Nuclear Theory·nucl-th

Tuesday·March 8, 2016

18 papers—9 primary·9 cross-listed

01
Low-energy $^{6}$ He scattering in a microscopic model
P. Descouvemont
A microscopic version of the Continuum Discretized Coupled Channel (CDCC) method is used to investigate $^{6}$ He scattering on $^{27}$ Al, $^{58}$ Ni, $^{120}$ Sn, and $^{208}$ Pb at energies around the Coulomb barrier. The $^{6}$ He nucleus is described by an antisymmetric 6-nucleon wave function, defined in the Resonating Group Method. The $^{6}$ He continuum is simulated by square-integrable positive-energy states. The model is based only on well known nucleon-target potentials, and is therefore does not depend on any adjustable parameter. I show that experimental elastic cross sections are fairly well reproduced. The calculation suggests that breakup effects increase for high target masses. For a light system such as $^{6}$ He+ $^{27}$ Al, breakup effects are small, and a single-channel approximation provides fair results. This property is explained by a very simple model, based on the sharp-cut-off approximation for the scattering matrix. I also investigate the $^{6}$ He-target optical potentials, which confirm that breakup channels are more and more important when the mass increases. At large distances, polarization effects increase the Coulomb barrier, and provide a long-tail absorption component in the imaginary part of the nucleus-nucleus interaction.
nucl-thPRC(2016)·11 citations
02
Separation of elastic and inelastic processes in the relaxation time approximation for collision integral
Wojciech Florkowski🇵🇱 · Radoslaw Ryblewski🇵🇱
We introduce a generalised relaxation-time-approximation form of the collision term in the Boltzmann kinetic equation that allows for using different relaxation times for elastic and inelastic collisions. The efficacy of the proposed framework is demonstrated with the numerical calculations that describe systems with different relations between the two relaxation times and the evolution time of the system.
nucl-thhep-phPRC(2016)·25 citations
03
Extracting the resonance parameters from experimental data on scattering of charged particles
P. Vaandrager · S. A. Rakityansky
A new parametrization of the multi-channel S-matrix is used to fit scattering data and then to locate the resonances as its poles. The S-matrix is written in terms of the corresponding "in" and "out" Jost matrices which are expanded in the Taylor series of the collision energy E around an appropriately chosen energy E0. In order to do this, the Jost matrices are written in a semi-analytic form where all the factors (involving the channel momenta and Sommerfeld parameters) responsible for their "bad behaviour" (i.e. responsible for the multi-valuedness of the Jost matrices and for branching of the Riemann surface of the energy) are given explicitly. The remaining unknown factors in the Jost matrices are analytic and single-valued functions of the variable E and are defined on a simple energy plane. The expansion is done for these analytic functions and the expansion coefficients are used as the fitting parameters. The method is tested on a two-channel model, using a set of artificially generated data points with typical error bars and a typical random noise in the positions of the points.
nucl-thnucl-exIJMPE(2016)·6 citations
04
Consistency between the monopole strength of the Hoyle state determined by structural calculation and that extracted from reaction observables
Kosho Minomo · Kazuyuki Ogata
We analyze the $α$ - $^{12}$ C inelastic scattering to the $0_{2}^{+}$ state of $^{12}$ C, the Hoyle state, in a fully microscopic framework. With no free adjustable parameter, the inelastic cross sections at forward angles are well reproduced by the microscopic reaction calculation using the transition density of $^{12}$ C obtained by the resonating group method and the nucleon-nucleon $g$ matrix interaction developed by the Melbourne group. It is thus shown that the monopole transition strength obtained by the structural calculation is consistent with that extracted from the reaction observable, suggesting no missing monopole strength of the Hoyle state.
nucl-thPRC(2016)·20 citations
05
Open Questions On Nuclear Collective Motion
S. Frauendorf
The status of the macroscopic and microscopic description of the collective quadrupole modes is reviewed, where limits due to non-adiabaticity and decoherence are exposed. The microscopic description of the yrast states in vibrator-like nuclei in the framework of the rotating mean field is presented.
nucl-thAIP Conf.Proc.(2016)·3 citations
06
Study of compound nucleus formation via bremsstrahlung emission in proton $α$ -particle scattering
Sergei P. Maydanyuk (1 and 2)🇺🇦 · Peng-Ming Zhang (1 and 3) ((1) Institute of Modern Physics, Chinese Academy of Sciences, (2) Institute for Nuclear Research, National Academy of Sciences of Ukraine, (3) State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences)🇨🇳
In this paper a role of many-nucleon dynamics in formation of the compound $^{5} Li$ nucleus in the scattering of protons off $α$ -particles at the proton incident energies up to 20 MeV is investigated. We propose a bremsstrahlung model allowing to extract information about probabilities of formation of such nucleus on the basis of analysis of experimental cross-sections of the bremsstrahlung photons. In order to realize this approach, the model includes elements of microscopic theory and also probabilities of formation of the short-lived compound nucleus. Results of calculations of the bremsstrahlung spectra are in good agreement with the experimental cross-sections.
nucl-thhep-phnucl-ex0 citations
07
Heavy baryonic resonances, multistrange hadrons, and equilibration at energies available at the GSI Schwerionensynchrotron, SIS18
J. Steinheimer🇩🇪 · M. Lorenz🇩🇪 · F. Becattini🇮🇹 · R. Stock🇩🇪 · M. Bleicher🇩🇪
We study the details and time dependence of particle production in nuclear collisions at a fixed target beam energy of $E_{lab} = 1.76$ A GeV with the UrQMD transport model. We find that the previously proposed production mechanism for multi strange hadrons, $ϕ$ and $Ξ$ , are possible due to secondary interactions of incoming nuclei of the projectile and target with already created nuclear resonances, while the Fermi momenta of the nuclei play only a minor role. We also show how the centrality dependence of these particle multiplicities can be used to confirm the proposed mechanism, as it strongly depends on the number of participants in the reaction. Furthermore we investigate the time dependence of particle production in collisions of Ca+Ca at this beam energy, in order to understand the origins of the apparent chemical equilibration of the measured particle yields. We find that indeed the light hadron yields appear to be in equilibrium already from the very early stage of the collision while in fact no local equilibration, through multi collision processes, takes place. The apparent equilibrium is reached only through the decay of the resonances according to available phase space.
nucl-thPRC(2016)·19 citations
08
The hard-sphere model of strongly interacting fermion systems
Angela Mecca🇮🇹
The formalism based on Correlated Basis Functions (CBF) and the cluster-expansion technique has been recently employed to derive an effective interaction from a realistic nuclear Hamiltonian. One of the main objectives of the work described in this Thesis is establishing the accuracy of this novel approach--that allows to combine the flexibility of perturbation theory in the basis of eigenstates of the noninteracting system with a realistic description of short-range correlations in coordinate space--by focusing on the hard-sphere fermion system. As a first application of the formalism, the quasiparticle properties of hard spheres of degeneracy four have been determined from the two-point Green's function. The calculation has been performed carrying out a perturbative expansion of the self-energy, up to the second order in the CBF effective interaction. The main results of this study are the momentum distributions, the quasiparticle spectra and their description in terms of effective mass. The investigation of the hard-sphere fermion fluid has been extended to study the shear viscosity and thermal conductivity coefficients of the system with degeneracy two, that can be regarded as a model of pure neutron matter. The resulting transport coefficients show a strong sensitivity to the quasiparticle effective mass, reflecting the effect of second order contributions to the self-energy which are not taken into account in nuclear matter studies available in the literature. The difference between first and second order results is likely to play an important role in astrophysical applications and needs to be carefully investigated extending our analysis to nuclear matter.
nucl-th0 citations
09
Fluctuations as a test of chemical non-equilibrium at the LHC
Viktor Begun🇵🇱
It is shown that large chemical potential leads to the significant increase of multiplicity fluctuations for bosons, and makes the fluctuations infinite in the case of Bose-Einstein condensation. It allows to distinguish between the models that explain the anomalous proton to pion ratio and the low transverse momentum enhancement of pion spectra in Pb+Pb collisions at the LHC within chemical equilibrium or non-equilibrium models. The effects of resonance decays, finite size of the system, requirements to the event statistics, different momentum cuts, and limited detector acceptance are considered. The obtained results show the possibility to observe a substantial increase of the normalized kurtosis for positively or negatively charged pions in the case of non-equilibrium or partial pion condensation using currently measured data.
nucl-thhep-phnucl-exPRC(2016)·28 citations

Affiliations

first authorsco-authorsvia INSPIRE