Staff

R. Brugnera, A. Garfagnini, I. Lippi (INFN), C. Sada

PostDoc

M. Redchuk, K. von Sturm (INFN)

PhD students

V. Biancacci

INFN associated

A. Bettini

Research Activity

The results of neutrino oscillation experiments have proved that neutrinos are massive.
However, neutrino oscillations are sensitive to the squared-mass differences of the mass eigenstates.
So many problems remain unsolved: the absolute energy scale of the neutrino masses, their nature (Dirac or Majorana particles).
The neutrinoless double beta decay can give an answer to both the questions.
While the double beta decay with two neutrinos emission, (N(A,Z) --> N(A,Z+2) + 2e- + 2 electron antineutrinos), is permitted by the Standard Model, the neutrinoless double beta decay is forbidden in the Standard Model: in this process lepton number is violated by two units.
Its observation could be explained through the fact that the neutrino is a Majorana particle (i.e. neutrino and antineutrino are the same particle). The measured half-life would give an information about the Majorana mass of the electron neutrino.
The GERDA (GERmanium Detector Array) is looking for the neutrinoless double beta decay using the isotope Ge76. The detector is located at the Laboratori Nazionali del Gran Sasso. Being an extremely rare decay, it is absolutely necessary to avoid and eliminate all possible events which could mimic and cover the signal (background events). This is the reason why the detector was built below the Gran Sasso mountain (the mountain protects the detector from the cosmic rays) and why it has its very peculiar layout. In GERDA the high-purity germanium detectors made from isotopically modified material with Ge76 enriched to ~86% are mounted naked inside a cryostat filled with ultra-pure liquid argon. To further shield the detectors the cryostat is inside a large water tank filled ultra-pure water.
In its first phase GERDA succeded to reject a discovery claim made by a part of the Heidelberg-Moscow collaboration. In its present second phase GERDA has obtained the lowest level of background events ever reached, ~10ˆ^-3 events /(keV kg yr), around the Q-value of the neutrinoless double beta decay.
If GERDA will be able to collect an exposure of 100 kg yr and to maintain such very low background level and in case of no signal, it will put a lower limit to the half-life of the neutrinoless double beta decay of Ge76 of: 2*10^{^26} yr (90% confidence level).
If this process is due to the exchange of massive Majorana neutrinos, the experiment will be sensitive to effective Majorana masses from 0.09 to 0.29 eV.
After the completion of the second phase, the experimental setup of GERDA will be used to develop the experimental techniques necessary for a future big experiment able to cover the entire region of the so called inverted mass hierarchy (i.e. to go down to 10 meV for the effective
Majorana mass).

Further information:
https://www.mpi-hd.mpg.de/gerda/home.html
http://gerda.pd.infn.it/

Some recent publications:
M. Agostini et al. (GERDA Collaboration), Background-free search for neutrinoless double-beta decay of Ge76 with GERDA, Nature 6 April 2017, Vol. 544 Issue No 7648, pag. 47-52, doi: 10.1038/nature27117
M. Agostini et al. (GERDA Collaboration), Results on double beta decay with emission of two neutrinos or Majorons in Ge76 from GERDA Phase I, Eur. Phys. J. C (2015) 75:416, doi: 10.1140/epjc/s10052-015-3627-y
K.-H. Ackermann et al. (GERDA Collaboration), The GERDA experiment for the search of neutrinoless double beta decay in Ge76, Eur. Phys. J. C (2013) 73:2330, doi: 10.1140/epjc/s10052-013-2330-0