Average and recommended half-life values for two-neutrino double beta decay

We report on an improved measurement of the 2νββ half-life of ^(136)Xe performed by EXO-200. The use of a large and homogeneous time-projection chamber allows for the precise estimate of the fiducial mass used for the measurement, resulting in a small systematic uncertainty. We also discuss in detail the data-analysis methods used for double-β decay searches with EXO-200, while emphasizing those directly related to the present measurement. The ^(136)Xe 2νββ half-life is found to be T^(2νββ)_(1/2) = 2.165±0.016(stat)±0.059(sys)×10^(21) yr. This is the most precisely measured half-life of any 2νββ decay to date.

Improved measurement of the 2νββ half-life of 136 Xe with the EXO-200 detector

In the past ten years, neutrino oscillation experiments have provided the incontrovertible evidence that neutrinos mix and have finite masses. These results represent the strongest demonstration that the electroweak Standard Model is incomplete and that new Physics beyond it must exist. In this scenario, a unique role is played by the Neutrinoless Double Beta Decay searches which can probe lepton number conservation and investigate the Dirac/Majorana nature of the neutrinos and their absolute mass scale (hierarchy problem) with unprecedented sensitivity. Today Neutrinoless Double Beta Decay faces a new era where large-scale experiments with a sensitivity approaching the so-called degenerate-hierarchy region are nearly ready to start and where the challenge for the next future is the construction of detectors characterized by a tonne-scale size and an incredibly low background. A number of new proposed projects took up this challenge. These are based either on large expansions of the present experiments or on new ideas to improve the technical performance and/or reduce the background contributions. In this paper, a review of the most relevant ongoing experiments is given. The most relevant parameters contributing to the experimental sensitivity are discussed and a critical comparison of the future projects is proposed.

/pdf/challenges-in-double-beta-decay-2we1rx6bkm.pdf

Challenges in Double Beta Decay

We use the axially deformed Skyrme quasiparticle random-phase approximation (QRPA) together with the ${\mathrm{SkM}}^{*}$ energy-density functional, both as originally presented and with the time-odd part adjusted to reproduce the Gamow-Teller resonance energy in ${}^{208}\mathrm{Pb}$, to calculate the matrix elements that govern the neutrinoless double-$\ensuremath{\beta}$ decay of ${}^{76}\mathrm{Ge},\phantom{\rule{0.28em}{0ex}}{}^{130}\mathrm{Te},\phantom{\rule{0.28em}{0ex}}{}^{136}\mathrm{Xe}$, and ${}^{150}\mathrm{Nd}$. Our matrix elements in ${}^{130}\mathrm{Te}$ and ${}^{136}\mathrm{Xe}$ are significantly smaller than those of previous QRPA calculations, primarily because of the difference in pairing or deformation between the initial and the final nuclei. In ${}^{76}\mathrm{Ge}$ and ${}^{150}\mathrm{Nd}$, our results are similar to those of less computationally intensive QRPA calculations. We suspect the ${}^{76}\mathrm{Ge}$ result, however, because we are forced to use a spherical ground state, even though our mean-field theory indicates a deformed minimum.

Large-scale calculations of the double- β decay of 76 Ge , 130 Te , 136 Xe , and 150 Nd in the deformed self-consistent Skyrme quasiparticle random-phase approximation

After nearly 80 years since the first guess on its existence, neutrino still escapes our insight: the mass and the true nature (Majorana or Dirac) of this particle is still unknown. In the past ten years, neutrino oscillation experiments have finally provided the incontrovertible evidence that neutrinos mix and have finite masses. These results represent the strongest demonstration that the Standard Model of electroweak interactions is incomplete and that new Physics beyond it must exist. None of these experimental efforts could however shade light on some of the basic features of neutrinos. Indeed, absolute scale and ordering of the masses of the three generations as well as charge conjugation and lepton number conservation properties are still unknown. In this scenario, a unique role is played by the Neutrinoless Double Beta Decay searches: these experiments can probe lepton number conservation, investigate the Dirac/Majorana nature of the neutrinos and their absolute mass scale (hierarchy problem) with unprecedented sensitivity. Today Neutrinoless Double Beta Decay faces a new era where large scale experiments with a sensitivity approaching the so-called degenerate-hierarchy region are nearly ready to start and where the challenge for the next future is the construction of detectors characterized by a tonne-scale size and an incredibly low background, to fully probe the inverted-hierarchy region. A number of new proposed projects took up this challenge. These are based either on large expansions of the present experiments or on new ideas to improve the technical performance and/or reduce the background contributions. n this paper, a review of the most relevant ongoing experiments is given. The most relevant parameters contributing to the experimental sensitivity are discussed and a critical comparison of the future projects is proposed.

The primary goal of the GERmanium Detector Array (GERDA) experiment at the Laboratori Nazionali del Gran Sasso of INFN is the search for the neutrinoless double beta decay of 76Ge. High-purity germanium detectors made from material enriched in 76Ge are operated directly immersed in liquid argon, allowing for a substantial reduction of the background with respect to predecessor experiments. The first 5.04 kg yr of data collected in Phase I of the experiment have been analyzed to measure the half-life of the neutrino-accompanied double beta decay of 76Ge. The observed spectrum in the energy range between 600 and 1800 keV is dominated by the double beta decay of 76Ge. The half-life extracted from GERDA data is T2ν1/2 = (1.84+0.14−0.10) × 1021 yr.

/pdf/measurement-of-the-half-life-of-the-two-neutrino-double-beta-511bq2mk3o.pdf

X Liu

Papers

Measurement of the half-life of the two-neutrino double beta decay of 76Ge with the Gerda experiment