Pair-transfer probability in open- and closed-shell Sn isotopes

Approximations made to estimate two-nucleon transfer probabilities in ground-state to ground-state transitions and physical interpretation of these probabilities are discussed. Probabilities are often calculated by approximating both ground states, of the initial nucleus A and of the final nucleus A\pm 2 by the same quasiparticle vacuum. We analyze two improvements of this approach. First, the effect of using two different ground states with average numbers of particles A and A\pm2 is quantified. Second, by using projection techniques, the role of particle number restoration is analyzed. Our analysis shows that the improved treatment plays a role close to magicity, leading to an enhancement of the pair-transfer probability. In mid-shell regions, part of the error made by approximating the initial and final ground states by a single vacuum is compensated by projecting onto good particle number. Surface effects are analyzed by using pairing interactions with a different volume-to-surface mixing. Finally, a simple expression of the pair-transfer probability is given in terms of occupation probabilities in the canonical basis. We show that, in the canonical basis formulation, surface effects which are visible in the transfer probability are related to the fragmentation of single-particle occupancies close to the Fermi energy. This provides a complementary interpretation with respect to the standard quasiparticle representation where surface effects are generated by the integrated radial profiles of the contributing wave functions.Comment: 12 pages, 7 figure

Numerical comparison between a Gyrofluid and Gyrokinetic model investigating collisionless magnetic reconnection

The first detailed comparison between gyrokinetic and gyrofluid simulations of collisionless magnetic reconnection has been carried out. Both the linear and nonlinear evolution of the collisionless tearing mode have been analyzed. In the linear regime, we have found a good agreement between the two approaches over the whole spectrum of linearly unstable wave numbers, both in the drift kinetic limit and for finite ion temperature. Nonlinearly, focusing on the small-$\Delta '$ regime, with $\Delta '$ indicating the standard tearing stability parameter, we have compared relevant observables such as the evolution and saturation of the island width, as well as the island oscillation frequency in the saturated phase.The results are basically the same, with small discrepancies only in the value of the saturated island width for moderately high values of $\Delta '$. Therefore, in the regimes investigated here, the gyrofluid approach can describe the collisionless reconnection process as well as the more complete gyrokinetic model.Comment: Accepted for publication on Physics of Plasma

Treatment of pairing correlations in nuclei close to drip lines

We discuss the HFB equations in coordinate representation,a suitable method for handling the full effects of the continuous quasiparticle spectrum. We show how the continuum HFB equations can be solved with the correct asymptotic conditions instead of the discretization conditions which are commonly used in the literature. The continuum HFB method is illustrated with a model where the mean field and pairing field have simple forms. The relationship with the continuum Hartree-Fock-BCS (HF-BCS) approximation is also discussed. Realistic HFB and HF-BCS calculations based on Skyrme interactions are compared for the case of a neutron-rich nucleus.Comment: 12 pages, 4 Postscript figures, uses KapProc.cls, to appear in Proceedings of NATO School " Nuclei far from stability and astrophysics", Predeal, Romania, 200

Quasiparticle Resonances in the BCS Approach

We present a simple method for calculating the energies and the widths of quasiparticle resonant states. The method is based on BCS equations solved in the Berggren representation. In this representation the quasiparticle resonances are associated to the Gamow states of the mean field. The method is illustrated for the case of neutron-rich nuclei $^{20-22}$O and $^{84}$Ni. It is shown that the contribution of the continuum coupling to the pairing correlations is small and largely dominated by a few resonant states close to the continuum threshold.Comment: 14 pages, 2 figure

Poteri e funzioni dello Stato: una voce per un dizionario di storia costituzionale

Il lavoro si propone di studiare le nozioni di potere e di funzione dello Stato, mediante una ricostruzione storico dommatica che parte dal pensiero politico di Locke e di Montesquieu per giungere sino alle sfide del costituzionalismo del Novecento. Esso accoglie, in particolare, una concezione oggettiva di funzione ed una concezione soggettiva di potere, ricostruendo poi, in chiave critica, i caratteri della teoria politica e giuridica della separazione dei poteri, per verificarne la perdurante validità in funzione di tutela delle libertà fondamentali dei cittadini

Revisiting Nucleosynthesis Constraints on Primordial Magnetic Fields

In view of several conflicting results, we reanalyze the effects of magnetic fields on the primordial nucleosynthesis. In the case the magnetic field is homogeneous over a horizon volume, we show that the main effects of the magnetic field are given by the contribution of its energy density to the Universe expansion rate and the effect of the field on the electrons quantum statistics. Although, in order to get an upper limit on the field strength, the weight of the former effect is numerically larger, the latter cannot be neglected. Including both effects in the PN code we get the upper limit $B \le 1\times 10^{11}$ Gauss at the temperature $T = 10^9~^oK$. We generalize the considerations to cases when instead the magnetic is inhomogeneous on the horizon length. We show that in these cases only the effect of the magnetic field on the electrons statistics is relevant. If the coherence length of the magnetic field at the end of the PN is in the range $10 \ll L_0 \ll 10^{11}$ cm our upper limit is $B \le 1\times 10^{12}$ Gauss.Comment: 12 pages LaTex file with two included ps fig

Second-order equation of state with the full Skyrme interaction: toward new effective interactions for beyond mean-field models

In a quantum Fermi system the energy per particle calculated at the second order beyond the mean-field approximation diverges if a zero-range interaction is employed. We have previously analyzed this problem in symmetric nuclear matter by using a simplified nuclear Skyrme interaction, and proposed a strategy to treat such a divergence. In the present work, we extend the same strategy to the case of the full nuclear Skyrme interaction. Moreover we show that, in spite of the strong divergence ($\sim$ $\Lambda^5$, where $\Lambda$ is the momentum cutoff) related to the velocity-dependent terms of the interaction, the adopted cutoff regularization can be always simultaneously performed for both symmetric and nuclear matter with different neutron-to-proton ratio. This paves the way to applications to finite nuclei.Comment: 15 figure

The European Union and its energy security challenges: engagement through and with networks

Energy security remains a vital issue for the European Union (EU), even more so in the wake of the events that unfolded in early 2014 in Ukraine. The EU’s already fragile position in the international energy arena in terms of security of supply appears to be more uncertain than ever after its umpteenth fallout with its historic energy supplier, Russia. This situation is untenable and calls for swift and decisive action to adequately tackle the issue once and for all. The article looks at the creation of a single EU energy market through integration of energy networks in the EU. It then examines various ways to diversify the EU’s energy supply, whether through increasing the import of liquefied natural gas, through its relations with the Eurasian Union, the promotion of renewable energy or the construction of alternative pipelines and energy routes. The article then offers an analysis of the latest developments of the Energy Charter Conference. The article concludes that from energy transit, to technology transfer, to investment protection, energy and trade present interplays across various fields. Improvements can be made to the EU trading system to ensure greater energy security and more efficient energy markets