Abstract. The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neutrinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 − 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation 76 Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale 76 Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 10 28 years, using existing resources as appropriate to expedite physics results.

A 71 Ga(3 He, t) 71 Ge charge-exchange experiment was performed to extract with high precision the Gamow-Teller (GT) transition strengths to the three lowest-lying states in 71 Ge, i.e., the ground state (1/2 −), the 175 keV (5/2 −) and the 500 keV (3/2 −) excited states. These are the relevant states, which are populated via a charged-current reaction induced by neutrinos from reactor-produced 51 Cr and 37 Ar sources. A precise measurement of the GT transition strengths is an important input into the calibration of the SAGE and GALLEX solar neutrino detectors and addresses a long-standing discrepancy between the measured and evaluated capture rates from the 51 Cr and 37 Ar neutrino calibration sources, which has recently spawned new ideas about unconventional neutrino properties.

The NEMO-3 detector, which had been operating in the Modane Underground Laboratory from 2003 to 2010, was designed to search for neutrinoless double-β (0νββ) decay. We report the final results of a search for 0νββ decays with 6.914 kg of 100 Mo using the entire NEMO-3 data set with a detector live time of 4.96 yr, which corresponds to an exposure of 34.3 kg · yr. We perform a detailed study of the expected background in the 0νββ signal region and find no evidence of 0νββ decays in the data. The level of observed background in the 0νββ signal region [2.8-3.2] MeV is 0.44 AE 0.13 counts=yr=kg, and no events are observed in the interval [3.2-10] MeV. We therefore derive a lower limit on the half-life of 0νββ decays in 100 Mo of * Deceased PHYSICAL REVIEW D 92, 072011 (2015) 1550-7998=2015=92 (7)=072011 (23) 072011-1 © 2015 American Physical Society T 1=2 ð0νββÞ > 1.1 × 10 24 yr at the 90% confidence level, under the hypothesis of decay kinematics similar to that for light Majorana neutrino exchange. Depending on the model used for calculating nuclear matrix elements, the limit for the effective Majorana neutrino mass lies in the range hm ν i < 0.33-0.62 eV. We also report constraints on other lepton-number violating mechanisms for 0νββ decays.

We show that the dominant Gamow-Teller part, M 0ν GT , of the nuclear matrix element governing the neutrinoless ββ decay is related to the matrix element M 2ν cl governing the allowed two-neutrino ββ decay. That relation is revealed when these matrix elements are expressed as functions of the relative distance r between the pair of neutrons that are transformed into a pair of protons in the ββ decay. Analyzing this relation allows us to understand the contrasting behavior of these matrix elements when A and Z is changed; while M 0ν GT changes slowly and smoothly, M 2ν has pronounced shell effects. We also discuss the possibility of phenomenological determination of the M 2ν cl and from them of the M 0ν GT values from the experimental study of the β ± strength functions.

The full data set of the NEMO-3 experiment has been used to measure the half-life of the two-neutrino double beta decay of 100 Mo to the ground state of 100 Ru,