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Séminaires 2014 (physique nucléaire)

Mercredi18 Juin 2014
Static and dynamic octupole correlations : a theoretical perspective
Luis ROBLEDO (Departamento de Fisica Teorica, Universidad Autonoma de Madrid, Spain)
The role played by octupole correlations in the nuclear dynamics is analyzed from a theoretical perspective. Results of mean field calculations with modern effective interactions as well as the corresponding beyond mean field extensions (symmetry restoration and configuration mixing) will be used in the discussion. Excitation energies of negative parity states and transition strengths to the ground state will be the observables to be described. A variety of situations, like the existence of permanent octupole deformation at both low and high spins, the impact of dynamical octupole correlations in binding energies or the description of cluster emission as a very asymmetric fission process driven by the octupole moment will be addressed. At the end, results of ongoing mean field calculations concerning odd-A nuclei will be presented.

Mercredi 11 Juin 2014
New hyperon equation of state for supernovae and neutrons stars
Debades BANDYOPADHYAY (Saha Institute of Nuclear Physics, Kolkata, India)
We develop new hyperon equations of state (EoS) tables for core-collapse supernova simulations and neutron stars. These EoS tables are based on a density dependent (DD) relativistic hadron field theory where baryon-baryon interaction is mediated by mesons, using the DD2 parameter set for nucleons . Furthermore, light and heavy nuclei along with the interacting nucleons are treated in the nuclear statistical equilibrium model of Hempel and Schaffner- Bielich which includes excluded volume effects. Of all possible hyperons, we consider only the contribution of Lamdas. We include the repulsive hyperon- hyperon interaction mediated by the strange phi meson in this case. The EoS tables encompass wide range of density (1012 to 1 fm3), temperature (0.1 to 158.48 MeV) and proton fraction (0.01 to 0.60). The effects of Lambda hyperons on thermodynamic quantities such as free energy per baryon, pressure, or entropy per baryon are investigated and found to be significant at higher densities. The cold, beta-equilibrated EoS with the crust included self-consistently is the first supernova EoS table involving hyperons which is directly compatible with the recently measured 2 M solar-mass neutron star. We also discuss results of supernova simulations using these new hyperon EoS tables.

Mercredi 28 Mai 2014
Towards the improvement of modern nuclear energy density functionals
Xavier ROCA-MAZA (Università degli Studi di Milano and INFN, Milano, Italy)
Density functional theory is a successful approach extensively used in physics, chemistry and material sciences. No other method achieves comparable accuracy at the same computational costs. In this seminar, I will present a new Skyrme energy density functional named SAMi [1]. This interaction has been accurately calibrated to reproduce properties of doubly-magic nuclei and infinite nuclear matter. In this respect, the novelties introduced in the fitting protocol of SAMi are the two-component spin-orbit potential and the careful description of the empirical hierarchy and positive values of spin (G0) and spin-isospin (G’0) Landau parameters : 0 < G0 < G’0, a feature that most of available Skyrme forces fail to reproduce. The presented fitting protocol goes a step forward in setting the bases for more precise description of the Gamow Teller Resonances (GTR), spin-dipole resonances (SDR) and of including a criteria in nuclear matter calculations to avoid some instabilities of the functional. Currently we are studying possible ways of determining the parameters of a Skyrme functional that includes tensor and spin-density dependent terms. On the other side, covariant energy density functionals have traditionally neglected the effect of Fock terms. This has serious consequences for the description GTR and SDR which can only be cured by adding further phenomenological parameters. Relativistic Hartree-Fock models do not suffer from these problems. They can successfully describe these resonances as well as the effective mass splitting between neutrons and protons without any additional parameter. However, they are non-local and require considerable numerical effort. By using the zero-range reduction of the interaction and the Fierz transformations, I will discuss a method to take into account the Fock terms in local functionals [2], which retains the simplicity of conventional models and provides proper descriptions of the spin-isospin channels and the Dirac masses. Nowadays, we are working to build a local covariant point-coupling functional that includes all terms in the Lagrangian allowed by the symmetries. We aim at considering as free parameters the ones corresponding to the scalar-isoscalar, vector-isoscalar and vector-isovector channels and determine the rest of the channels by using the Fierz transformations. Finally, if there is enough time, I would like to devote few words to some related issues regarding fitting procedures of new energy density functionals.
[1] New Skyrme interaction with improved spin-isospin properties, X. Roca-Maza, G. Colò, and H. Sagawa, Phys. Rev. C 86, 031306(R).
[2] Localized form of Fock terms in nuclear covariant density functional theory, Haozhao Liang, Pengwei Zhao, Peter Ring, Xavier Roca-Maza, and Jie Meng, Phys. Rev. C 86, 021302(R).

Mardi 13 Mai 2014
(Cours du groupe Théorie)
Neutron Stars as Physics Gold Mines
Jorge PIEKAREWICZ (Florida State University)
A neutron star is a gold mine for the study of physical phenomena that cut across a variety of disciplines, from particle physics to general relativity. Although the most common perception of a neutron star is that of a uniform assembly of neutrons packed to enormous densities, the reality is different and far more interesting. In these lectures---targeted at students---I will focus on the equation of state of neutron-rich matter with special emphasis on its impact on the structure and composition of neutron stars. In particular, I will discuss the many exotic states of matter that are speculated to exist in a neutron star.

Mercredi 7 Mai 2014
Pygmies, Giants, and Skins
Jorge PIEKAREWICZ (Florida State University)
Understanding the equation of state (EOS) of neutron-rich matter is a central goal of nuclear physics that cuts across a variety of disciplines. Indeed, the limits of nuclear existence, the collision of energetic heavy ions, the structure of neutron stars, and the dynamics of core-collapse supernova all depend critically on the nuclear-matter EOS. In this talk I will concentrate on the fundamental role that laboratory experiments on neutron skins and Pygmy/Giant resonances will play in helping us constrain the EOS of neutron-star matter and ultimately the structure and dynamics of neutron stars.

Mercredi 30 Avril 2014
Dynamical exploration of complex states of matter in neutron star crusts
Virginia de la MOTA (SUBATECH, Nantes / IN2P3 (CNRS), Université de Nantes et Ecole des Mines de Nantes))
Modern nuclear physics as well as many-body theories are valuable tools which may allow us to improve our understanding of the physics of compact objects. The interest is twofold : on one side, learning about stellar matter properties as its transport coefficients, may have important astrophysical consequences. On the other side, probing the equation of state to which the occurrence of exotic structures in neutron stars crusts is expected to be sensitive, is in direct connection with heavy ion reactions aspects as the processes leading to fragment formation. We investigate the occurrence of exotic structures in the outermost layers of neutron stars within the framework of a microscopic model describing the nucleonic dynamics through a time-dependent mean field. In this model starting from an initial crystalline lattice of nuclei at subnuclear densities the system evolves and selforganizes in various low-lying energy structures without assumption of final shapes. The effect on the behavior of these structures due to different lattice symmetries, densities, nuclear composites and lattice perturbations are analyzed. We investigate their sensitivity to the equation of state, in particular to the isospin-dependent part of the effective force. The impact of exotic structures on the transport properties of the crustal nuclear medium is also studie.

Mercredi 9 Avril 2014
Relativistic Nuclear Energy Density Functionals : Mean-Field and Beyond
Tamara NIKSIC (University of Zagreb)
Relativistic energy density functionals (EDF) have become a standard tool for nuclear structure calculations, providing a complete and accurate, global description of nuclear ground states and collective excitations. Guided by the medium dependence of the microscopic nucleon self-energies in nuclear matter, semi-empirical functionals have been adjusted to the nuclear matter equation of state and to bulk properties of finite nuclei, and applied to studies of arbitrarily heavy nuclei, exotic nuclei far from stability, and even systems at the nucleon drip-lines. REDF-based structure models have also been developed that go beyond the static mean-field approximation, and include collective correlations related to the restoration of broken symmetries and to fluctuations of collective variables. These models are employed in analyses of structure phenomena related to shell evolution, including detailed predictions of excitation spectra and electromagnetic transition rates. The seminar will highlight some of the recent applications of the collective Hamiltonian model based on the relativistic EDF framework and also, a new implementation of the finite amplitude method for the solution of the relativistic quasiparticle random-phase approximation, based on the relativistic Hartree-Bogoliubov model for deformed nuclei, will be presented.

Mercredi 2 avril 2014
Cluster correlations in dense matter and equation of state
Stefan TYPEL (GSI, Darmstadt)
Correlations are an essential feature of strongly interacting matter that affect the thermodynamic properties and the chemical composition of the system. At densities below the nuclear saturation density, a homogeneous distribution of constituents cannot be assumed since inhomogeneous structures on different length scales develop, e.g. clusters of nucleons can appear or macroscopic phases (liquid-gas phase transition) at not too high temperatures. These effects can be studied in a generalized relativistic density functional (gRDF) approach with density-dependent meson-nucleon couplings. In the gRDF model, clusters are introduced as explicit degrees of freedom. Besides light (2H, 3H, 3He, 4He) and heavy nuclei (A > 4), nucleon-nucleon correlations in the continuum are included in an effective way. They are necessary in order to reproduce the model- independent low-density benchmark, the virial equation of state. The formation and dissolution of cluster correlations are modeled by medium-dependent mass shifts, which are mainly driven by the action of the Pauli principle. This more microscopic approach replaces the widely used excluded-volume mechanism to describe the dissolution of clusters at high densities.
The appearance of clusters has to be taken into account in the preparation of global equation of state tables for astrophysical applications that are used, e.g., in simulations of core-collapse supernovae. In such stellar matter, electrons and muons have to be considered in addition to the baryonic particle species in order to respect the charge neutrality condition. The non-negligible Coulomb interaction is responsible for further correlations that lead to the formation of a crystal structure at low temperatures as it occurs in the crust of neutron stars. The gRDF approach can be adapted to the description of cluster correlations at the surface of heavy nuclei. Here the a-particle naturally appears in the low-density matter region, a prerequisite for the a-decay of nuclei. Applying an extended relativistic Thomas-Fermi approximation, the systematic variation of the neutron skin thickness of nuclei with the neutron number is studied allowing for the alpha-particle formation at the surface. A reduction of the neutron skin thickness is found that modifies the correlation with the slope parameter L of the symmetry energy, which was observed in relativistic and non-relativistic mean-field models of nuclei.

Mercredi 26 Mars 2014
Neutron pair transfer in sub-barrier capture process
Gurgen ADAMIAN (BLTP, JINR, Dubna Russia)
The strong effect of the dineutron-like cluster transfer stemming from the surface of magic and non-magic nuclei O-18, Ca-48, Ni-64, Mo-94,96, Ru-100,102,104, Pd-104,106,108, and Sn-112,114,116,118,120,124,132 is demonstrated. The dominance of two-neutron transfer channel at the vicinity of the Coulomb barrier is supported by time-dependent mean-field approaches. The sub-barrier capture reactions following the neutron pair transfer are proposed to be used for the indirect study of neutron-neutron correlations in the surface region of nuclei.

Mardi 25 mars 2014
Cours-Séminaire informel : Self-Consistent RPA with Coupled-Cluster ground state
Peter SCHUCK ((Groupe Théorie, IPN, Orsay)
RPA is a hybrid theory in the sense that at some stage of its derivation a HF ground state is supposed whereas RPA yields in the end ground state correlations (besides excitation energies). This is the famous quasi-boson approximation of RPA. Recently we made progress in avoiding this `bosonisation’, i.e. violation of the Pauli principle, to a great extent in using as a ground state the one of Coupled Cluster Theory (CCT) at so-called sub-two level. Since this CCT ground state now depends on the RPA solution, naturally the RPA becomes a non-linear, i.e., self-consistent theory. Applications of this theory to several non-trivial but exactly solvable model cases will be given and discussed. The `expose’ necessarily will be largely formal and addresses to people with special interest and some knowledge in the subject.

Mercredi 12 février 2014
Using neutron stars to constrain the composition of dense matter
Debarati CHATTERJEE (Laboratoire Univers et Théories (LUTH), Meudon)
Using neutron stars to constrain the composition of dense matter Résumé Mass measurement of neutron stars has proved to be a useful tool to probe the composition of dense matter above nuclear saturation density. It is believed that the presence of strangeness-bearing exotic components such as hyperons in the neutron star interior would result in a lower maximum neutron star mass. However, the recent discovery of massive compact stars such as PSR J1614-2230 and more recently J0348+0432 contradicts the existence of exotic matter in the core. On the other hand, kaon production at subthreshold densities in heavy-ion collisions, which is another robust tool to probe stiffness of nuclear matter at suprasaturation densities, indicates a low compression modulus for matter up to 2-3 times nuclear saturation density. Motivated by the recent observational data on neutron star masses, we carefully re-examined the conditions for appearance of hyperons in the interior of neutron stars. Our investigation pointed to the result that maximum neutron star masses decrease with the strangeness content of the core linearly, independent of the nuclear EoS. Pulsar mass measurements can then be used to constrain hyperon fractions in neutron stars. We also studied the implications of the heavy-ion results on properties of compact stars. Starting from the soft EoS and connecting it smoothly to the stiffest causal EoS, we calculated the maximally allowed gravitational mass of a compact star.

Mercredi 5 février 2014
Correlations and light element abundances in nuclear systems of astrophysical relevance
Gerd ROEPKE (Institut für Physik, University of Rostock)
The correct many-particle approach for nuclear systems at subsaturation densities is an important ingredient for the investigation of astrophysical compact objects, in particular supernova explosions. Correlations and cluster formation are essential for the nuclear matter equation of state, the symmetry energy, and further properties. Quantum condensates is an interesting phenomenon with new properties, that can also be found in nuclei.

Mercredi 22 janvier 2014
Less-empirical nuclear density functional theory
Density functionals of Skyrme, Gogny or relativistic type are the most general quantum-mechanical tool for predicting properties of medium-mass and heavy nuclei. However, progress in the experimental study of heavy and neutron-rich sytems challenges their accuracy and predictive power. Construction of nuclear density functionals has been, up to now, mostly phenomenological, while current efforts aim to supplement this by connecting this theory with the nuclear many-body problem in its first-principles formulation. I will present progress towards a rigorous formulation of an exact density functional theory (DFT) allowing for spatial-symmetry breaking/restoration and explicit treatment of collective motion, as well as functionals based on a factorized form of soft realistic two- and three-body interactions which are a promising ingredient to be added to this model

Mercredi 15 Janvier 2014
A review on stochastic methods beyond the independent particle picture
Denis LACROIX (Groupe Théorie, IPN, Orsay)
Time-dependent Mean-field theories or its time-dependent density functional are among the most versatile approaches to the many-body problems in different fields of physics. The strong simplification that makes these approaches so attractive is the replacement of the initial correlated problem by an independent particle problem. The price to pay is the loss of information on complex correlations. While these techniques can provide approximate description of single-particle degrees of freedom, they are not designed to describe dissipative or fluctuating phenomena. Many aspects like quantum fluctuations, quantum tunneling in collective space, spontaneous symmetry breaking, ... are poorly addressed if not completely missed. Several approaches have been proposed in the last decade where noise has been added to mean-field to improve the treatment of correlations. All these approaches are motivated by a unique question : Can we describe a complex many-body problem by a set of simpler non- linear one-body evolutions. Three stochastic mean-field (SMF) will be presented, some of them being an exact reformulation of the N-body problem. Difficulties and/or successes of these SMF will be illustrated in model cases of interest in condensed matter, atomic physics and/or nuclear physics. To quote some of the application, illustration with the Lipkin model, Richardson pairing model, lattice model (Hubbard-like) models, Bose condensates, nuclei will be shown.



Institut de Physique Nucléaire Orsay - 15 rue Georges CLEMENCEAU - 91406 ORSAY (FRANCE)
UMR 8608 - CNRS/IN2P3

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