Download Referee report of the GERDA

Referee report of the GERDA-MAJORANA project
It is now possible for next-generation neutrinoless double beta decay (DBD) experiments to
access the neutrino mass range of interest suggested by recent studies of neutrino oscillations. Actually, a
well-designed germanium detector array, such as the new GERDA project, can find the effective mass, if
the massive neutrinos are Majorana particles and the neutrino mass spectrum is quasi-degenerate or
inverted hierarchy. In fact, many theories of the fundamental particle interactions predict that massive
neutrinos are Majorana in nature. Hence such type of the experiments may establish the Majorana nature,
the mass spectrum and the absolute mass scale of the neutrino. The implemented in GERDA methods and
technique have been augmented by the availability of isotopic enrichment facilities, dramatic
improvements in germanium detector performances, and underground laboratory location. The realization
that the technology is available to achieve such a fundamental physics goal provides the basic motivation
for the GERDA experiment.
Recent results of experiments with solar, atmospheric and reactor neutrinos show compelling
evidence of neutrino flavor oscillations and, correspondently, nonzero neutrino mass. Still unanswered,
however, are the very fundamental questions, namely, whether the neutrino is a Majorana or Dirac
particle, which scenario of the neutrino mass scale (mass hierarchy) is realized in nature, and what are the
absolute values of neutrino masses. The investigation of neutrinoless double beta decay is the only viable
experimental approach to discriminate between Majorana and Dirac character of neutrinos. The
observation of neutrinoless double beta decay would also provide a measurement of its effective mass
mee, and possibly in the future, open the window to study CP violation in the lepton sector induced by
Majorana phases. Therefore, neutrinoless double beta decay is a process which, when experimentally
observed, will be a manifestation of new physics beyond the Standard Model.
So far, the most sensitive results have been obtained with Ge-76. The HdM and the IGEX
experiments have placed lower bounds on the half-life for this process of 1.91025 y and 1.61025 y,
respectively, and a part of the HdM team has claimed for the first time positive indications for
neutrinoless DBD in Ge-76 (T1/2 = 1.51025 y) which still is considered as very controversial.
The new GERDA experiment will operate in the Gran Sasso underground laboratory an array of
bare enriched Ge-76 diodes in a shield of liquid argon. This shield together with an additional water layer
will reduce the external backgrounds by more than two orders of magnitude as compared to previous
DBD experiments. It is planned to develop and use innovative techniques to reduce the backgrounds by
two orders of magnitude with respect to predecessor experiments, which will make it possible to explore
the neutrino mass scale down to 100 – 300 meV in the first two phases of the experiment. The
experimental techniques developed for the GERDA experiment will guide the way to an ultimate
germanium experiment with a sensitivity of 10 meV.
The experimental set up will be located deep underground at 3400 meter water equivalent in the
Gran Sasso National Laboratory (LNGS) to shield against cosmic ray background. About 2 m of liquid
argon serves as a primary shield, contained in a vacuum-isolated stainless steel (+ copper) cryostat, and
followed by about 3 m of highly purified water. The outer water shield complements the shielding against
the rock and concrete. It also serves as a neutron shield. Equipped with photomultipliers, it serves as a
veto against cosmic muons. The scintillation light of liquid argon (LAr) can be used as an additional veto
signal. Internal backgrounds must also be reduced to reach the desired sensitivity level. For germanium,
these backgrounds are understood to come primarily from cosmogenic 68Ge and 60Co. Though their
activities can be reduced in an optimized production procedure, it is critical to distinguish these types of
background events from those resulting from double beta decay. Research and development will be
carried out to produce new segmented and BeGe types of germanium detectors which can resolve multisite energy deposits. Another effective approach is to discriminate multi-site deposits from the pulse
shape analysis of the signal. Both techniques will likely be necessary in order to reach the specified
background levels of 10−3 cts/(keV·kg·y). Background discrimination by liquid argon scintillation light
readout would be orthogonal with respect to pulse shape and segmentation methods.
The GERDA is an international collaboration where 13 institutions from 5 countries participate.
The team from Joint Institute for Nuclear Research, Dubna, participates in the most parts of the GERDA
collaboration tasks from the beginning of the project realization. All members of the JINR team have
extensive experience in carrying out different types of experiments aimed to search for very rare decays
in ultra-low counting conditions. These experiments have been carried out in underground low
background facilities of Baksan Neutrino Observatory (Russia), LSM (France), Canfranc (Spain) and
Homestake (USA). Double beta decay and dark matter candidates have been investigated by the team
utilizing various types of nuclear detectors, in particular, scintillators and semiconductor Ge diodes. The
team also has significant experience with international collaborative projects, such as IGEX, NEMO-2,
NEMO-3, TGV. The team members have deals with many aspects of research and development activities,
including experimental set up, data acquisition system design, Monte Carlo simulation, as well as
electronics development and engineering. For the material screening task assigned to the team several
high sensitive gamma-spectrometers placed in the JINR and in the low background underground facility
of Baksan Neutrino Observatory are available as well as the set of calibration and reference sources. The
JINR team has well equipped laboratories for producing, assembling and testing both plastic scintillators
and semicinductor detectors.
There are two R&D projects that are currently underway to optimize the design of the
proposed MAJORANA experiment. These projects are called SEGA and MEGA. The
MAJORANA proposal is based on well-established technology that does not require proof-ofprinciple research and development. The MAJORANA experiment will consist of a few hundred
crystals enriched in 76Ge grouped into an assembly of modules constructed from electroformed
copper with the total mass up to 1 ton. The MAJORANA experiment is planned to start from
2014.
The physics results of the GERDA and MAJORANA experiments will be of fundamental
relevance for the advance of science, in particular for elementary particle physics, as well as for nuclear
physics, astrophysics and cosmology. The Majorana nature is a precondition for the seesaw mechanism
explaining the lightness of the neutrinos with respect to the other fermions. Through the seesaw
mechanism, neutrinos would also shed light on GUT scale physics. The Majorana nature of neutrinos
seems as well to be the key to the understanding of the origin of matter in today’s Universe. The
hypothesis of Leptogensis, i.e. the generation of a primordial lepton-antilepton asymmetry early on in the
history of the Universe, is possible only, if neutrinos are Majorana particles.
The new methods and technological capabilities used by the GERDA and MAJORANA
collaborations demonstrate a real possibility to make rapid strides toward the measurement of the
effective Majorana neutrino mass, as well as impacting other science areas.
I have no doubt that the GERDA-MAJORANA project deserves all possible support and that
these experiments will be realized soon with achievements of the proposed goals.
Leading Research Scientist
MPI-K, Heidelberg / Kurchatov Institute, Moscow
E. Kh. Akhmedov