Nuclear Theory

We study the properties of a spin-down neutron impurity immersed in a low-density free Fermi gas of spin-up neutrons. In particular, we analyze its energy ( E ??), effective mass ( m ????) and quasiparticle residue ( Z ??). Results are compared with those of state-of-the-art quantum Monte Carlo calculations of the attractive Fermi polaron realized in ultracold atomic gases experiments, and with those of previous studies of the neutron polaron. Calculations are performed within the Brueckner--Hartree--Fock approach using the chiral two-body nucleon-nucleon interaction of Entem and Machleidt at N 3 LO with a 500 MeV cut-off and the Argonne V18 phenomenological potential. Only contributions from the 1 S 0 partial wave, which is the dominant one in the low-density region considered, are included. Contributions from three-nucleon forces are expected to be irrelevant at these densities and, therefore, are neglected in the calculation. Our results show that for Fermi momenta between ??.25 and ??.45 fm ?? the energy, effective mass and quasiparticle residue of the impurity vary only slightly, respectively, in the ranges ??.604 E F < E ??<??.635 E F , 1.300m< m ????<1.085m and 0.741< Z ??<0.836 in the case of the chiral interaction, and ??.621 E F < E ??<??.643 E F , 1.310m< m ????<1.089m and 0.739< Z ??<0.832 when using the Argonne V18 potential. These results are compatible with those derived from ultracold atoms and show that a spin-down neutron impurity in a free Fermi gas of spin-up neutrons with a Fermi momentum in the range 0.25??k F ??.45 fm ?? exhibits properties very similar to those of an attractive Fermi polaron in the unitary limit.

Pionless effective field theory in a finite volume (FVEFT ?/ ) is investigated as a framework for the analysis of multi-nucleon spectra and matrix elements calculated in lattice QCD (LQCD). By combining FVEFT ?/ with the stochastic variational method, the spectra of nuclei with atomic number A?�{2,3} are matched to existing finite-volume LQCD calculations at heavier-than-physical quark masses corresponding to a pion mass m ? =806 MeV, thereby enabling infinite-volume binding energies to be determined using infinite-volume variational calculations. Based on the variational wavefunctions that are constructed in this approach, the finite-volume matrix elements of various local operators are computed in FVEFT ?/ and matched to LQCD calculations of the corresponding QCD operators in the same volume, thereby determining the relevant one and two-body EFT counterterms and enabling an extrapolation of the LQCD matrix elements to infinite volume. As examples, the scalar, tensor, and axial matrix elements are considered, as well as the magnetic moments and the isovector longitudinal momentum fraction.

We present the code HF-SHELL for solving the self-consistent mean-field equations for configuration-interaction shell model Hamiltonians in the proton-neutron formalism. The code can calculate both ground-state and finite-temperature properties in the Hartree-Fock (HF), HF+Bardeen-Cooper-Schrieffer (HF+BCS), and the Hartree-Fock-Bogoliubov (HFB) mean-field approximations. Particle-number projection after variation is incorporated to reduce the grand-canonical ensemble to the canonical ensemble, making the code particularly suitable for the calculation of nuclear state densities. The code does not impose axial symmetry and allows for triaxial quadrupole deformations. The self-consistency cycle is particularly robust through the use of the heavy-ball optimization technique and the implementation of different options to constrain the quadrupole degrees of freedom.

"Hybrid Hadronization" is a new Monte Carlo package to hadronize systems of partons. It smoothly combines quark recombination applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems, following the picture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates with PYTHIA 8 and can be applied to a variety of systems from e + + e − to A+A collisions. It takes systems of partons and their color flow information, for example from a Monte Carlo parton shower generator, as input. In addition, if for A+A collisions a thermal background medium is provided, the package allows sampling thermal partons that contribute to hadronization. Hybrid Hadronization is available for use as a standalone code and is also part of JETSCAPE since the 2.0 release. In these proceedings we review the physics concepts underlying Hybrid Hadronization and demonstrate how users can use the code with various parton shower Monte Carlos. We present calculations of hadron chemistry and fragmentation functions in small and large systems when Hybrid Hadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular, we discuss observable effects of the recombination of shower partons with thermal partons.

Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A >140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties. Here, we introduce new improvements to our scission-point model, called SPY, to derive the fission fragment distribution for all neutron-rich fissioning nuclei of relevance in r-process calculations. These improvements include a phenomenological modification of the scission distance and a smoothing procedure of the distribution. Such corrections lead to a much better agreement with experimental fission yields. Those yields are also used to estimate the number of neutrons emitted by the excited fragments on the basis of different neutron evaporation models. Our new fission yields are extensively compared to those predicted by the so-called GEF model. The impact of fission on the r-process nucleosynthesis in binary neutron mergers is also reanalyzed. Two scenarios are considered, the first one with low initial electron fraction is subject to intense fission recycling, in contrast to the second one which includes weak interactions on nucleons. The various regions of the nuclear chart responsible for fission recycling during the neutron irradiation as well as after freeze-out are discussed. The contribution fission processes may have to the final abundance distribution is also studied in detail in the light of newly defined quantitative indicators describing the fission recycling, the fission seeds and the fission progenitors. In particular, those allow us to estimate the contribution of fission to the final abundance distribution stemming from specific heavy nuclei. Calculations obtained with SPY and GEF fission fragment distributions are compared for both r-process scenarios.

Fission properties of the actinide nuclei are deduced from theoretical analysis. We investigate potential energy surfaces and fission barriers and predict the fission fragment mass-yields of actinide isotopes. The results are compared with experimental data where available. The calculations were performed in the macroscopic-microscopic approximation with the Lublin-Strasbourg Drop (LSD) for the macroscopic part and the microscopic energy corrections were evaluated in the Yukawa-folded potential. The Fourier nuclear shape parametrization is used to describe the nuclear shape, including the non-axial degree of freedom. The fission fragment mass-yields of considered nuclei are evaluated within a 3D collective model using the Born-Oppenheimer approximation.

Astrophysical nucleosynthesis is a family of diverse processes by which atomic nuclei undergo nuclear reactions and decays to form new nuclei. The complex nature of nucleosynthesis, which can involve as many as tens of thousands of interactions between thousands of nuclei, makes it difficult to study any one of these interactions in isolation using standard approaches. In this work, we present a new technique, nucleosynthesis tracing, that we use to quantify the specific role of individual nuclear reaction, decay, and fission processes in relationship to nucleosynthesis as a whole. We apply this technique to study fission and β − -decay as they occur in the rapid neutron capture ( r ) process of nucleosynthesis.

In this article we review the perfect-fluid hydrodynamics with spin framework proposed recently. This framework generalises the standard relativistic hydrodynamics framework to include spin degrees of freedom and provides a natural method to describe the spin polarization evolution of massive spin 1/2 particles. This formalism is based on the GLW (de Groot - van Leeuwen - van Weert) energy-momentum tensor and spin tensor. We show here using Bjorken model that how this spin hydrodynamics framework may be used for the determination of the observables which describes the particle polarization measured in the experiment.

The thermal description of light nuclei at the chemical freeze-out has been investigated. First, I have verified the equilibration of the light nuclei and then introduced a new method to investigate the light nuclei formation. One can study the proximity between the phase space density of light nuclei ratios and their hadronic constituents e.g d ¯ /d and ( p ¯ n ¯ /pn) . I have found that with the exclusion of the decay feed-down from the hadronic yields in the thermal model, the hadronic representations have good agreement with the light nuclei ratios. I have performed a similar analysis with the ratio of Λ -hypernuclei and 3 He , which is related to the ratio Λ/p . In this context, the strangeness population factor S 3 has been studied also. These results indicate that the nuclei and hypernuclei formation may occur near the standard chemical freeze-out and before the decay of the hadronic resonances. This method will serve as a guideline to discuss the light nuclei formation and the inclusion of decay into their hadronic constituents.

In this work, we start with chiral kinetic theory and construct the spin hydrodynamic framework for a chiral spinor system. Using the 14-moment expansion formalism, we obtain the equations of motion of second-order dissipative relativistic fluid dynamics with non-trivial spin polarization density. In a chiral spinor system, the spin alignment effect could be treated in the same framework as the Chiral Vortical Effect (CVE). However, the quantum corrections due to fluid vorticity induce not only CVE terms in the vector/axial charge currents but also corrections to the stress tensor. In this framework, viscous corrections to the hadron spin polarization are self-consistently obtained, which will be important for the precise prediction of the polarization rate for the observed hadrons, e.g. Λ -hyperon.