download report - Sapienza

Scientific Report 2007-2009
Particle physics
P25. Neutrinoless double beta decay search with CUORE experiment
and scintillating bolometry developments
In the field of fundamental particle physics the neutrino
has become more and more important in the last
few years, since the discovery of its mass. In particular,
the ultimate nature of the neutrino (if it is a Dirac or a
Majorana particle) plays a crucial role not only in neutrino
physics, but in the overall framework of fundamental
particle interactions. The only way to disentangle its
ultimate nature is to search for the so-called Neutrinoless
Double Beta Decay (DBD) [(A, Z) → (A, Z + 2) + 2e − ].
The DBD is an extremely rare processes. In the two neutrino
decay mode their halflives range from T1/2 2ν ∼ 1018
y to 10 25 y. The technique used by our group to search
this process is the bolometric one. A thermal detector
is a sensitive calorimeter which measures the energy
deposited by a single interacting particle through the
corresponding temperature rise. This is accomplished
by using suitable materials (dielectric crystals) and by
running the detector at very low temperatures (in the
10 mK range ) in a suitable cryostat (e.g. dilution refrigerators).
In such a condition a small energy release
in the crystal results in a measurable temperature rise.
This temperature change can be measured by means of
a proper thermal sensor, a NTD germanium thermistor.
We have run a pilot experiment (CUORICINO) at
Laboratori Nazionali del Gran Sasso and we are preparing
a large mass (an array of 988 5 × 5 × 5 cm 3 TeO 2
crystals) experiment that will be sensitive to an halflife
in excess of 10 26 y. CUORICINO experiment, ended in
2008, has set an upper limit for the halflife of the process
T1/2 0ν (130 Te)≥ 3.0 × 10 24 y (90% C.L.) (Fig. 1).
in charge of the entire process of crystal procurement
from specifications to final acceptance tests through
the qualifications of the materials. CUORE has the
goal of probing the entire degenerate region of the
neutrino mass spectrum being able to penetrate although
partially the region of the inverted hyerarchy
(m ββ ∼ 50 meV). Our expertise has allowed to present
to ERC-AdG program an innovative and extremely
ambitious project (LUCIFER), a double readout (heat
and scintillation) demonstrator composed of a few
dozens ZnSe detectors with the goal of gaining two
orders of magnitude with respect to present experiments
in the background suppression. The principle of this
detector is sketched in Fig. 2. The steps toward this
experiments are the enrichment of about 15 Kg of 82 Se,
the growth of ultrapure ZnSe crystals of about 500
grams each, the qualification of light detectors (Ge or
Si wafers) with the resolution required, the assembly in
the former CUORICINO cryostat and the developments
of sophisticated analysis techniques for the background
abatement. A success of this project would open the
way for an experiment capable of exploring the entire
region of the inverted hyerarchy of neutrino masses
(O(10 meV)).
Figure 2: The concept of the detector (left) and of the analysis
(right). The ability to tag α particles is a formidable
asset in the search for 0νββ in high Q-value candidates.
References
1. C. Arnaboldi et al., Phys. Rev. C78, 035502 (2008).
2. I. Dafinei et al., Phys. Status Solidi A204, 1567 (2007).
3. M. Pedretti et al., Int. J. Mod. Phys. A23, 3395 (2008).
4. E. Andreotti et al. , JINST 4, P09003 (2009).
Figure 1: Background spectrum from 2470 to 2590 keV in
CUORICINO. The solid lines are the best fit to the region
and bounds (68% and 90%) CL on the number of candidate
0νββ decay events respectively.
Authors
F. Bellini, C. Cosmelli, I. Dafinei 1 , R. Faccini, , F. Ferroni,
E. Longo, S. Morganti 1 , M. Vignati
http://www.roma1.infn.it/exp/cuore/
In the preparation of CUORE (A Cryogenic Underground
Observatory for Rare Events) experiment
our group is on charge of several crucial tasks, from
automatized detector assembly in ultrapure atmosphere
to the software of the experiment. Our main expertise
though is in the crystal developments. Our group is
Sapienza Università di Roma 132 Dipartimento di Fisica