Nuclear Theory·nucl-th

Tuesday·June 5, 2018

14 papers—10 primary·4 cross-listed

01
[Submitted on 1 Jun 2018]
Polarized EMC Effect in the QMC Model
Stephen Tronchin🇦🇺 · Hrayr H. Matevosyan🇦🇺 · Anthony W. Thomas🇦🇺
The ratios of the in-medium to free nucleon structure functions for the unpolarized and polarized cases are obtained using the MIT bag model for the free case, along with the QMC model to incorporate the in-medium modifications of the structure functions. As discussed in earlier work, the observed nuclear EMC effect is reasonably well described. This gives us confidence to investigate the predictions of the model for the polarized EMC effect, the ratio $g_{1}^{*} (x) / g_{1} (x)$ for a bound proton to that of a free proton. This ratio is found to be substantially different from unity and very similar in shape and size to that found in the unpolarized case. This prediction of such a fundamental change in the valence structure of a bound nucleon needs to be tested experimentally at the earliest opportunity.
Comments:
11 pages, 4 figures
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph); Nuclear Experiment (nucl-ex)
arXiv:
1806.00481 [pdf]
PLB(2018)·12 citations
02
[Submitted on 1 Jun 2018]
Bayesian approach to model-based extrapolation of nuclear observables
Léo Neufcourt · Yuchen Cao · Witold Nazarewicz · Frederi Viens
The mass, or binding energy, is the basis property of the atomic nucleus. It determines its stability, and reaction and decay rates. Quantifying the nuclear binding is important for understanding the origin of elements in the universe. The astrophysical processes responsible for the nucleosynthesis in stars often take place far from the valley of stability, where experimental masses are not known. In such cases, missing nuclear information must be provided by theoretical predictions using extreme extrapolations. Bayesian machine learning techniques can be applied to improve predictions by taking full advantage of the information contained in the deviations between experimental and calculated masses. We consider 10 global models based on nuclear Density Functional Theory as well as two more phenomenological mass models. The emulators of S2n residuals and credibility intervals defining theoretical error bars are constructed using Bayesian Gaussian processes and Bayesian neural networks. We consider a large training dataset pertaining to nuclei whose masses were measured before 2003. For the testing datasets, we considered those exotic nuclei whose masses have been determined after 2003. We then carried out extrapolations towards the 2n dripline. While both Gaussian processes and Bayesian neural networks reduce the rms deviation from experiment significantly, GP offers a better and much more stable performance. The increase in the predictive power is quite astonishing: the resulting rms deviations from experiment on the testing dataset are similar to those of more phenomenological models. The empirical coverage probability curves we obtain match very well the reference values which is highly desirable to ensure honesty of uncertainty quantification, and the estimated credibility intervals on predictions make it possible to evaluate predictive power of individual models.
Subjects:
Nuclear Theory (nucl-th); Machine Learning (stat.ML)
arXiv:
1806.00552 [pdf]
PRC(2018)·180 citations
03
[Submitted on 2 Jun 2018]
Fission Dynamics of 240Pu from Saddle-to-Scission and Beyond
Aurel Bulgac · Shi Jin · Kenneth Roche · Nicolas Schunck · Ionel Stetcu
Calculations are presented for the time evolution of $^{240}$ Pu from the proximity of the outer saddle point until the fission fragments are well separated, using the time-dependent density functional theory extended to superfluid systems. We have tested three families of nuclear energy density functionals and found that all functionals exhibit a similar dynamics: the collective motion is highly dissipative and with little trace of inertial dynamics, due to the one-body dissipation mechanism alone. This finding justifies the validity of using the overdamped collective motion approach and to some extent the main assumptions in statistical models of fission. This conclusion is robust with respect to the nuclear energy density functional used. The configurations and interactions left out of the present theory framework only increase the role of the dissipative couplings. An unexpected finding is varying the pairing strength within a quite large range has only minor effects on the dynamics. We find notable differences in the excitation energy sharing between the fission fragments in the cases of spontaneous and induced fission. With increasing initial excitation energy of the fissioning nucleus more excitation energy is deposited in the heavy fragment, in agreement with experimental data on average neutron multiplicities.
Comments:
Published version, 26 pages, 15 figures
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.00694 [pdf]
PRC(2019)·122 citations
04
[Submitted on 3 Jun 2018]
Microscopic description of proton-induced spallation reactions with the Constrained Molecular Dynamics (CoMD) Model
A. Assimakopoulou · G.A. Souliotis · A. Bonasera · A. Botvina · N.G. Nicolis · M. Veselsky
We studied the complete dynamics of the proton-induced spallation process with the microscopic framework of the Constrained Molecular Dynamics (CoMD) Model. We performed calculations of proton-induced spallation reactions on 181Ta, 208Pb, and 238U targets with the CoMD model and compared the results with a standard two-step approach based on an intranuclear cascade model (INC) followed by a statistical deexcitation model. The calculations were also compared with recent experimental data from the literature. Our calculations showed an overall satisfactory agreement with the experimental data and suggest further improvements in the models. We point out that this CoMD study represents the first complete dynamical description of spallation reactions with a microscopic N-body approach and may lead to advancements in the physics-based modelling of the spallation process.
Comments:
11 pages, 7 figures, submitted to Phys. Rev. C
Subjects:
Nuclear Theory (nucl-th); Nuclear Experiment (nucl-ex)
arXiv:
1806.00767 [pdf]
J.Phys.G(2019)·4 citations
05
[Submitted on 4 Jun 2018]
Neutron to Dark Matter Decay in Neutron Stars
T. F. Motta🇦🇺 · P. A. M. Guichon🇫🇷 · A. W. Thomas🇦🇺
Recent proposals have suggested that a previously unknown decay mode of the neutron into a dark matter particle could solve the long lasting measurement problem of the neutron decay width. We show that, if the dark particle in neutron decay is the major component of the dark matter in the universe, this proposal is in disagreement with modern astro-physical data concerning neutron star masses.
Comments:
Invited talk presented at the Conference on Particles and Cosmology at NTU Singapore, March 2018
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.00903 [pdf]
Int.J.Mod.Phys.A(2018)·35 citations
06
[Submitted on 4 Jun 2018]
KN scattering amplitude revisited in a chiral unitary approach and a possible broad resonance in S=+1 channel
Kenji Aoki🇯🇵 · Daisuke Jido🇯🇵
We revisit the $K N$ scattering amplitude in order to investigate the possibility for the existence of a broad resonance in the $I = 0$ $K N$ channel around the energy of 1617 MeV with 305 MeV width. We use the chiral unitary model to describe the $K N$ scattering amplitudes and determine the model parameters so as to reproduce the differential cross sections of the $K^{+} N$ scatterings and the $I = 0$ and 1 total cross sections up to $p_{l ab} = 800$ MeV/c, from which inelastic contributions start to be significant. Performing analytic continuation of the determined amplitude to the complex energy plane, we find a pole for a broad resonance state. We point out that the rapid increase appearing in the $I = 0$ total cross section around $p_{l ab} = 500$ MeV/c is a hint of the possible broad resonance of strangeness $S = + 1$ .
Comments:
26 pages, 14 figures
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph)
arXiv:
1806.00925 [pdf]
PTEP(2019)·10 citations
07
[Submitted on 4 Jun 2018]
Hartree-Fock Gamow basis from realistic nuclear forces
Qiang Wu · Furong Xu
We present a simplified method to generate the Hartree-Fock Gamow basis from realistic nuclear forces. The Hartree-Fock iteration in the harmonic-oscillator basis is first performed, and then the obtained HF potential is analytically continued to the complex-k plane, finally by solving the Schrödinger equation in the complex-k plane the Gamow basis is obtained. As examples, the method is applied to 4He and 22O with the renormalized chiral N3LO potential. The basis obtained which includes bound, resonant and scattering states can be further used in many-body calculations to study weakly bound nuclei.
Comments:
11 pages, 5 figures
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.01014 [pdf]
Chin.Sci.Bull.(2018)·3 citations
08
[Submitted on 4 Jun 2018]
Proton-Nucleus Elastic Scattering: Comparison between Phenomenological and Microscopic Optical Potentials
Matteo Vorabbi · Paolo Finelli · Carlotta Giusti
Elastic scattering is a very important process to understand nuclear interactions in finite nuclei. Despite decades of efforts, the goal of reaching a coherent description of this physical process in terms of microscopic forces is still far from being completed. In previous papers (Phys. Rev. C93, 034619 (2016), Phys. Rev. C96, 044001 (2017)) we derived a nonrelativistic theoretical optical potential from nucleon-nucleon chiral potentials at fourth (N3LO) and fifth order (N4LO). We checked convergence patterns and established theoretical error bands. With this work we study the performances of our optical potential in comparison with those of a successful nonrelativistic phenomenological optical potential in the description of elastic proton scattering data on several isotopic chains at energies around and above 200 MeV. We use the same framework and the same approximations of our previous papers, where the nonrelativistic optical potential is derived at the first-order term within the spectator expansion of the multiple scattering theory and adopting the impulse approximation and the optimum factorization approximation. The cross sections and analyzing powers for elastic proton scattering off calcium, nickel, tin, and lead isotopes are presented for several incident proton energies, exploring the range $156 \leq E \leq 333$ MeV, where experimental data are available. In addition, we provide theoretical predictions for Ni56 at 400 MeV, which is of interest for the future experiments at EXL. Our results indicate that microscopic optical potentials derived from nucleon-nucleon chiral potentials at N4LO can provide reliable predictions for the cross section and the analyzing power both of stable and exotic nuclei, even at energies where the reliability of the chiral expansion starts to be questionable.
Comments:
26 pages, 10 figures, submitted to PRC
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.01037 [pdf]
PRC(2018)·23 citations
09
[Submitted on 4 Jun 2018]
Energy-density functionals inspired by effective-field theories: Applications to neutron drops
Jérémy Bonnard🇫🇷 · Marcella Grasso🇫🇷 · Denis Lacroix🇫🇷
New energy-density functionals (EDFs) inspired by effective-field theories (EFTs) have been recently proposed. The present work focuses on three of such functionals which were developed to produce satisfactory equations of state for nuclear matter. We aim to extend these functionals to treat finite systems including a spin-orbit contribution and pairing correlations. We illustrate here a first step towards this direction, namely a generalization of such functionals tailored to perform applications to neutron gases confined in harmonic traps. Sets of available \textit{ab initio} results are used as benchmark pseudo-data for adjusting the additional parameters (with respect to the nuclear matter case) that have to be introduced for finite-size systems. Several quantities are predicted and compared to \textit{ab initio} and other EDF results such as, for instance, total energies, potentials, and density profiles. The associated effective masses are also analyzed. In cases where \textit{ab initio} results are available, two of these functionals globally provide predictions which are close one to the other as well as to \textit{ab initio} values. It is shown that, in general, this is not the case for several currently used Skyrme functionals. Directions for improving the third functional are discussed.
Comments:
14 pages, 10 figures
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.01084 [pdf]
PRC(2018)·29 citations
10
[Submitted on 4 Jun 2018]
Halo structure of $^{17}$ C
Jonas Braun · Hans-Werner Hammer · Lucas Platter
$^{17}$ C has three states below the $^{16}$ C + $n$ threshold with quantum numbers $J^{P} = 3/ 2^{+}, 1/ 2^{+}, 5/ 2^{+}$ . These states have relatively small neutron separation energies compared to the neutron separation and excitation energies of $^{16}$ C. This separation of scales motivates our investigation of $^{17}$ C in a Halo effective field theory (Halo EFT) with a $^{16}$ C core and a valence neutron as degrees of freedom. We discuss various properties of the three states such as electric radii, magnetic moments, electromagnetic transition rates and capture cross sections. In particular, we give predictions for the charge radius and the magnetic moment of the $1/ 2^{+}$ state and for neutron capture on $^{16}$ C into this state. Furthermore, we discuss the predictive power of the Halo EFT approach for the $3/ 2^{+}$ and $5/ 2^{+}$ states which are described by a neutron in a $D$ -wave relative to the core.
Comments:
19 pages, 9 figures, section on M1 capture revised, published version
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1806.01112 [pdf]
EPJA(2018)·10 citations

Affiliations

first authorsco-authorsvia INSPIRE