Double beta decay, Majorana neutrinos, and neutrino mass

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.

Figure 1

A generic level diagram for $\beta \beta$.

Figure 2

Feynman diagrams for $\beta \beta \left(2\nu \right)$ (top) and $\beta \beta \left(0\nu \right)$ (bottom).

Figure 3

Majorana propagator resulting from $\beta \beta \left(0\nu \right)$ amplitude (185).

Figure 4

(Color online) The effective Majorana mass $⟨m_{\beta \beta }⟩$ as a function of the mass of the lightest neutrino $m_{\mathit{\text{lightest}}}$. In making the plot, we have used the best fit values for the parameters in Table . The filled areas represent the range possible because of the Majorana phases and are irreducible. If one incorporates the uncertainties in the mixing parameters, the regions widen. See 48 for an example of how the mixing parameter uncertainty affects the regions.

Figure 5

The distribution of the sum of electron energies for $\beta \beta \left(2\nu \right)$ (dotted curve) and $\beta \beta \left(0\nu \right)$ (solid curve). The curves were drawn assuming that ${\Gamma }_{0\nu }$ is 1% of ${\Gamma }_{2\nu }$ and for a $1-\sigma$ energy resolution of 2%.

Figure 6

(Color online) Distribution of the electron energies taken one at a time for $\beta \beta \left(0\nu \right)$. The solid curve is for light-neutrino exchange and the dotted curve is for right-handed currents in both the leptonic and hadronic currents (79).

Figure 7

(Color online) The CUORICINO structure. Figure courtesy of the CUORE Collaboration.

Figure 8

(Color online) The CUORE structure. Figure courtesy of the CUORE Collaboration.

Figure 9

(Color online) The atomic level structure of a Ba ion indicating the levels of interest and a picture of the emission of a lone ion within an ion trap. Figure courtesy of the EXO Collaboration.

Figure 10

(Color online) An artist’s conception of the EXO-200 experiment. Figure courtesy of the EXO Collaboration.

Figure 11

(Color online) The left panel shows a concept drawing of 1 of 19 strings of Ge detectors that would be contained within a MAJORANA module shown in the right panel. Figure courtesy of the MAJORANA Collaboration.

Figure 12

The top panel shows the wave form of a candidate single-site energy deposit in a Ge detector. The middle panel shows a candidate multiple-site energy deposit of similar energy. The bottom panel shows how the spectrum in the region near $1.6\mathrm{MeV}$ changes as pulse-shape analysis and segmentation cuts are employed. Figure courtesy of the MAJORANA Collaboration.

Figure 13

(Color online) An artist’s conception of the GERDA experiment. Figure courtesy of the GERDA Collaboration.

Figure 14

(Color online) An artist’s conception of the GERDA phase II tower. Figure courtesy of the GERDA Collaboration.

Figure 15

(Color online) An artist’s conception of COBRA 64-detector setup. Figure courtesy of the COBRA Collaboration.

Figure 16

(Color online) A photograph of a detector holder containing 16 detectors. Figure courtesy of the COBRA Collaboration.

Figure 17

(Color online) A schematic of the MOON-I scintillator layout. Figure courtesy of the MOON Collaboration.

Figure 18

A schematic of the MOON-I configuration. Figure courtesy of the MOON Collaboration.

Figure 19

(Color online) A schematic of the NEMO detector. Figure courtesy of the NEMO Collaboration.

Figure 20

(Color online) A schematic of a SuperNEMO module. Figure courtesy of the NEMO Collaboration.

Figure 21

(Color online) A photograph of the CANDLES-III installation. Figure courtesy of the CANDLES Collaboration.