Speaker
Dr.
Natalia Di Marco
(LNGS-INFN)
Description
The GERmanium Detector Array (GERDA) experiment at the INFN Gran Sasso Laboratory, Italy, is searching for the neutrinoless double beta (0νββ) decay of the isotope $^{76}$Ge. High-purity germanium crystals enriched in $^{76}$Ge, simultaneously used as source and detector, are directly deployed into ultra-pure cryogenic liquid argon, which acts both as cooling medium and shield against the external radiation. The signature of the 0νββ decay would be a mono-energetic peak at the Q$_{\beta\beta}$-value of the process, namely 2039 keV for $^{76}$Ge.
Data from the first phase of GERDA (Phase I), collected between 2011 and 2013, gave no
positive indication of the 0νββ decay of $^{76}$Ge with an exposure of about 20 kg$\cdot$yr and a background index at the Q$_{\beta\beta}$-value of 10$^{−2}$ counts/(keV$\cdot$kg$\cdot$yr). A lower limit on the half-life of the process of T$_{1/2} > 2.1\cdot10^{25}$ yr (90$\%$ C.L.) was set.
The second phase of the experiment is taking data since end of 2015: newly developed custom-made BEGe-type Germanium detectors add 20 kg of mass and allow for a superior background rejection by pulse shape discrimination while the instrumentation of the cryogenic liquid surrounding the detectors acts as additional active veto and assures a further background suppression.
Initial results from Phase II with about 10 kg$\cdot$yr exposure (published in Nature vol. 544, April 6th 2017) allow to improve the limit on the half-life of 0νββ decay of $^{76}$Ge to T$_{1/2} > 5.3\cdot10^{25}$ yr (90$\%$ C.L.) and indicate that the target background of 10$^{−3}$ counts/(keV$\cdot$kg$\cdot$yr) is achieved, thus making GERDA the first experiment in the field which will be “background free” up to the design exposure of 100 kg$\cdot$yr. At present, while the data taking is in progress, a valid exposure of 34 kg$\cdot$yr has been accumulated taking into account the runs until April 15th 2017. In this talk I will summarize the basic concept of the GERDA design, the data taking and the physics results obtained in Phase II. A special focus will be given to the background achieved at Q$_{\beta\beta}$ and to the analysis of the residual background components. I will then show the results of the data unblinding of June 2017 and the expected performances for the full 100 kg$\cdot$yr exposure.
Primary author
Dr.
Natalia Di Marco
(LNGS-INFN)
