GERDA results and the future perspectives for the neutrinoless double beta decay search using 76Ge
Authors:   GERDA Collaboration, Agostini M. , Bakalyarov A. M. , Balata M. , Barabanov I. , Baudis L. , Bauer C. , Bellotti E. , Belogurov S. , Bettini A. , Bezrukov L. , Biernat J. , Bode T. , Borowicz D. , Brudanin V. , Brugnera R. , Caldwell A. , Cattadori C. , Chernogorov A. , Comellato T. , D’Andrea V. , Demidova E. V. , Di Marco N. , Domula A. , Doroshkevich E. , Egorov V. , Gangapshev A. , Garfagnini A. , Giordano M. , Grabmayr P. , Gurentsov V. , Gusev K. , Hakenmüller J. , Heisel M. , Hemmer S. , Hiller R. , Hofmann W. , Hult M. , Inzhechik L. V. , Janicskó Csáthy J. , Jochum J. , Junker M. , Kazalov V. , Kermaidic Y. , Kihm T. , Kirpichnikov I. V. , Kirsch A. , Klimenko A. , Kneißl R. , Knöpfle K. T. , Kochetov O. , Kornoukhov V. N. , Kuzminov V. V. , Laubenstein M. , Lazzaro A. , Lindner M. , Lippi I. , Lubashevskiy A. , Lubsandorzhiev B. , Lutter G. , Macolino C. , Majorovits B. , Maneschg W. , Miloradovic M. , Mingazheva R. , Misiaszek M. , Moseev P. , Nemchenok I. , Panas K. , Pandola L. , Pelczar K. , Pertoldi L. , Pullia A. , Ransom C. , Riboldi S. , Rumyantseva N. , Sada C. , Salamida F. , Schneider B. , Schönert S. , Schreiner J. , Schütz A.-K. , Schulz O. , Schwingenheuer B. , Selivanenko O. , Shevchik E. , Shirchenko M. , Simgen H. , Smolnikov A. , Stanco L. , Vanhoefer L. , Vasenko A. A. , Veresnikova A. , von Sturm K. , Wagner V. , Wegmann A. , Wester T. , Wiesinger C. , Wojcik M. , Yanovich E. , Zhitnikov I. , Zhukov S. V. , Zinatulina D. , Zschocke A. , Zsigmond A. J. , Zuber K. , Zuzel G. 
Published: April 2018

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay ([Formula: see text]) of [Formula: see text]Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of [Formula: see text] was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the [Formula: see text] decay of [Formula: see text]Ge [Formula: see text] yr (90% CL), while the corresponding median sensitivity was [Formula: see text] yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to [Formula: see text]. After analyzing 23.2 kg[Formula: see text]⋅[Formula: see text]yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the [Formula: see text]Ge [Formula: see text] decay was obtained: [Formula: see text] yr (90% CL), reaching a sensitivity of [Formula: see text] yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg[Formula: see text]yr. If no signal is found by then the GERDA sensitivity will have reached [Formula: see text] yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of [Formula: see text] decay of [Formula: see text]Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of [Formula: see text]Ge. After ten years of data taking, it will reach a sensitivity beyond 10[Formula: see text] yr, and hence fully cover the inverted hierarchy region.