Double beta decay, Majorana neutrinos, and neutrino mass

Abstract

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.

Nuclear matrix elements for neutrinoless double-beta decay and double-electron capture

“…is different from its antiparticle). The best sensitivity on small Majorana neutrino masses can be reached in the investigation of neutrinoless double-beta decay (0νββ-decay) [1,2], (A, Z) → (A, Z + 2) + e − + e − (1) and the resonant neutrinoless double-electron capture (0νECEC) [2,3],…”

“…The effective Majorana neutrino mass can be calculated by using neutrino oscillation parameters: an assumption about the mass of lightest neutrino, by chosing a type of spectrum (normal or inverted) and values of CP-violating phases. In future experiments, CUORE ( 130 T e), EXO, KamLAND-Zen ( 136 Xe), MAJORANA ( 76 Ge), SuperNEMO ( 82 Se), SNO+ ( 150 Nd), and others [1,2], a sensitivity |m ββ | ≃ a few tens of meV (4) is planned to be reached. This is the region of the inverted hierarchy of neutrino masses.…”

X-ray photons from late-decaying majoron dark matter

“…that follow from beta [7] and double beta decay studies [8], together with cosmological observations of the cosmic microwave background (CMB) and large scale structure [9] preclude neutrinos from playing a direct role as dark matter. However, the mechanism of neutrino mass generation may provide the clue to the origin and nature of dark matter.…”

Neutrino Oscillation Experiments

“…A significant unknown, not addressed here, is whether the nature of the neutrino is Majorana or Dirac, i.e., whether the neutrino is its own antiparticle. The only real way of going after this question is via searches for neutrinoless double beta decay, and a worldwide effort is underway [26,27]. A second significant unknown is the absolute mass scale of the neutrino; there are kinematic and cosmological approaches to this question [27].…”