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(M. Agostini et al, Background-free search for neutrinoless double-beta decay of 76Ge with GERDA)

One unsolved cosmological question is matter dominance over antimatter in the Universe, unexplained by the Standard Model (SM). Many extensions of the SM resolve this by proposing that neutrinos are their own antiparticles. This could be validated via the observation of a rare radioactive decay called neutrinoless double-beta decay (0nbb). Here, a nucleus decays, releasing two electrons (beta particles) and no electron neutrinos. The signature would be the observation of a narrow peak in the energy spectrum of the electrons. Current limits on the half-life of 0nbb decay are at least 15 orders of magnitude longer than the age of the Universe, so high target masses and a low background are needed to detect this rare decay.

GERDA (GERmanium Detector Array) is an experiment designed to search for 0nbb decay located under the Gran Sasso massif in central Italy. Uniquely, it operates in the “zero-background” limit where no events are expected in the region of interest up to its design exposure.The array is composed of germanium detectors in radio-pure liquid argon (LAr), cooled to 90 K. The detectors are enriched in the isotope Ge-76, predicted to decay via 0nbb decay, such that the germanium acts as both source and detector of the decay.

The detectors are assembled in strings, contained in the LAr cryostat, which acts as active shielding. The cryostat is within a water tank, which provides additional passive shielding. There are additional panels of plastic scintillator above the cryostat, to veto muons. This arrangement vetoes background muons, already suppressed by the rock overhead, and effectively suppresses other backgrounds

GERDA is the first low-background germanium experiment to introduce the active LAr shielding. The LAr is now instrumented with photodetectors to veto external background events. These events deposit energy in the liquid argon, producing scintillation light. The light is shifted to the visible region, and detected by the photodetectors. A signal in a photodetector indicates that coincident events detected in the germanium detectors are likely background events, and therefore can be excluded from 0nbb decay analyses.

In the region of interest around Qbb, which defines the amount of energy released by the decay, the function used to fit the data is a combination of a flat background and a peak with width corresponding to the energy resolution. The best fit is no signal events, i.e. no signal for 0nbb decay is observed, with a limit on the half-life at 90% confidence level of T½ > 5.3 x 10^25 yr. GERDA will continue taking data until mid-2019. In the meantime, plans for a next-generation experiment are being made. The recently formed LEGEND collaboration then seeks to upgrade the facility, anticipating a ten-fold initial increase in sensitivity.

 Figure 1: Energy spectra for the two detector types. The top panel shows the coaxial detector (used in the former Heidelberg-Moscow and IGEX experiments) spectrum, whereas the BEGe (Broad Energy Germanium) detector spectrum is displayed in the bottom. The blue line indicates an expected 2nbb contribution, and the histograms represent the data before the LAr veto (white) and after (grey). The bottom panel contains labels with known background contributions. Inset in the bottom panel shows the magnified region between two potassium lines in the BEGe spectrum.