The experimental and theoretical evidence for intrinsic reflection-asymmetric shapes in nuclei is reviewed. The theoretical methods discussed cover a wide spectrum, from mean-field theory and its extensions to algebraic and cluster approaches. The experimental data for nuclear ground states and at low and high spin, cited as evidence for reflection asymmetry, are collected and categorized. The extensive data on electric dipole transition moments and their theoretical interpretation are surveyed, along with available data on electric octupole moments. The evidence for reflection-asymmetric molecular states in light nuclei is summarized. The application of reflection-asymmetric theories to descriptions of the fission barrier, bimodal fission, superdeformation, and hyperdeformations is reviewed, and some other perspectives in the wider context of nuclear physics are also given. [S0034-6861(96)00102-X]

Intrinsic reflection asymmetry in atomic nuclei

Signatures of criticality in the evolution of the nuclear ground-state shapes across the NxZ plane are discussed. Attention is paid to specific data indicating sudden structural changes in various isotopic and isotonic chains of medium-mass and heavy even-even nuclei, as well as to diverse theoretical aspects of the models used to describe these changes. The interacting boson model and the geometric collective model, in particular, are discussed in detail, the former providing global predictions for the evolution of collective observables in nuclei between closed shells and the latter yielding a parameter-efficient description of nuclei at the critical points of shape transitions. Some issues related to the mechanism of first- and second-order quantum phase transitions in general many-body systems are also outlined.

Quantum phase transitions in the shapes of atomic nuclei

Motivated by the recent results of Hansen et al. concerning a noticeable hemispherical power asymmetry in the Wilkinson Microwave Anisotropy Probe (WMAP) data on small angular scales, we revisit the dipole-modulated signal model introduced by Gordon et al.. This model assumes that the true cosmic microwave background signal consists of a Gaussian isotropic random field modulated by a dipole, and is characterized by an overall modulation amplitude, A, and a preferred direction, ṕ. Previous analyses of this model have been restricted to very low resolution (i.e., 3°.6 pixels, a smoothing scale of 9° FWHM, and l ≾40) due to computational cost. In this paper, we double the angular resolution (i.e., 1°.8 pixels and 4°.5 FWHM smoothing scale), and compute the full corresponding posterior distribution for the five-year WMAP data. The results from our analysis are the following: the best-fit modulation amplitude for l ≤ 64 and the ILC data with the WMAP KQ85 sky cut is A = 0.072 ± 0.022, nonzero at 3.3σ, and the preferred direction points toward Galactic coordinates (l, b) = (224°, – 22°) ± 24°. The corresponding results for l ≾ 40 from earlier analyses were A = 0.11 ± 0.04 and (l, b) = (225°, – 27°). The statistical significance of a nonzero amplitude thus increases from 2.8σ to 3.3σ when increasing l_(max) from 40 to 64, and all results are consistent to within 1σ. Similarly, the Bayesian log-evidence difference with respect to the isotropic model increases from Δln E = 1.8 to Δln E = 2.6, ranking as "strong evidence" on the Jeffreys' scale. The raw best-fit log-likelihood difference increases from Δln L=6.1 to Δln L=7.3. Similar, and often slightly stronger, results are found for other data combinations. Thus, we find that the evidence for a dipole power distribution in the WMAP data increases with l in the five-year WMAP data set, in agreement with the reports of Hansen et al.

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Increasing Evidence for Hemispherical Power Asymmetry in the Five-Year WMAP Data

The last decades brought impressive progress in synthesizing and studying properties of nuclides located very far from the beta stability line. Among the most fundamental properties of such exotic nuclides, the ones usually established first are the half-life, possible radioactive decay modes, and their relative probabilities. When approaching limits of nuclear stability, new decay modes set in. First, beta decays are accompanied by emission of nucleons from highly excited states of daughter nuclei. Second, when the nucleon separation energy becomes negative, nucleons start being emitted from the ground state. A review of the decay modes occurring close to the limits of stability is presented. The experimental methods used to produce, identify, and detect new species and their radiation are discussed. The current theoretical understanding of these decay processes is reviewed. The theoretical description of the most recently discovered and most complex radioactive process---the two-proton radioactivity---is discussed in more detail.

P.O. Lipas

Papers

IBA consistent-Q formalism extended to the vibrational region

Beta decay of 31Ar

IBM-2 description of M1 properties in deformed nuclei

Unrestricted shapes of light nuclei in the local-density approximation: Comparison with jellium clusters

Microscopic IBA calculations of (e, e') M1 form factors☆