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.

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Theory of neutrinoless double-beta decay.

The nuclear matrix elements that govern the rate of neutrinoless double beta decay must be accurately calculated if experiments are to reach their full potential. Theorists have been working on the problem for a long time but have recently stepped up their efforts as ton-scale experiments have begun to look feasible. Here we review past and recent work on the matrix elements in a wide variety of nuclear models and discuss work that will be done in the near future. Ab initio nuclear-structure theory, which is developing rapidly, holds out hope of more accurate matrix elements with quantifiable error bars.

https://iopscience.iop.org/article/10.1088/1361-6633/aa5bc5/pdf

Status and future of nuclear matrix elements for neutrinoless double-beta decay: a review.

Topological insulators are a class of materials with an unusual band structure that makes them metallic at the surface and insulating in the bulk. Okamoto and co-workers use electronic structure calculations to predict a new family of possible topological insulators based on transition-metal oxides.

/pdf/interface-engineering-of-quantum-hall-effects-in-digital-55et9jid0e.pdf

Interface engineering of quantum Hall effects in digital transition metal oxide heterostructures

Status and Future of Nuclear Matrix Elements for Neutrinoless Double-Beta Decay: A Review

The dominant decay mode of atomic nuclei is beta decay (β-decay), a process that changes a neutron into a proton (and vice versa). This decay offers a window to physics beyond the standard model, and is at the heart of microphysical processes in stellar explosions and element synthesis in the Universe1–3. However, observed β-decay rates in nuclei have been found to be systematically smaller than for free neutrons: this 50-year-old puzzle about the apparent quenching of the fundamental coupling constant by a factor of about 0.75 (ref. 4) is without a first-principles theoretical explanation. Here, we demonstrate that this quenching arises to a large extent from the coupling of the weak force to two nucleons as well as from strong correlations in the nucleus. We present state-of-the-art computations of β-decays from light- and medium-mass nuclei to 100Sn by combining effective field theories of the strong and weak forces5 with powerful quantum many-body techniques6–8. Our results are consistent with experimental data and have implications for heavy element synthesis in neutron star mergers9–11 and predictions for the neutrino-less double-β-decay3, where an analogous quenching puzzle is a source of uncertainty in extracting the neutrino mass scale12. The difference between the β-decay rate predicted for free neutrons and that measured in real nuclei is explained by first-principles calculations to arise from strong correlations and the weak-force coupling between nucleons.

Discrepancy between experimental and theoretical β-decay rates resolved from first principles

We have investigated the changes occurring in the electronic structure of digitally controlled SrVO(3) ultrathin films across the metal-insulator transition (MIT) by the film thickness using in situ photoemission spectroscopy. With decreasing film thickness, a pseudogap is formed at E(F) through spectral weight transfer from the coherent part to the incoherent part. The pseudogap finally evolves into an energy gap that is indicative of the MIT in a SrVO(3) ultrathin film. The observed spectral behavior is reproduced by layer dynamical-mean-field-theory calculations, and it indicates that the observed MIT is caused by the reduction in the bandwidth due to the dimensional crossover.

Dimensional-Crossover-Driven Metal-Insulator Transition in SrVO3 Ultrathin Films

The double-differential cross sections for the 48Ca(p,n) and 48Ti(n,p) reactions were measured at 300 MeV. A multipole decomposition technique was applied to the spectra to extract the Gamow-Teller (GT) components. The integrated GT strengths up to an excitation energy of 30 MeV in 48Sc are 15.3+/-2.2 and 2.8+/-0.3 in the (p,n) and (n,p) spectra, respectively. In the (n,p) spectra additional GT strengths were found above 8 MeV where shell models within the fp shell-model space predict almost no GT strengths, suggesting that the present shell-model description of the nuclear matrix element of the two-neutrino double-beta decay is incomplete.

T. H. Okabe

Papers

Dimensional-Crossover-Driven Metal-Insulator Transition in SrVO3 Ultrathin Films

Gamow-teller strength distributions in Sc48 by the Ca48(p,n) and Ti48(n,p) reactions and two-neutrino double-β decay nuclear matrix elements