The discovery of neutrino masses through the observation of oscillations boosted the importance of neutrinoless double beta decay (). In this paper, we review the main features of this process, underlining its key role from both the experimental and theoretical point of view. In particular, we contextualize the in the panorama of lepton number violating processes, also assessing some possible particle physics mechanisms mediating the process. Since the existence is correlated with neutrino masses, we also review the state of the art of the theoretical understanding of neutrino masses. In the final part, the status of current experiments is presented and the prospects for the future hunt for are discussed. Also, experimental data coming from cosmological surveys are considered and their impact on expectations is examined.

/pdf/neutrinoless-double-beta-decay-2015-review-3iw4l9ikse.pdf

Neutrinoless Double Beta Decay: 2015 Review

The GERDA experiment searches for the lepton-number-violating neutrinoless double-β decay of ^{76}Ge (^{76}Ge→^{76}Se+2e^{-}) operating bare Ge diodes with an enriched ^{76}Ge fraction in liquid argon. The exposure for broad-energy germanium type (BEGe) detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of 1.0_{-0.4}^{+0.6}×10^{-3}  counts/(keV kg yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0νββ experiment. No signal is observed and a new 90% C.L. lower limit for the half-life of 8.0×10^{25}  yr is placed when combining with our previous data. The expected median sensitivity assuming no signal is 5.8×10^{25}  yr.

/pdf/improved-limit-on-neutrinoless-double-beta-decay-of-76-ge-4uq9ij83ih.pdf

Improved Limit on Neutrinoless Double-$\beta$ Decay of $^{76}$Ge from GERDA Phase II

If neutrinos are their own antiparticles, neutrinoless double-β decay of 76Ge should occur; a new lower-limit half-life of 5 × 1025 years for this process has now been determined under background-free conditions.

Background-free search for neutrinoless double-beta decay of Ge-76 with GERDA

The discovery of neutrino masses through the observation of oscillations boosted the importance of neutrinoless double beta decay ($0\nu\beta\beta$). In this paper, we review the main features of this process, underlining its key role both from the experimental and theoretical point of view. In particular, we contextualize the $0\nu\beta\beta$ in the panorama of lepton-number violating processes, also assessing some possible particle physics mechanisms mediating the process. Since the $0\nu\beta\beta$ existence is correlated with neutrino masses, we also review the state-of-art of the theoretical understanding of neutrino masses. In the final part, the status of current $0\nu\beta\beta$ experiments is presented and the prospects for the future hunt for $0\nu\beta\beta$ are discussed. Also, experimental data coming from cosmological surveys are considered and their impact on $0\nu\beta\beta$ expectations is examined.

/pdf/neutrinoless-double-beta-decay-2015-review-18h0oim6h4.pdf

Neutrinoless double beta decay: 2015 review

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay of $^{76}$Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in $^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of $5.2\times10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) in the signal region and met the design goal to collect an exposure of 100 kg$\cdot$yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg$\cdot$yr of total exposure. A limit on the half-life of $0\nu\beta\beta$ decay in $^{76}$Ge is set at $T_{1/2}>1.8\times10^{26}$ yr at 90% C.L., which coincides with the sensitivity assuming no signal.

/pdf/final-results-of-gerda-on-the-search-for-neutrinoless-double-33cpuxyjsx.pdf

Final Results of GERDA on the Search for Neutrinoless Double-β Decay.

The Gerda collaboration is performing a search for neutrinoless double beta decay of 76Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori Nazionali del Gran Sasso and has started operation in November 2011. The design, construction and first operational results are described, along with detailed information from the R&D phase.

/pdf/the-gerda-experiment-for-the-search-of-0nbb-decay-in-76ge-3v7rx1hlux.pdf

F. Ritter

Papers

The Gerda experiment for the search of 0νββ decay in 76Ge