Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems

The finite size dependent enhancement of pairing in mesoscopic Fermi systems is studied under the assumption that the BCS approach is valid and that the two body force is size independent. Different systems are investigated such as superconducting metallic grains and films as well atomic nuclei. It is shown that the finite size enhancement of pairing in these systems is in part due to the presence of a surface which accounts quite well for the data of nuclei and explains a good fraction of the enhancement in Al grains.Comment: Updated version 17/02/0

Neutrino mean free path and in-medium nuclear interaction

Neutrinos produced during the collapse of a massive star are trapped in a nuclear medium (the proto-neutron star). Typically, neutrino energies (10-100 MeV) are of the order of nuclear giant resonances energies. Hence, neutrino propagation is modified by the possibility of coherent scattering on nucleons. We have compared the predictions of different nuclear interaction models. It turns out that their main discrepancies are related to the density dependence of the k-effective mass as well as to the onset of instabilities as density increases. This last point had led us to a systematic study of instabilities of infinite matter with effective Skyrme-type interactions. We have shown that for such interactions there is always a critical density, above which the system becomes unstable.Comment: 4 pages, 4 figures, Proceedings of the 17th Divisional Conference on Nuclear Physics in Astrophysics (NPDC17), 30th September - 4th October 2002, ATOMKI, Debrecen, Hungary, to appear in Nuclear Physics

Optical detection of a BCS transition of Lithium-6 in harmonic traps

We study the detection of a BCS transition within a sample of Lithium--6 atoms confined in a harmonic trap. Using the local density approximation we calculate the pair correlation function in the normal and superfluid state at zero temperature. We show that the softening of the Fermi hole associated with a BCS transition leads to an observable increase in the intensity of off--resonant light scattered from the atomic cloud at small angles.Comment: 7 pages, 3 figures, submitted to Europhysics Letter

Experimentally induced innovations lead to persistent culture via conformity in wild birds.

Journal ArticleResearch Support, Non-U.S. Gov't© 2014 Macmillan Publishers Limited. All Rights ReservedThis a post-print, author-produced version of an article accepted for publication in Nature. The definitive version is available at http://www.nature.com/nature/index.htmlIn human societies, cultural norms arise when behaviours are transmitted through social networks via high-fidelity social learning. However, a paucity of experimental studies has meant that there is no comparable understanding of the process by which socially transmitted behaviours might spread and persist in animal populations. Here we show experimental evidence of the establishment of foraging traditions in a wild bird population. We introduced alternative novel foraging techniques into replicated wild sub-populations of great tits (Parus major) and used automated tracking to map the diffusion, establishment and long-term persistence of the seeded innovations. Furthermore, we used social network analysis to examine the social factors that influenced diffusion dynamics. From only two trained birds in each sub-population, the information spread rapidly through social network ties, to reach an average of 75% of individuals, with a total of 414 knowledgeable individuals performing 57,909 solutions over all replicates. The sub-populations were heavily biased towards using the technique that was originally introduced, resulting in established local traditions that were stable over two generations, despite a high population turnover. Finally, we demonstrate a strong effect of social conformity, with individuals disproportionately adopting the most frequent local variant when first acquiring an innovation, and continuing to favour social information over personal information. Cultural conformity is thought to be a key factor in the evolution of complex culture in humans. In providing the first experimental demonstration of conformity in a wild non-primate, and of cultural norms in foraging techniques in any wild animal, our results suggest a much broader taxonomic occurrence of such an apparently complex cultural behaviour.BBSRCERCUppsala UniversityAustralian Postgraduate Awar

Search for 2νββ decay of ^(136)Xe to the 0^+^1 excited state of ^(136)Ba with the EXO-200 liquid xenon detector

EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless ββ decay of ^(136)Xe to the ground state of ^(136)Ba. We report here on a search for the two-neutrino ββ decay of 136Xe to the first 0+ excited state, 0^+_1, of ^(136)Ba based on a 100 kg yr exposure of ^(136)Xe. Using a specialized analysis employing a machine learning algorithm, we obtain a 90% CL half-life sensitivity of 1.7 × 10^(24) yr. We find no statistically significant evidence for the 2νββ decay to the excited state resulting in a lower limit of T^(2ν)_(1/2)(0^+ → 0^+_1) > 6.9 ×10^(23) yr at 90% CL. This observed limit is consistent with the estimated half-life of 2.5 × 10^(25) yr

Isospin non-equilibrium in heavy-ion collisions at intermediate energies

We study the equilibration of isospin degree of freedom in intermediate energy heavy-ion collisions using an isospin-dependent BUU model. It is found that there exists a transition from the isospin equilibration at low energies to non-equilibration at high energies as the beam energy varies across the Fermi energy in central, asymmetric heavy-ion collisions. At beam energies around 55 MeV/nucleon, the composite system in thermal equilibrium but isospin non-equilibrium breaks up into two primary hot residues with N/Z ratios closely related to those of the target and projectile respectively. The decay of these forward-backward moving residues results in the strong isospin asymmetry in space and the dependence of the isotopic composition of fragments on the N/Z ratios of the target and projectile. These features are in good agreement with those found recently in experiments at NSCL/MSU and TAMU, implications of these findings are discussed.Comment: 9 pages, latex, + 3 figures available upon reques

Surface Incompressibility from Semiclassical Relativistic Mean Field Calculations

By using the scaling method and the Thomas-Fermi and Extended Thomas-Fermi approaches to Relativistic Mean Field Theory the surface contribution to the leptodermous expansion of the finite nuclei incompressibility has been self-consistently computed. The validity of the simplest expansion, which contains volume, volume-symmetry, surface and Coulomb terms, is examined by comparing it with self-consistent results of the finite nuclei incompressibility for some currently used non-linear sigma-omega parameter sets. A numerical estimate of higher-order contributions to the leptodermous expansion, namely the curvature and surface-symmetry terms, is made.Comment: 18 pages, REVTeX, 3 eps figures, changed conten

Systematics of Fission Barriers in Superheavy Elements

We investigate the systematics of fission barriers in superheavy elements in the range Z = 108-120 and N = 166-182. Results from two self-consistent models for nuclear structure, the relativistic mean-field (RMF) model as well as the non-relativistic Skyrme-Hartree-Fock approach are compared and discussed. We restrict ourselves to axially symmetric shapes, which provides an upper bound on static fission barriers. We benchmark the predictive power of the models examining the barriers and fission isomers of selected heavy actinide nuclei for which data are available. For both actinides and superheavy nuclei, the RMF model systematically predicts lower barriers than most Skyrme interactions. In particular the fission isomers are predicted too low by the RMF, which casts some doubt on recent predictions about superdeformed ground states of some superheavy nuclei. For the superheavy nuclei under investigation, fission barriers drop to small values around Z = 110, N = 180 and increase again for heavier systems. For most of the forces, there is no fission isomer for superheavy nuclei, as superdeformed states are in most cases found to be unstable with respect to octupole distortions.Comment: 17 pages REVTEX, 12 embedded eps figures. corrected abstrac

Mean field theory for global binding systematics

We review some possible improvements of mean field theory for application to nuclear binding systematics. Up to now, microscopic theory has been less successful than models starting from the liquid drop in describing accurately the global binding systematics. We believe that there are good prospects to develop a better global theory, using modern forms of energy density functionals and treating correlation energies systematically by the RPA.Comment: RevTex, 17 pages, 5 eps figures. To be published in Yadernaya Fizika, special edition for the 90th birthday of Professor A.B. Migda