeter. This technique allows a high background rejection, full reconstruction of event
kinematics, and a wide choice of isotopes. NEMO 3 accommodates ~10kg of enriched double
beta source isotopes, mainly 100Mo (7kg) and 82Se (1 kg), and smaller quantities of 116Cd,
130
Te, 150Nd , 96Zr and 48Ca.
NEMO-3 demonstrated the perfect experimental ability in detection of two neutrino
double beta decay (2. In particular, for the first time a very large statistics of about 105
2 events from 100Mo practically without background has been collected. The accurate
2 half-life measurements for the transition to ground state of daughter nucleus have been
performed by NEMO-3 for the seven available isotopes. Also NEMO-3 has studied 100Mo
double beta decay to excited states of 100Ru. The precise measurement of two-neutrino double
beta decay half-lives is important since 2 events constitute the ultimate background for
0 search. Moreover 2 is used to adjust the relevant model parameters for calculation
of 0 nuclear matrix elements.
No evidence for the 0 signal has been observed with NEMO-3 data yet. From the
analysis of the collected data the new lower limits on 0 decay half-life for the available
isotopes have been obtained. The best NEMO-3 result over these isotopes is obtained for
100
Mo : T1/2(0) > 5.8 x 1023 years (90% C.L) . This translates into the range of upper
limits on the effective Majorana neutrino mass, <m> < 0.6 – 1.3 eV according to the most
recent nuclear matrix element calculations. It is planned to 2011 to achieve the NEMO-3
half-life sensitivity T1/2(0) > 2 x 1024 years, or sensitivity for neutrino mass in the range
<m> < 0.3 – 0.7 eV. This upper mass limit is comparable with the world best results.
NEMO-3 has also explored the possible mechanisms of the 0 decay with right-handed
weak currents, Majoron emission, supersymmetric particle exchange. The limits on the
corresponding coupling constants, model parameters have been obtained.
The next generation 0 detector SuperNEMO is intended to improve the
sensitivity on the half-life of 0 decay by two orders of magnitude compared to NEMO-3.
SuperNEMO is conceived to extend the successful experimental technique used by NEMO-3
to a larger detector, able to accommodate ~100kg of  source isotope. It will use the
technology of calorimeter and tracking in modular structure, whilst improving critical
performance parameters. Design studies address the most critical issues, such as energy
resolution, tracker optimization and source radiopurity. To reach the planned SuperNEMO
sensitivity the energy resolution should be improved by a factor of two compared to NEMO3 up to 7% at 1 MeV (FWHM) . The R&D studies have successfully demonstrated the
feasibility of achieving the target energy resolution with the use of the latest achievements in
PMT, reflector and scintillator technology. Significant progress has also been attained in the
other directions of the design studies.
The main candidate isotopes for SuperNEMO are 82Se and 150Nd. The SuperNEMO
R&D program has started in 2006, the goal is to produce the Technical Design Report in
2009. Finally SuperNEMO is aiming to reach the sensitivity on effective Majorana neutrino
mass of 50 - 100 meV in the year 2017. This is competitive compared to other existing
projects of the next generation 0 experiments.
The JINR team played a key role in realization of NEMO-3 project, for instance,
JINR provided a major contribution is the scintillation calorimeter. The JINR team is
participating in the data taking, plays essential role in software development and data analysis.
The JINR team activity in SuperNEMO R&D program concerns generally the same aspects as
in NEMO-3. The main direction of this activity is the calorimeter energy resolution
improvement. JINR also is involved in software development and the MC simulations.
The considered project is one of the most advanced in the search for the neutrinooless
double beta decay. The NEMO 3 experiment is expected to provide new important results.
The SuperNEMO R&D program tasks are challenging while realistic. The good experience of
the team members ensures the successful fulfillment of the design studies for the competitive
next generation 0 detector.
I have no doubt that the proposed project merits approval and all possible support.
Director
of the Baksan Neutrino Observatory INR RAS
V.V.Kuzminov