Search for neutrinoless double beta decay of 76Ge with the GERDA experiment at LNGS

Wednesday 17 May 2017, 14:00 → 16:00 Asia/Shanghai

A415

演讲人简介 Luciano Pandola is a staff researcher at the Laboratori Nazionali del Sud (LNS) of the Italian National Institute for Nuclear Physics, in Catania. He earned his Ph.D in 2005 from the University of L'Aquila and was involved in experiments at the Gran Sasso Laboratory searching for solar neutrinos, dark matter and neutrinoless double beta decay. He is presently the Analysis Coordinator for the GERDA double beta decay experiment at Gran Sasso. He is participating in the DarkSide experiment, searching for WIMP dark matter, and in the NUMEN project at LNS, which is measuring double-charge-exchange nuclear matrix elements. Luciano Pandola is involved since more than 10 years in the development and validation of the Geant4 physics models, specifically in the domains which are of interest for rare-event, astroparticle, space and medical physics. He is currently the coordinator of the Working Group of the Geant4 Collaboration devoted to the development and maintenance of the so-called "Advanced Examples", that are released with the Geant4 toolkit. He gained a large experience in Monte Carlo studies of performance and expected background of physics experiments. 摘要 The GERmanium Detector Array (GERDA) experiment at the Gran Sasso Laboratory (LNGS) of INFN is searching for the neutrinoless double beta decay of 76Ge, i.e. 76Ge  76Se + 2e-. The neutrinoless double beta decay has been searched for in many nuclei, and the lower limits of the half-lives are of the order of 1025 yr. While the neutrino-accompanied version of the double beta decay is an allowed (and rare) process in the Standard Model of particle physics, the neutrinoless double beta decay is forbidden because of the violation of the lepton number conservation. The observation of this nuclear process, which is predicted by models beyond the Standard Model, would imply that neutrinos have a Majorana mass component. Provided that the exchange of light Majorana neutrinos is the leading mechanism of neutrino-less double beta decay, measuring or constraining the half-life also sheds light on the absolute neutrino mass scale. The experiment makes use of high-purity Germanium (HPGe) made of material isotopically enriched in 76Ge. The experimental signature for the neutrinoless double beta decay is a mono-energetic peak at the Qββ-value of the 76Ge decay (2039 keV). The detectors are operated naked in liquid argon, which serves as cooling medium and as shielding against the external radiation. The GERDA setup is complemented by a 5m-thick water buffer, which is equipped with photo-multipliers and operated as a Cherenkov muon veto. The first phase of the experiment was completed between November 2011 and May 2013, totaling an total exposure of 21.6 kg·yr. A blind analysis was performed, such that all calibrations and event selection criteria had been finalized before the data were processed around the Qββ-value of the 76Ge decay. Data of GERDA Phase I gave no positive indication for the neutrinoless double beta of 76Ge and allowed to place a lower limit on the process half-life T1/2 > 2.1 1025 yr (90% CL), with a background level of 10-2 counts/(keV kg yr). The second Phase of the experiment is taking data since 2015 with a doubled mass of enriched Ge detectors. The goal of Phase II is to achieve a sensitivity in the range of the 1026 yr by collecting a “background-free” exposure of 100 kg yr, i.e. reducing the background to 10-3 counts/(keV kg yr). Newly developed custom-made BEGe-type Germanium detectors add 20 kg of mass and allow for a superior background rejection by pulse shape discrimination. The other key handle for the background suppression in Phase II is the instrumentation of the cryogenic liquid surrounding the detectors for light detection serving as additional active veto. The same blinding procedure of Phase I is being followed. Initial results from Phase II with about 10 kg yr exposure (published in Nature 544, April 6th 2017) allow to improve the limit on the half-life of 0νββ decay of 76Ge to T1/2 > 5.3·1025 yr (90% C.L.) and indicate that the target background is achieved, thus making GERDA the first experiment in the field which will be “background free” up to the design exposure. Data are being taken and about 30 kg yr of Phase II exposure have been collected so far after the first data release. The new data are currently blinded, i.e. events in the range Qββ±25 keV are not available for analysis: the preliminary analysis of the new data outside the blinded region confirms the excellent background performance. This presentation will summarize the basic concept of the GERDA design, the data taking and the physics results obtained in Phase II so far. A special focus will be given to the background achieved at Qββ and to the analysis of the residual background components.

Slides Pandola-Seminar-IHEP.pdf