Nuclear Theory·nucl-th

Friday·May 8, 2020

11 papers—7 primary·4 cross-listed

01
Reduction of SO in $^{34}$ Si: weak binding or density-depletion effect ?
O. Sorlin🇫🇷 · F. de Oliveira🇫🇷 · J.P. Ebran🇫🇷
The reduction of the neutron spin-orbit splitting $2 p_{3/2} - 2 p_{1/2}$ between the $^{41}$ Ca and $^{35}$ Si isotones is a unique feature throughout the chart of nuclides, as the spin-orbit splitting usually increases with $A$ . Moreover, its way of decrease, gradual between $^{41}$ Ca and $^{35}$ Si or abrupt between $^{37}$ S and $^{35}$ Si, as well as its origin, caused by the weak binding energy of the $p$ states or by the sudden central proton density depletion in $^{35}$ Si, are subject of debate. The results reported here using the self-consistent Covariant Energy Density Functional calculations with the DD-ME2 parametrization rather point to an abrupt, local decrease in $^{35}$ Si, and to the large dominance of the central density depletion effect. It is concluded that weak binding, central density depletion as well as correlations must be taken into account to fully evaluate the amplitude and causes of this spin-orbit reduction.
nucl-thnucl-exPLB(2020)·7 citations
02
The Dirac oscillator: an alternative basis for nuclear structure calculations
Junjie Yang · J. Piekarewicz
Background: The isotropic harmonic oscillator supplemented by a strong spin-orbit interaction has been the cornerstone of nuclear structure since its inception more than seven decades ago. In this paper we introduce---or rather re-introduced---the "Dirac Oscillator", a fully relativistic basis that has all the desired attributes of the ordinary harmonic oscillator while naturally incorporating a strong spin-orbit coupling. Purpose: To assess---to our knowledge for the first time---the power and flexibility of the Dirac Oscillator basis in the solution of nuclear structure problems within the framework of covariant density functional theory. Methods: Self-consistent calculations of binding energies and ground-state densities for a selected set of doubly-magic magic are performed using the Dirac-oscillator basis and are then compared against results obtained with the often-used Runge-Kutta method. Results: Results obtained using the Dirac oscillator basis reproduced with high accuracy those derived using the Runge-Kutta method and suggest a clear path for a generalization to systems with axial symmetry. Conclusions: Although the three-dimensional harmonic oscillator with spin-orbit corrections has been the staple of the nuclear shell model since the beginning, the Dirac oscillator is practically unknown among the nuclear physics community. In this paper we illustrate the power and flexibility of the Dirac oscillator and suggest extensions to the study of systems without spherical symmetry as required in constrained calculations of nuclear excitations.
nucl-thPRC(2020)·13 citations
03
Quantifying Uncertainties on Fission Fragment Mass Yields With Mixture Density Networks
A.E. Lovell · A.T. Mohan · P. Talou
Probabilistic machine learning techniques can learn both complex relations between input features and output quantities of interest as well as take into account stochasticity or uncertainty within a data set. In this initial work, we explore the use of one such probabilistic network, the Mixture Density Network (MDN), to reproduce fission yields and their uncertainties. We study mass yields for the spontaneous fission of $^{252}$ Cf, exploring the number of training samples needed for converged predictions, how different levels of uncertainty propagate from the training set to the MDN predictions, and how well physical constraints of the yields - such as normalization and symmetry - are upheld by the algorithm. Finally, we test the ability of the MDN to interpolate between and extrapolate beyond samples in the training set using energy-dependent mass yields for the neutron-induced fission on $^{235}$ U. The MDN provides a reliable way to include and predict uncertainties and is a promising path forward for supplementing sparse sets of nuclear data.
nucl-thnucl-exJ.Phys.G(2020)·32 citations
04
Correlations Between Fission Fragment and Neutron Anisotropies in Neutron-Induced Fission
A.E. Lovell · P. Talou · I. Stetcu · K.J. Kelly
Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are several potential sources of angular anisotropies that are more difficult to disentangle: the angular distributions of prompt neutrons from fully accelerated fragments or from scission neutrons, and the emission of neutrons from fission fragments that are not fully accelerated. In this work, we study the effects of the first group of anisotropy sources, particularly exploring the correlations between the fission fragment anisotropy and the resulting neutron anisotropy. While kinematic effects were already accounted for in our Hauser-Feshbach Monte Carlo code, $CGMF$ , anisotropic angular distributions for the fission fragments and pre-equilibrium neutrons resulting from neutron-induced fission on $^{233, 234, 235, 238}$ U, $^{239, 241}$ Pu, and $^{237}$ Np have been introduced for the first time. The effects of these sources of anisotropy are examined over a range of incident neutron energies, from thermal to 20 MeV, and compared to experimental data from the Chi-Nu liquid scintillator array. The anisotropy of the fission fragments is reflected in the anisotropy of the prompt neutrons, especially as the outgoing energy of the prompt neutrons increases, allowing for an extraction of the fission fragment anisotropy to be made from a measurement of the neutrons.
nucl-thnucl-exPRC(2020)·11 citations
05
Hartree-Fock-Bogoliubov theory for odd-mass nuclei with a time-odd constraint and application to deformed halo nuclei
Haruki Kasuya · Kenichi Yoshida
We show that the lowest-energy solution of the Hartree-Fock-Bogoliubov (HFB) equation has the even particle-number parity as long as the time-reversal symmetry is conserved in the HFB Hamiltonian without null eigenvalues. Based on this finding, we give a rigorous foundation of a method for solving the HFB equation to describe the ground state of odd-mass nuclei by employing a time-reversal anti-symmetric constraint operator to the Hamiltonian, where one obtains directly the ground state as a self-consistent solution of the cranked-HFB-type equation. Numerical analysis is done for the neutron-rich Mg isotopes with a reasonable choice for the operator, and it is demonstrated that the anomalous increase in the matter radius of $^{37}$ Mg is well described when the last neutron occupies a low angular-momentum orbital in the framework of the nuclear energy-density-functional method, revealing the deformed halo structure.
nucl-thnucl-exPTEP(2021)·30 citations
06
Heavy flavor quenching and flow: the roles of initial condition, pre-equilibrium evolution and in-medium interaction
Shu-Qing Li🇨🇳 · Wen-Jing Xing🇨🇳 · Feng-Lei Liu🇨🇳 · Shanshan Cao🇺🇸 · Guang-You Qin🇨🇳
Within an advanced Langevin-hydrodynamics framework coupled to a hybrid fragmentation-coalescence hadronization model, we study heavy flavor quenching and flow in relativistic heavy-ion collisions. We investigate how the initial heavy quark spectrum, the energy loss and hadronization mechanisms of heavy quarks in medium, the evolution profile of pre-equilibrium stage, the flow of medium and the temperature dependence of heavy quark diffusion coefficient influence the suppression and elliptic flow of heavy mesons at RHIC and the LHC. Our result shows that different modeling of initial conditions, pre-equilibrium evolution and in-medium interaction can individually yield about 10-40% uncertainties in D meson suppression and flow at low transverse momentum. We also find that a proper combination of collisional versus radiative energy loss, coalescence versus fragmentation in hadronization, and the inclusion of medium flow are the most important factors for describing the suppression and elliptic flow of heavy mesons.
nucl-thhep-phnucl-exCPC(2020)·29 citations
07
Exact representations of many body interactions with RBM neural networks
Ermal Rrapaj🇺🇸 · Alessandro Roggero🇺🇸
Restricted Boltzmann Machines (RBM) are simple statistical models defined on a bipartite graph which have been successfully used in studying more complicated many-body systems, both classical and quantum. In this work, we exploit the representation power of RBMs to provide an exact decomposition of many-body contact interactions into one-body operators coupled to discrete auxiliary fields. This construction generalizes the well known Hirsch's transform used for the Hubbard model to more complicated theories such as Pionless EFT in nuclear physics, which we analyze in detail. We also discuss possible applications of our mapping for quantum annealing applications and conclude with some implications for RBM parameter optimization through machine learning.
nucl-thphysics.comp-phquant-phPRE(2021)·23 citations
08
Was GW170817 a canonical neutron star merger? Bayesian analysis with a third family of compact stars
David Blaschke🇷🇺 · Alexander Ayriyan🇷🇺 · David Alvarez-Castillo🇷🇺 · Hovik Grigorian🇷🇺
We investigate the possibility that GW170817 has not been the merger of two conventional neutron stars (NS) but involved at least one if not two hybrid stars with a quark matter core which might even belong to a third family of compact stars. To this end, we develop a Bayesian analysis method for selecting the most probable equation of state (EoS) under a set of constraints from compact star physics, which now also include the tidal deformability from GW170817 and the first result for the mass and radius determination for PSR J0030+0451 by NICER. We apply this method for the first time to a two-parameter family of hybrid EoS based on the DD2 model with nucleonic excluded volume for hadronic matter and the color superconducting generalized nlNJL model for quark matter. The model has a variable onset of deconfinement and can mimic the effects of pasta phases with the possibility of a third family of hybrid stars in the mass-radius ( $M - R$ ) diagram. The main findings of this study are that: 1) the presence of multiple configurations for a given mass (twins) corresponds to a set of disconnected lines in the diagram of tidal deformabilities for binary mergers, so that merger events from the same mass range may result in a probability landscape with different peak positions; 2) the Bayesian analysis with the above observational constraints favors an early onset of the deconfinement transition, at masses of $M_{onset} \leq 0.8 M_{⊙}$ with a $M - R$ relationship that in the range of observed neutron star masses is almost indistinguishable from that of a soft hadronic APR EoS; 3) a few yet fictitious measurements of the NICER experiment with a $1 σ$ range that is half of the present value and different mass and radius would change the posterior likelihood so that hybrid EoS with a phase transition onset in the range $M_{onset} = 1.1 - 1.6 M_{⊙}$ would be favored.
↳ astro-ph.HEhep-phnucl-thUniverse(2020)·84 citations
09
Investigating the dependence of collective dynamics on n/p asymmetry for light nuclei
R.T. deSouza · Varinderjit Singh · S. Hudan · Z. Lin · C.J. Horowitz
The dynamics present in the fusion of neutron-rich nuclei is explored through the comparison of experimental cross-sections at above-barrier energies with measurements of the interaction cross-section at relativistic energies. The increase of fusion dynamics with increasing neutron excess is clearly demonstrated. Experimental cross-sections are compared with the predictions of a Sao Paulo model using relativistic mean field density distributions and the impact of different interactions is explored.
↳ nucl-exnucl-th0 citations
10
Phase diagram of helically imbalanced QCD matter
M. N. Chernodub🇫🇷 · Victor E. Ambrus🇷🇴
We discuss the influence of a helicity imbalance on the phase diagram of dense QCD at finite temperature. We argue that the helical chemical potential is a thermodynamically relevant quantity in theories with the mass gap generation. Using the linear sigma model coupled to quarks, we show that the presence of the helical density substantially affects the phase diagram of dense quark matter. A moderate helical density makes the chiral phase transition softer while shifting the critical endpoint towards lower temperatures and higher baryon chemical potentials. As the helical density increases, the segment of the first-order transition disappears, and the chiral transition becomes a soft crossover. At even higher helical chemical potentials, the first-order transition reappears again at the zero-density finite-temperature transition and extends into the interior of the phase diagram. This evolution of the chiral transition reflects the existence of a thermodynamic duality between helical and vector (baryonic) chemical potentials. We also show that the presence of the helicity imbalance of quark matter increases the curvature of the chiral pseudocritical line in QCD.
↳ hep-thhep-phnucl-thPRD(2021)·15 citations
11
Inclusive and effective bulk viscosities in the hadron gas
J.-B. Rose🇩🇪 · J. M. Torres-Rincon🇩🇪 · H. Elfner🇩🇪
We estimate the temperature dependence of the bulk viscosity in a relativistic hadron gas. Employing the Green-Kubo formalism in the SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) transport approach, we study different hadronic systems in increasing order of complexity. We analyze the (in)validity of the single exponential relaxation ansatz for the bulk-channel correlation function and the strong influence of the resonances and their lifetimes. We discuss the difference between the inclusive bulk viscosity of an equilibrated, long-lived system, and the effective bulk viscosity of a short-lived mixture like the hadronic phase of relativistic heavy-ion collisions, where the processes whose inverse relaxation rate are larger than the fireball duration are excluded from the analysis. This clarifies the differences between previous approaches which computed the bulk viscosity including/excluding the very slow processes in the hadron gas. We compare our final results with previous hadron gas calculations and confirm a decreasing trend of the inclusive bulk viscosity over entropy density as temperature increases, whereas the effective bulk viscosity to entropy ratio, while being lower than the inclusive one, shows no strong dependence to temperature.
↳ hep-phnucl-exnucl-thJ.Phys.G(2020)·31 citations

Affiliations

first authorsco-authorsvia INSPIRE