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