The GERDA experiment at Gran Sasso: Search for neutrinoless double beta decay in germanium 76

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) designed to search for the rare neutrinoless double beta decay (0v beta beta) of 76 Ge. In the first phase of the experiment, high purity germanium diodes inherited from the former Heidelberg-Moscow and IGEX experiments are operated "bare" and immersed in liquid argon, with an overall background environment of 2 center dot 10(-2) cts/(keVkg yr), a factor of ten better than its predecessors. Preliminary measurements on two neutrinos double beta decay (2v beta beta) giving T-1/2(2v) = (1.88 +/- 0.10) center dot 10(21) yr and a recently published background model are discussed in the paper. Results on 0v beta beta are expected in summer 2013 and, in the absence of a signal, the expected sensitivity is T-1/2(0v) > 1.9 x 10 25 yr. Phase II of the experiment is scheduled to start at the end of 2013, after an upgrade shutdown, with an additional set of new detectors. Thanks to the new design of the diodes and to the introduction of liquid argon instrumentation techniques, the experiment aims to reduce further the expected background to about 1 center dot 10(-3) cts/(keVkg yr), and improve the 0v beta beta sensitivity to T-1/2(0v) > 1.35 center dot 10(26)yr.