The EVIDENCE FOR THE OBSERVATION OF 0νββ DECAY: THE IDENTIFICATION OF 0νββ EVENTS FROM THE FULL SPECTRA

Abstract

In this brief review, a description of the observed evidence for neutrinoless double beta 3-5 in the 76 Ge experiment in Gran Sasso (Heidelberg-Moscow experiment) which has been operated with 11 kg enriched 76 Ge detectors in the period 1990-2003, is provided. Two different methods of pulse shape analysis have been used to select potential 0νββ events from the γ background of the measured spectrum -a selection by a neuronal net approach, 3,4,16 and a selection by a new method comparing measured pulses with a library of pulse shapes of point-like events calculated from simulation of the electric field distribution in the detectors (see Refs. 6-8 and 37). The latter method also allows spatial localization of measured events. Both methods lead to selections of events at Q ββ with almost no γ-background. The observed line at Q ββ is identified as a 0νββ signal. It has a confidence level of more than 6σ.

LeptonicCPviolation

“…Up to now, no indications in favor of this process have been obtained, although some members of the Heidelberg-Moscow collaboration claim to have observed 0νββ with a lifetime which corresponds to m ee ≃ 0.4 eV (Klapdor- Kleingrothaus and Krivosheina, 2006). This result will be soon checked by an independent experiment.…”

Limits on the neutrino mass from cosmology

“…For example, inverted hierarchy could be ruled out if we can exclude that ∑ m ν > 2 (∆ m 2 23 ) where ∆ m 2 23 ∼ 2.4 · 10 −3 eV 2 is the squared mass difference between neutrino mass eigenstates [55]. We also note that the current limit on the sum of neutrino masses from cosmological observations could be quite important for ongoing and future experiments which aim to measure neutrino masses: for example there is a tension between our results and the limits of 0.16 < m β β < 0.52(2σ ) on the neutrino mass coming from the analysis of part of the Heidelberg-Moscow experiment on neutrinoless double beta decay [56]: moreover, the next-generation tritium β -decay experiment KATRIN [57] would not be able to measure the absolute value of neutrino mass because its detectability threshold is at ∼ 0.2 eV. …”

Upgrade for Phase II of the Gerda experiment

“…A background index (BI) of about 10 −2 cts/(keV kg year) was achieved, one order of magnitude lower than in the best previous 0νββ decay searches with 76 Ge. No signal was found for 0νββ decay, and a new 90% confidence level (CL) limit of T 0ν 1/2 > 2.1 × 10 25 year was derived (median sensitivity 2.4×10 25 year) that strongly disfavored a previous claim of observation [8,9]. Further Phase I results include a much improved half-life for 2νββ decay of 76 Ge and improved limits for Majoron ββ decay modes [10], as well as 2νββ decays of 76 Ge into excited states of 76 Se [11].…”