The GERDA experiment has been proposed in 2004 as a new 76Ge double-beta decay experiment at LNGS. GERDA has installed in Hall A of LNGS a facility where germanium detectors made out of isotopically enriched material are operated inside a cryogenic fluid shield. The setup has probed the neutrinoless double beta decay of 76Ge with a sensitivity of T1/2 > 10^25 yr at 90% confidence level. With respect to previous experiments based on Ge the background has been reduced by an order of magnitude to 10^-2 counts/(keV kg x yr) due to the new design, to selection of pure material and to the employment of pulse shape analysis. After an exposure of 21.6 kg x yr during a period 2011 to 2013 and a blind analysis no signal is observed and a lower limit is derived for the half life of neutrinoless double beta decay of 76Ge yielding T1/2 > 2.1 x 10^25 x yr (90% C.L.). The corresponding effective mass of the neutrino amounts to 0.2-0.4 eV in case the light neutrino exchange is the dominant process. The combination with selected results from previous experiments with 76Ge yields T1/2 > 3.0 x 10^25 x yr (90% C.L.). Presently, Phase II is in preparation with the main aim of further background reduction by installation of an active veto for the liquid argon. Thirty new detectors with a total mass of about 20 kg will be deployed additionally to those used in Phase I. These new detectors are of BEGe (Broad Energy Germanium) type which excels in pulse shape discrimination for additional background reduction. With an exposure of 100 kg x yr a sensitivity of about 10^26 x yr for the half life of neutrinoless double beta decay will be reached.
The study of neutrinoless double beta decay is the most sensitive approach to answer the question whether the neutrino is a Majorana particle like most extensions of the Standard Model assume. The potential of this method has increased considerably during the last years since a non-zero mass of the neutrinos has been established by the observation of neutrino flavor oscillations. In fact, the observation of neutrinoless double beta decay would not only establish the Majorana nature of the neutrino but also establish lepton number violation and physics beyond the Standard Model of particle physics. Together with other results this may shed light on the baryon asymmetry and leptogenesis. Depending on the existence of sterile neutrinos, the mass hierarchy and the detailed nuclear models the half life provides a determination of effective neutrino mass. Past double beta decay experiments, Heidelberg-Moscow and IGEX, have used 76Ge both as source and as detector. Both collaborations have reported almost the same lower limit on the lifetime of 1.6 x 10^25 x yr, corresponding to a neutrino mass range of 0.33 to 1.3 eV. A subgroup of the Heidelberg-Moscow collaboration claimed a 4 sigma excess in the spectrum near the energy expected for neutrinoless double beta decay and gave a neutrino mass range from 0.2 eV to 0.6 eV. Other double beta decay experiments such as CUORICINO or NEMO used other isotopes and reached the similar sensitivity region. A new generation of experiments like CUORE, EXO, Kamland-ZEN, Majorana and GERDA are on the way to increase the sensitivity drastically by employing new techniques for background reduction and by increasing the mass and exposure. Phase I of GERDA has been taking data during the period 2011 to 2013. It has demonstrated that the concept of operating bare germanium detectors directly in liquid argon is feasible and leads to a significant background reduction. The facility where germanium detectors made out of isotopically enriched material (86% in 76Ge) are operated inside a cryogenic fluid shield is located in Hall A of LNGS. The stainless steel cryostat has a diameter of 4 m and is lined with OFRP copper on the inside; it holds 64 m^3 of liquid argon which is cooled actively. The cryostat itself stands in a water tank of 10 m diameter and 8.3 m height. It is filled with 590 m^3 of ultrapure water from the BOREXINO plant. Water and argon shield against environmental radioactivity from rock and concrete. Water is instrumented with photomultiplier to acts a muon veto in order to reduce the cosmic background. For Phase II the argon will also be equipped with light sensors to further reduce the background. A clean room, a sophisticated lock and suspension systems on top of the cryostat allow to insert and remove detectors without introducing contamination into the vessel.
M. Agostini o, M. Allardt d, A.M. Bakalyarov m, M. Balata a, I. Barabanov k, L. Baudis s, C. Bauer g, N. Becerici-Schmidt n, E. Bellotti h,i, S. Belogurov l,k , S.T. Belyaev m, G. Benato s, A. Bettini p,q, L. Bezrukov k, T. Bode o, D. Borowicz c,e, V. Brudanin e, R. Brugnera p,q, A. Caldwell n, C. Cattadori i, A. Chernogorov l, V. D’Andrea a, E.V. Demidova l, A. di Vacri a, A. Domula d, E. Doroshkevich k, V. Egorov e, R. Falkenstein r, O. Fedorova k, K. Freund r, N. Frodyma c, A. Gangapshev k,g, A. Garfagnini p,q, C. Gooch n, P. Grabmayr r, V. Gurentsov k, K. Gusev m,e,o, C. Hahne d, A. Hegai r, M. Heisel g, S. Hemmer p,q, G. Heusser g, W. Hofmann g, M. Hult f , L.V. Inzhechik k, L. Ioannucci a, J. Janicsko Csathy o, J. Jochum r, M. Junker a, V. Kazalov k, T. Kihm g, I.V. Kirpichnikov l, A. Kirsch g, A. Kish s, A. Klimenko g,e, K.T. Knoepfle g, O. Kochetov e, V.N. Kornoukhov l,k, V.V. Kuzminov k, M. Laubenstein a, A. Lazzaro o, V.I. Lebedev m, B. Lehnert d, H.Y. Liao n, M. Lindner g, I. Lippi q, A. Lubashevskiy g,e, B. Lubsandorzhiev k, G. Lutter f, C. Macolino a, B. Majorovits n, W. Maneschg g, G. Marissens f, E. Medinaceli p,q, M. Miloradovic s, M. Misiaszek c, P. Moseev k, I. Nemchenok e, S. Nisi a, D. Palioselitis n, K. Panas c, L. Pandola b, K. Pelczar c, A. Pullia j, M. Reissfelder g, S. Riboldi j, N. Rumyantseva e, C. Sada p,q, F. Salamida i, M. Salathe g, C. Schmitt r, B. Schneider d, J. Schreiner g, O. Schulz n, B. Schwingenheuer g, S. Schoenert o, A-K. Schutz r, H. Seitz n, O. Selivanenko k, E. Shevchik e, M. Shirchenko m,e, H. Simgen g, A. Smolnikov g, L. Stanco q, M. Stepaniuk g, H. Strecker g, L. Vanhoefer n, A.A. Vasenko l, A. Veresnikova k, K. von Sturm p,q, V. Wagner g, M. Walter s, A. Wegmann g, T. Wester d, C. Wiesinger o, H. Wilsenach d, M. Wojcik c, E. Yanovich k, I. Zhitnikov e, S.V. Zhukov m, D. Zinatulina e, K. Zuber d, G. Zuzel c.
a INFN Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, Assergi, Italy
b INFN Laboratori Nazionali del Sud, Catania, Italy
c Institute of Physics, Jagiellonian University, Cracow, Poland
d Institut fuer Kern und Teilchenphysik, Technische Universitaat Dresden, Germany
e Joint Institute for Nuclear Research, Dubna, Russia
f Institute for Reference Materials and Measurements, Geel, Belgium
g Max-Planck-Institut fuer Kernphysik, Heidelberg, Germany
h Dipartimento di Fisica, Università Milano Bicocca, Italy
i INFN Milano Bicocca, Milano, Italy
j Dipartimento di Fisica, Universita` degli Studi di Milano e INFN Milano, Italy
k Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
l Institute for Theoretical and Experimental Physics, Moscow, Russia
m National Research Centre “Kurchatov Institute”, Moscow, Russia
n Max-Planck-Institut fuer Physik, Muenchen, Germany
o Physik Department and Excellence Cluster Universe, Technische Universitaat Muenchen, Germany
p Dipartimento di Fisica e Astronomia dell’Università di Padova, Italy
q INFN Padova, Italy
r Physikalisches Institut, Eberhard Karls Universitaat Tuebingen, Germany
s Physik Institut der Universitaat Zurich, Switzerland
Chair of collaboration board: R. Brugnera