Nuclear Theory·nucl-th

Wednesday·April 3, 2019

12 papers—6 primary·6 cross-listed

01
Neutron skins as laboratory constraints on properties of neutron stars and on what we can learn from heavy ion fragmentation reactions
C.A. Bertulani · J. Valencia
The equation of state (EOS) of infinite nuclear matter with a small proton/neutron fraction is a crucial input to determine the properties of neutron stars and compare model predictions to astronomical observations. The so-called `symmetry energy' is the part of the EOS accounting for the difference in the number of neutrons and protons. Numerous experiments have been devised to assess the symmetry energy and constrain its functional dependence with the nucleon density. Further constraints follow from a stellar modeling using the EOS to reproduce astronomical observations such as neutron star masses and radii. The recent detection of gravitational waves emitted from neutron star mergers and the nucleosynthesis ensuing from these events caused a surge of interest for such studies. Several types of nuclear reactions have been proposed to study the symmetry energy part of the EOS. Some of them consist in determining the neutron skin in nuclei and exploit its correlation with the slope parameter of the symmetry energy. In this article we explore a particular set of reactions using high energy ( $E_{l ab} \sim 1$ GeV/nucleon) neutron-rich projectiles. We explore measurements of all reaction fragments (a) in the same isotopic chain, i.e., only by removal of neutrons, (b) in all charge-changing channels, and (c) total interaction cross sections. Using Hartree-Fock-Bogoliubov (HBF) predictions for neutron and proton densities with Skyrme interactions, we explore the sensitivity of these cross sections with the neutron skin in nuclei.
nucl-thPRC(2019)·28 citations
02
$A b ini t i o$ description of collectivity for $s d$ shell nuclei
A. Saxena · A. Kumar · V. Kumar · P.C. Srivastava · T. Suzuki
In the present work, we have reported shell model results for open shell nuclei Ne, Mg and Si isotopes with $10 \leq N \leq 20$ in $s d$ -shell model space. We have performed calculations in $s d$ shell with two $ab ini t i o$ approaches: in-medium similarity renormalization group (IM-SRG) and coupled-cluster (CC) theory. We have also performed calculations with phenomenological USDB interaction and chiral effective field theory based CEFT interaction. The results for rotational spectra and $B (E 2; 2_{1}^{+} \to 0_{1}^{+})$ transitions are reported for even-mass isotopes. The IM-SRG and CC results are in reasonable agreement with the experimental data except at $N$ =20. This demonstrates a validity of $ab ini t i o$ description of deformation for doubly open-shell nuclei for $s d$ shell. To see the importance of $p f$ orbitals, we have also compared our results with SDPF-MU interaction by taking account of $2 p - 2 h$ and $4 p - 4 h$ configurations in $s d$ - $p f$ -shell model space.
nucl-thHyperfine Interact.(2019)·4 citations
03
Microscopic study of K-Selection Rule Violation in $^{182}$ W K=10 $^{+}$ Isomer Decay
C. R. Praharaj · Swagatika Bhoi · Z. Naik · S. K. Ghorui · S. K Patra
In this paper we study the microscopic mechanism for the retarded decay of K-isomers to lower K bands. We do angular momentum projection from suitable intrinsic states. The retardation arises from poor overlap between the low K and high K bands in the integral over Euler angles. Deformed HF and angular momentum projection calculations are done for the decay of the $K = 1 0^{+}$ isomer band to the ground band of $^{182} W$ . K-mixing is unimportant and the K quantum numbers of the bands are quite good. There is significant difference in the reduced matrix elements of transition operators in our formalism and that in the rotational model. Angular momentum projection gives J-selection rule but there is no K selection rule for reduced matrix elements of electromagnetic multipole operators. Thus, E2 and M1 transitions from the K isomer to the ground band of $^{182} W$ are finite but retarded in angular momentum projection theory, as in experiments. This provides a theoretical basis for the study of K-isomers and their decay modes. Quantitative results are presented. The microscopic model gives J-selection rule and angular momentum conservation for combined matter and radiation systems.
nucl-th1 citation
04
Effect of shell structure on the fission of sub-lead nuclei
Guillaume Scamps · Cédric Simenel
Fission of atomic nuclei often produces mass asymmetric fragments. However, the origin of this asymmetry was believed to be different in actinides and in the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 252502 (2010)]. It has recently been argued that quantum shell effects stabilising pear shapes of the fission fragments could explain the observed asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564}, 382 (2018)]. This interpretation is tested in the sub-lead region using microscopic mean-field calculations of fission based on the Hartree-Fock approach with BCS pairing correlations. The evolution of the number of protons and neutrons in asymmetric fragments of mercury isotope fissions is interpreted in terms of deformed shell gaps in the fragments. A new method is proposed to investigate the dominant shell effects in the pre-fragments at scission. We conclude that the mechanisms responsible for asymmetric fissions in the sub-lead region are the same as in the actinide region, which is a strong indication of their universality.
nucl-thnucl-exPRC(2019)·91 citations
05
Two-neutron transfer reactions and quantum-chaos measure of nuclear spectra
A.I. Levon · A.G. Magner
A new statistical interpretation of the nuclear collective states is suggested and applied recently in rare earths and actinide nuclei by the two-neutron transfer reactions in terms of the nearest neighbor-spacing distributions (NNSDs). Experimental NNSDs were obtained by using the complete and pure sequences of the collective states through an unfolding procedure. The two-neutron transfer reactions allow to obtain such a sequence of the collective states that meets the requirements for a statistical analysis. Their theoretical analysis is based on the linear approximation of a repulsion level density within the Wigner-Dyson theory. This approximation is successful to evaluate separately the Wigner chaos and Poisson order contributions. We found an intermediate behavior of NNSDs between the Wigner and Poisson limits. NNSDs turn out to be shifted from a chaos to order with increasing the length of spectra and the angular momentum of collective states. Perspectives for the statistical analysis of the symmetry breaking of states with the fixed projection of angular momenta $K$ are discussed.
nucl-thphysics.data-anNucl.Phys.Atom.Energy(2019)·1 citation
06
Description of the mass-asymmetric fission of the Pt isotopes, obtained in the reaction $^{36}$ Ar + $^{142}$ Nd within the two-stage fusion-fission model
V. L. Litnevsky · F. A. Ivanyuk · G. I. Kosenko · S. Chiba
The two stages dynamical stochastic model developed earlier for description of fusion-fission reactions is applied to the calculation of mass- and energy-distributions of fission fragments of platinum isotopes in reaction $^{36} Ar +^{142} Nd \to^{178 - x} Pt + xn$ . The first stage of this model is the calculation of the approaching of projectile nucleus to the target nucleus. On the second stage of the model, the evolution of the system formed after the touching of the projectile and target nuclei is considered. The evolution of the system on both stages is described by three-dimensional Langevin equations for the shape parameters of the system. The mutual orientation of the colliding ions and tunneling through the Coulomb barrier in the entrance channel are also taken into account. The potential energy of the system is calculated within the macroscopic-microscopic approach. The calculated mass-energy distributions of fission fragments are compared with the available experimental data. The impact of shell effects, rotation of the system and neutron evaporation on the calculated results is discussed.
nucl-thPRC(2019)·1 citation
07
Strange electromagnetic form factors of the nucleon with $N_{f} = 2 + 1$ $O (a)$ -improved Wilson fermions
Dalibor Djukanovic🇩🇪 · Konstantin Ottnad🇩🇪 · Jonas Wilhelm🇩🇪 · Hartmut Wittig🇩🇪
We present results for the strange contribution to the electromagnetic form factors of the nucleon computed on the CLS ensembles with $N_{f} = 2 + 1$ flavors of $O (a)$ -improved Wilson fermions and an $O (a)$ -improved vector current. Several source-sink separations are investigated in order to estimate the excited-state contamination. We calculate the form factors on six ensembles with lattice spacings in the range of $a = 0.049 - 0.086 fm$ and pion masses in the range of $m_{π} = 200 - 360 MeV$ , which allows for a controlled chiral and continuum extrapolation. In the computation of the quark-disconnected contributions we employ hierarchical probing as a variance reduction technique.
↳ hep-lathep-phnucl-thPRL(2019)·34 citations
08
Quantized first-order phase transition and two sets of critical end point in droplet quark matter
Kun Xu🇨🇳 · Mei Huang🇨🇳
The finite-size effect on the chiral phase transition is investigated in the Nambu--Jona-Lasinio model. To take into account finite-size effects, momentum integrals are replaced by momentum summations. The ground state of quark matter at finite size is favored when applying the periodic spatial boundary condition for quarks. The zero-momentum contribution is taken into account in the periodic boundary condition, and its contribution becomes important when the system size is comparable with the pion wavelength. When the zero-mode contribution becomes dominant, the conventional first-order chiral phase transition at high baryon chemical potential splits into two first-order phase transitions in small system of quark matter, and two sets of critical end point show up in the temperature and chemical potential plane.
↳ hep-phhep-latnucl-thPoS(2019)·5 citations
09
Phase structure of three flavor QCD in external magnetic fields using HISQ fermions
Akio Tomiya🇺🇸 · Heng-Tong Ding🇨🇳 · Xiao-Dan Wang🇨🇳 · Yu Zhang🇨🇳 · Swagato Mukherjee🇺🇸 · Christian Schmidt🇩🇪
We study the phase structure of QCD with three degenerate flavors in the external magnetic fields using highly improved staggered quarks (HISQ). The simulations are performed on $1 6^{3} \times 6$ lattice. In order to investigate the quark mass dependence of the chiral transition we choose the values of the bare quark masses 0.015 and 0.0009375 in the lattice unit, corresponding to $m_{π} = 320$ MeV and 80 MeV in the continuum limit. We found no indication of a first order phase transition in the current window of quark masses and external magnetic fields. Unlike to the case with standard staggered fermions inverse magnetic catalysis is always observed at about the critical temperature. The microscopic origin of this phenomena are further discussed by looking into the Dirac eigenvalue spectrum.
↳ hep-lathep-phnucl-thPoS(2019)·24 citations
10
Equation-of-state Constraints and the QCD Phase Transition in the Era of Gravitational-Wave Astronomy
Andreas Bauswein🇩🇪 · Niels-Uwe Friedrich Bastian🇵🇱 · David Blaschke🇵🇱 · Katerina Chatziioannou🇺🇸 · James Alexander Clark🇺🇸 · Tobias Fischer🇵🇱 · Hans-Thomas Janka🇩🇪 · Oliver Just🇯🇵 · Micaela Oertel🇫🇷 · Nikolaos Stergioulas🇬🇷
We describe a multi-messenger interpretation of GW170817, which yields a robust lower limit on NS radii. This excludes NSs with radii smaller than about 10.7 km and thus rules out very soft nuclear matter. We stress the potential of this type of constraints when future detections become available. A very similar argumentation may yield an upper bound on the maximum mass of nonrotating NSs. We also discuss simulations of NS mergers, which undergo a first-order phase transition to quark matter. We point out a different dynamical behavior. Considering the gravitational-wave signal, we identify an unambiguous signature of the QCD phase transition in NS mergers. The occurrence of quark matter through a strong first-order phase transition during merging leads to a characteristic shift of the dominant postmerger frequency. The frequency shift is indicative for a phase transition if it is compared to the postmerger frequency which is expected for purely hadronic EoS models. A very strong deviation of several 100 Hz is observed for hybrid EoSs in an otherwise tight relation between the tidal deformability and the postmerger frequency. We address the potential impact of a first-order phase transition on the electromagnetic counterpart of NS mergers. Our simulations suggest that there would be no significant qualitative differences between a system undergoing a phase transition to quark matter and purely hadronic mergers. The quantitative differences are within the spread which is found between different hadronic EoS models. This implies on the one hand that GW170817 is compatible with a possible transition to quark matter. On the other hand these considerations show that it may not be easy to identify quantitative differences between purely hadronic mergers and events in which quark matter occurs considering solely their electromagnetic counterpart or their nucleosynthesis products. (abridged)
↳ astro-ph.HEnucl-exnucl-thAIP Conf.Proc.(2019)·78 citations
11
Wounded nucleon, quark and quark-diquark emission functions versus experimental results from RHIC
Michał Barej🇵🇱 · Adam Bzdak🇵🇱 · Paweł Gutowski🇵🇱
Using the wounded nucleon, quark, and quark-diquark models, we extract the wounded source emission functions from the PHOBOS data for d+Au collisions at $s_{_{N N}} = 200$ GeV. We apply these models to compute charged particle multiplicity distributions as functions of pseudorapidity for p+p, p+Al, p+Au, d+Au, $^{3}$ He+Au, Cu+Cu, Cu+Au, Au+Au, and U+U collisions at the same energy and compare them with experimental data from the PHOBOS and PHENIX collaborations. In symmetric collisions of heavy nuclei, the obtained distributions differ among the tested models. On the other hand, in asymmetric collisions, all three models give essentially the same distributions. The wounded quark-diquark and quark models are in reasonable agreement with data for all the investigated systems.
↳ hep-phhep-exnucl-exnucl-thPRC(2019)·9 citations
12
Merger of compact stars in the two-families scenario
Roberto De Pietri🇮🇹 · Alessandro Drago🇮🇹 · Alessandra Feo🇮🇹 · Giuseppe Pagliara🇮🇹 · Michele Pasquali🇮🇹 · Silvia Traversi🇮🇹 · Grzegorz Wiktorowicz🇨🇳
We analyse the phenomenological implications of the two-families scenario on the merger of compact stars. That scenario is based on the coexistence of both hadronic stars and strange quark stars. After discussing the classification of the possible mergers, we turn to detailed numerical simulations of the merger of two hadronic stars, i.e., "first family" stars in which delta resonances and hyperons are present, and we show results for the threshold mass of such binaries, for the mass dynamically ejected and the mass of the disk surrounding the post-merger object. We compare these results with those obtained within the one-family scenario and we conclude that relevant signatures of the two-families scenario can be suggested, in particular: the possibility of a rapid collapse to a black hole for masses even smaller than the ones associated to GW170817; during the first milliseconds, oscillations of the postmerger remnant at frequencies higher than the ones obtained in the one-family scenario; a large value of the mass dynamically ejected and a small mass of the disk, for binaries of low total mass. Finally, based on a population synthesis analysis, we present estimates of the number of mergers for: two hadronic stars; hadronic star - strange quark star; two strange quark stars. We show that for unequal mass systems and intermediate values of the total mass, the merger of a hadronic star and a strange quark star is very likely (GW170817 has a possible interpretation into this category of mergers). On the other hand, mergers of two strange quark stars are strongly suppressed.
↳ astro-ph.HEastro-ph.SRhep-phnucl-thApJ(2019)·71 citations

Affiliations

first authorsco-authorsvia INSPIRE