The theoretical and experimental issues relevant to neutrinoless double beta decay are reviewed. The impact that a direct observation of this exotic process would have on elementary particle physics, nuclear physics, astrophysics, and cosmology is profound. Now that neutrinos are known to have mass and experiments are becoming more sensitive, even the nonobservation of neutrinoless double beta decay will be useful. If the process is actually observed, we will immediately learn much about the neutrino. The status and discovery potential of proposed experiments are reviewed in this context, with significant emphasis on proposals favored by recent panel reviews. The importance of and challenges in the calculation of nuclear matrix elements that govern the decay are considered in detail. The increasing sensitivity of experiments and improvements in nuclear theory make the future exciting for this field at the interface of nuclear and particle physics.

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Double Beta Decay, Majorana Neutrinos, and Neutrino Mass

Weak-interaction and nuclear-structure aspects of nuclear double beta decay

Neutrinoless double-beta decay, which is a very old and yet elusive process, is reviewed. Its observation will signal that the lepton number is not conserved and that the neutrinos are Majorana particles. More importantly it is our best hope for determining the absolute neutrino-mass scale at the level of a few tens of meV. To achieve the last goal certain hurdles must be overcome involving particle, nuclear and experimental physics. Nuclear physics is important for extracting useful information from the data. One must accurately evaluate the relevant nuclear matrix elements--a formidable task. To this end, we review the sophisticated nuclear structure approaches which have recently been developed, and which give confidence that the required nuclear matrix elements can be reliably calculated employing different methods: (a) the various versions of the quasiparticle random phase approximations, (b) the interacting boson model, (c) the energy density functional method and (d) the large basis interacting shell model. It is encouraging that, for the light neutrino-mass term at least, these vastly different approaches now give comparable results. From an experimental point of view it is challenging, since the life times are long and one has to fight against formidable backgrounds. One needs large isotopically enriched sources and detectors with high-energy resolution, low thresholds and very low background. If a signal is found, it will be a tremendous accomplishment. The real task then, of course, will be the extraction of the neutrino mass from the observations. This is not trivial, since current particle models predict the presence of many mechanisms other than the neutrino mass, which may contribute to or even dominate this process. In particular, we will consider the following processes: The neutrino induced, but neutrino-mass independent contribution. Heavy left and/or right-handed neutrino-mass contributions. Intermediate scalars (doubly charged, etc). Supersymmetric (SUSY) contributions. We will show that it is possible to disentangle the various mechanisms and unambiguously extract the important neutrino-mass scale, if all the signatures of the reaction are searched for in a sufficient number of nuclear isotopes.

/pdf/theory-of-neutrinoless-double-beta-decay-2c6bzk0uf6.pdf

Theory of neutrinoless double-beta decay.

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

Double beta (ββ) decay is a rare transition between two nuclei with
the same mass number that changes the nuclear charge number by two
units. It has been long recognized as a powerful tool for the study of
lepton conservation in general and of neutrino properties in particular.
Because the lifetimes of ββ decay are so long, the experimental study
of ββ decay is particularly challenging and has spawned a whole field
of experiments requiring very low background.

Double beta decay

The two-neutrino mode of double-beta decay in $^{82}\mathrm{Se}$ has been observed in a time-projection chamber at a half-life of ${(1.1}_{\mathrm{\ensuremath{-}}0.3}^{+0.8}$)\ifmmode\times\else\texttimes\fi{}${10}^{20}$ yr (68% confidence level). This result from direct counting confirms the earlier geochemical measurements and helps provide a standard by which to test the double-beta-decay matrix elements of nuclear theory. It is the rarest natural decay process ever observed directly in the laboratory.

Direct evidence for two-neutrino double-beta decay in 82Se.

Pairs of negative beta particles have been observed originating from a $^{82}\mathrm{Se}$ source during a cloud-chamber search for double beta decay. Backgrounds recognized in previous experiments were suppressed to well below the observed event rate, and no other significant backgrounds are apparent. Within the limited statistics of the small data sample, the observed single-electron energy spectrum, the two-electron sum energy spectrum, and the opening angle distribution are consistent with expectation for neutrino-accompanied double beta decay of $^{82}\mathrm{Se}$. The tentative assignment of the observed events to double beta decay, results in a $^{82}\mathrm{Se}$ half-life of (1.0\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ years, in good agreement with some very recent theoretical predictions. However, the result is in serious disagreement with the much longer half-lives measured in geochemical experiments. A planned follow-up experiment is described.RADIOACTIVITY $\ensuremath{\beta}\ensuremath{\beta}$-decay $^{82}\mathrm{Se}$; measured ${I}_{\ensuremath{\beta}\ensuremath{\beta}}$; deduced ${T}_{\frac{1}{2}}$.

Double beta decay of 82Se.

A time projection chamber with a selenium double-beta decay source as the central electrode has yielded a lower limit of 1.0 x 10/sup 20/ yr at 68% confidence level for the two-neutrino half-life of /sup 82/Se. This limit is consistent with the results of geochemical measurements, and disagrees with the shorter half-life predictions of nuclear theory. For the neutrinoless mode we find a corresponding lower limit of 7 x 10/sup 21/ yr, also at 68% confidence level.

Experimental Investigation of Double Beta Decay in $^{82}$Se

The University of California, Irvine $^{82}\mathrm{Se}$ double-beta-decay experiment has now accumulated 7960 h of live time. After imposition of kinematic cuts, thirteen candidate events remain in the sum-energy window between 2.0 and 3.0 MeV. If these are interpreted as neutrinoless double-beta decay with the emission of a Goldstone boson (known as the Majoron), we obtain a half-life limit of 4.4\ifmmode\times\else\texttimes\fi{}${10}^{20}$ yr (90% confidence level). This gives, dependent upon the various calculations of the nuclear matrix elements, a limit of the majoron coupling constant ${\mathrm{g}}_{\mathrm{ee}}$\ensuremath{\le}(2\char21{}30)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. .AE

Limit on neutrinoless double-beta decay with majoron emission in 82Se.

Fermilab experiment E-760 studies the resonant formation of Charmonium states in proton-antiproton interactions using a hydrogen gas-jet target in the antiproton Accumulator ring at Fermilab. Precision measurements of the mass and width of the Charmonium states, {chi}{sub 1}, {chi}{sub 2} and a new measurement of the {psi} mass are presented.

A. A. Hahn

Papers

Direct evidence for two-neutrino double-beta decay in 82Se.

Double beta decay of 82Se.

Experimental Investigation of Double Beta Decay in $^{82}$Se

Limit on neutrinoless double-beta decay with majoron emission in 82Se.

Precision measurements of charmonium states formed in p-p annihilation