Showing papers on "Nuclear matter published in 2022"

Constraining neutron-star matter with microscopic and macroscopic collisions

Abstract: Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not probed only in astrophysical observations, but also in terrestrial heavy-ion collision experiments. Here we use Bayesian inference to combine data from astrophysical multi-messenger observations of neutron stars1-9 and from heavy-ion collisions of gold nuclei at relativistic energies10,11 with microscopic nuclear theory calculations12-17 to improve our understanding of dense matter. We find that the inclusion of heavy-ion collision data indicates an increase in the pressure in dense matter relative to previous analyses, shifting neutron-star radii towards larger values, consistent with recent observations by the Neutron Star Interior Composition Explorer mission5-8,18. Our findings show that constraints from heavy-ion collision experiments show a remarkable consistency with multi-messenger observations and provide complementary information on nuclear matter at intermediate densities. This work combines nuclear theory, nuclear experiment and astrophysical observations, and shows how joint analyses can shed light on the properties of neutron-rich supranuclear matter over the density range probed in neutron stars.

Holographic modeling of nuclear matter and neutron stars

Abstract: Abstract I review holographic models for (dense and cold) nuclear matter, neutron stars, and their mergers. I start by a brief general discussion on current knowledge of cold QCD matter and neutron stars, and go on discussing various approaches to model cold nuclear and quark matter by using gauge/gravity duality, pointing out their strengths and weaknesses. Then I focus on recent results for a complex bottom-up holographic framework (V-QCD), which also takes input from lattice QCD results, effective field theory, and perturbative QCD. Dense nuclear matter is modeled in V-QCD through a homogeneous non-Abelian bulk gauge field. Feasible “hybrid” equations of state for cold nuclear (and quark) matter can be constructed by using traditional methods (e.g., effective field theory) at low densities and the holographic V-QCD model at higher densities. I discuss the constraints from this approach to the properties of the nuclear to quark matter transition as well as to properties of neutron stars. Using such hybrid equations of state as an input for numerical simulations of neutron star mergers, I also derive predictions for the spectrum of produced gravitational waves.

Finding Structure in the Speed of Sound of Supranuclear Matter from Binary Love Relations.

Abstract: Analyses that connect observations of neutron stars with nuclear-matter properties can rely on equation-of-state insensitive relations. We show that the slope of the binary Love relations (between the tidal deformabilities of binary neutron stars) encodes the baryon density at which the speed of sound rapidly changes. Twin stars lead to relations that present a signature "hill," "drop," and "swoosh" due to the second (mass-radius) stable branch, requiring a new description of the binary Love relations. Together, these features can reveal new properties and phases of nuclear matter.

Neural network reconstruction of the dense matter equation of state from neutron star observables

Abstract: The Equation of State (EoS) of strongly interacting cold and hot ultra-dense QCD matter remains a major challenge in the field of nuclear astrophysics. With the advancements in measurements of neutron star masses, radii, and tidal deformabilities, from electromagnetic and gravitational wave observations, neutron stars play an important role in constraining the ultra-dense QCD matter EoS. In this work, we present a novel method that exploits deep learning techniques to reconstruct the neutron star EoS from mass-radius (M-R) observations. We employ neural networks (NNs) to represent the EoS in a model-independent way, within the range of ∼1-7 times the nuclear saturation density. The unsupervised Automatic Differentiation (AD) framework is implemented to optimize the EoS, so as to yield through TOV equations, an M-R curve that best fits the observations. We demonstrate that this method works by rebuilding the EoS on mock data, i.e., mass-radius pairs derived from a randomly generated polytropic EoS. The reconstructed EoS fits the mock data with reasonable accuracy, using just 11 mock M-R pairs observations, close to the current number of actual observations.

De Broglie waves and nuclear element interaction; Abundant waves structures of the nonlinear fractional Phi-four equation

Abstract: This research study investigates the computational wave solutions of the nonlinear fractional Phi-four (NLFPF) equation. The NLFPF model describes the nuclear element interaction and is also a very effective unique form of the Klein–Gordon (KG) equation. Two recent analytical (Khater II (Khat II) and Novel Kudryashov (NKud)) schemes are applied to the NLFPF model for constructing some novel solitary wave solutions. These solutions are explained through some numerical simulation to explain more properties of the nuclear element interaction. The paper’s novelty, scientific contributions, and results’ physical explanations are demonstrated. All solutions’ accuracy has been checked by putting them back into the original model by using Mathematica 13.1.

Imposing multi-physics constraints at different densities on the neutron Star Equation of State

Abstract: Abstract Neutron star matter spans a wide range of densities, from that of nuclei at the surface to exceeding several times normal nuclear matter density in the core. While terrestrial experiments, such as nuclear or heavy-ion collision experiments, provide clues about the behaviour of dense nuclear matter, one must resort to theoretical models of neutron star matter to extrapolate to higher density and finite neutron/proton asymmetry relevant for neutron stars. In this work, we explore the parameter space within the framework of the Relativistic Mean Field model allowed by present uncertainties compatible with state-of-the-art experimental data. We apply a cut-off filter scheme to constrain the parameter space using multi-physics constraints at different density regimes: chiral effective field theory, nuclear and heavy-ion collision data as well as multi-messenger astrophysical observations of neutron stars. Using the results of the study, we investigate possible correlations between nuclear and astrophysical observables.

Implications of NICER for Neutron Star Matter: The QHC21 Equation of State

Abstract: The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation of radii from 1.4$M_\odot$ to 2.1$M_\odot$, reveal key features of the equation of state of neutron star matter. The pressure rises rapidly in the regime of baryon density $n \sim$ 2-4 times nuclear saturation density, $n_0$ -- the region where we expect hadronic matter to be undergoing transformation into quark matter -- and the pressure in the nuclear regime is greater than predicted by microscopic many-body variational calculations of nuclear matter. To incorporate these insights into the microscopic physics from the nuclear to the quark matter regimes, we construct an equation of state, QHC21, within the framework of quark-hadron crossover (QHC). We include nuclear matter results primarily based on the state-of-the-art chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces. We employ explicit nuclear degrees of freedom only up to $n \sim 1.5n_0$, in order to explore the possibility of further physical degrees of freedom than nucleonic here. The resulting QHC21, which has a peak in sound velocity in $\sim 2$-$4 n_0$, is stiffer than the earlier QHC19 below 2$n_0$, predicting larger radii in substantial agreement with the NICER data.

New constraints on the neutron-star mass and radius relation from terrestrial nuclear experiments

Abstract: Hajime Sotani1,2,*, Nobuya Nishimura1,3,4, and Tomoya Naito5,3 1Astrophysical Big Bang Laboratory, RIKEN, Saitama 351-0198, Japan ∗E-mail: sotani@yukawa.kyoto-u.ac.jp 2Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS), RIKEN, Saitama 351-0198, Japan 3RIKEN Nishina Center for Accelerator-Based Science, Saitama 351-0198, Japan 4Division of Science, National Astronomical Observatory of Japan, Tokyo 181-8588, Japan 5Department of Physics, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan

Bayesian reconstruction of nuclear matter parameters from the equation of state of neutron star matter

Abstract: The nuclear matter parameters (NMPs), those underlie in the construction of the equation of state (EoS) of neutron star matter, are not directly accessible. The Bayesian approach is applied to reconstruct the posterior distributions of NMPs from the EoS of neutron star matter. The constraints on lower-order parameters as imposed by the finite nuclei observables are incorporated through appropriately chosen prior distributions. The calculations are performed with two sets of pseudo data on the EoS whose true models are known. The median values of second or higher order NMPs show sizeable deviations from their true values and associated uncertainties are also larger. The sources of these uncertainties are intrinsic in nature, identified as (i) the correlations among various NMPs and (ii) the variations in the EoS of symmetric nuclear matter, symmetry energy, and the neutron-proton asymmetry in such a way that the neutron star matter EoS remain almost unaffected.

Glassy quantum nuclear pasta in neutron star crusts

Abstract: Nuclear pasta is an exotic form of nuclear matter that occurs in the crust of neutron stars below saturation density. Various exotic nuclear structures with cylindrical, planar, and more complicated geometries evolve from the Coulomb lattice of nuclei immersed in a fluid of neutrons. Such structures in the crust should not only affect how a neutron star cools and rotates but also the height of the ``mountains'' that the crust can sustain---potentially detectable as persistent sources of gravitational waves. By performing a large set of quantum calculations the authors show that the energy landscape of nuclear pasta consists of multiple local minima with very similar energies. Hence, at the characteristic temperatures nuclear pasta becomes a self-organized glassy amorphous solid similar to soft-matter systems found on Earth.

Asymmetric Nuclear Matter in Relativistic Mean-field Models with Isoscalar- and Isovector-meson Mixing

Abstract: Using the relativistic mean-field model with nonlinear couplings between the isoscalar and isovector mesons, we study the properties of isospin-asymmetric nuclear matter. Not only the vector mixing, ω μ ω μ ρ ν ρ ν , but also the quartic interaction due to the scalar mesons, σ 2 δ 2, is taken into account to investigate the density dependence of nuclear symmetry energy, E sym, and the neutron star properties. It is found that the δ meson increases E sym at high densities, whereas the σ–δ mixing makes E sym soft above the saturation density. Furthermore, the δ meson and its mixing have a large influence on the radius and tidal deformability of a neutron star. In particular, the σ–δ mixing reduces the neutron star radius; thus, the present calculation can simultaneously reproduce the dimensionless tidal deformabilities of a canonical 1.4 M ⊙ neutron star observed from the binary neutron star merger GW170817 and the compact binary coalescence GW190814.

Can fermion-boson stars reconcile multimessenger observations of compact stars?

Abstract: Mixed fermion-boson stars are stable, horizonless, everywhere regular solutions of the coupled Einstein-(complex, massive) Klein-Gordon-Euler system. While isolated neutron stars and boson stars are uniquely determined by their central energy density, mixed configurations conform an extended parameter space that depends on the combination of the number of fermions and (ultra-light) bosons. The wider possibilities offered by fermion-boson stars could help explain the tension in the measurements of neutron star masses and radii reported in recent multi-messenger observations and nuclear-physics experiments. In this work we construct equilibrium configurations of mixed fermion-boson stars with realistic equations of state for the fermionic component and different percentages of bosonic matter. We show that our solutions are in excellent agreement with multi-messenger data, including gravitational-wave events GW170817 and GW190814 and X-ray pulsars PSR J0030+0451 and PSR J0740+6620, as well as with nuclear physics constraints from the PREX-2 experiment.

The role of baryon structure in neutron stars

Abstract: Understanding the equation of state of dense nuclear matter is a fundamental challenge for nuclear physics. It is especially timely and interesting challenge as we have reached a period where neutron stars, which contain the most dense nuclear matter in the Universe, are now being studied in completely new ways, from gravitational waves to satellite based telescopes. We review the theoretical approaches to calculating this equation of state which involve a change in the structure of the baryons.

Universal trend of charge radii of even-even Ca–Zn nuclei

Abstract: Radii of nuclear charge distributions carry information about the strong and electromagnetic forces acting inside the atomic nucleus. Whereas the global behavior of nuclear charge radii is governed by the bulk properties of nuclear matter, their local trends are affected by quantum motion of proton and neutron nuclear constituents. The measured differential charge radii $\ensuremath{\delta}\ensuremath{\langle}{r}_{c}^{2}\ensuremath{\rangle}$ between neutron numbers $N=28$ and $N=40$ exhibit a universal pattern as a function of $n=N\text{--}28$ that is independent of the atomic number. Here we analyze this remarkable behavior in even-even nuclei from calcium to zinc using two state-of-the-art theories based on quantified nuclear interactions: the ab initio coupled cluster theory and nuclear density functional theory. Both theories reproduce the smooth rise of differential charge radii and their weak dependence on the atomic number. By considering a large set of isotopic chains, we show that this trend can be captured by just two parameters: the slope and curvature of $\ensuremath{\delta}\ensuremath{\langle}{r}_{c}^{2}\ensuremath{\rangle}(n)$. We demonstrate that these parameters show appreciable model dependence, and the statistical analysis indicates that they are not correlated with any single model property, i.e., they are impacted by both bulk nuclear properties as well as shell structure.

The Equation of State of Neutron-Rich Matter at Fourth Order of Chiral Effective Field Theory and the Radius of a Medium-Mass Neutron Star

Abstract: We report neutron star predictions based on our most recent equations of state. These are derived from chiral effective field theory, which allows for a systematic development of nuclear forces, order by order. We utilize high-quality two-nucleon interactions and include all three-nucleon forces up to fourth order in the chiral expansion. Our ab initio predictions are restricted to the domain of applicability of chiral effective field theory. However, stellar matter in the interior of neutron stars can be up to several times denser than normal nuclear matter at saturation, and its composition is essentially unknown. Following established practices, we extend our microscopic predictions to higher densities matching piecewise polytropes. The radius of the average-size neutron star, about 1.4 solar masses, is sensitive to the pressure at normal densities, and thus it is suitable to constrain ab initio theories of the equation of state. For this reason, we focus on the radius of medium-mass stars. We compare our results with other theoretical predictions and recent constraints.

Holographic QCD in the NICER era

Abstract: We analyze families of hybrid equations of state of cold QCD matter, which combine input from gauge/gravity duality and from various ab initio methods for nuclear matter at low density, and predict that all neutron stars are fully hadronic without quark matter cores. We focus on constraints from recent measurements by the NICER telescope on the radius and mass of the millisecond pulsar PSR J0740+6620. These results are found to be consistent with our approach: they set only mild constraints on the hybrid equations of state, and favor the most natural models which are relatively stiff at low density. Adding an upper bound on the maximal mass of neutron stars, as suggested by the analysis of the GW170817 neutron star merger event, tightens the constraints considerably. We discuss updated predictions on observables such as the transition density and latent heat of the nuclear to quark matter transition as well as the masses, radii, and tidal deformabilities of neutron stars.

Nearly model-independent constraints on dense matter equation of state in a Bayesian approach

Abstract: We apply a Bayesian approach to construct a large number of minimally constrained equations of state (EOSs) and study their correlations with a few selected properties of a neutron star (NS). Our set of minimal constraints includes a few basic properties of saturated nuclear matter and low-density pure neutron matter EOS which is obtained from a precise next-to-next-to-next-to-leading-order (${\mathrm{N}}^{3}\mathrm{LO}$) calculation in chiral effective field theory. The tidal deformability and radius of a NS with mass $1--2\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ are found to be strongly correlated with the pressure of $\ensuremath{\beta}$-equilibrated matter at densities higher than the saturation density (${\ensuremath{\rho}}_{0}=0.16\text{ }\text{ }{\mathrm{fm}}^{\ensuremath{-}3}$) in a nearly model-independent manner. These correlations are employed to parametrize the pressure for $\ensuremath{\beta}$-equilibrated matter, around $2{\ensuremath{\rho}}_{0}$, as a function of neutron star mass and the corresponding tidal deformability. The maximum mass of the neutron star is also found to be strongly correlated with the pressure of $\ensuremath{\beta}$-equilibrated matter at densities $\ensuremath{\sim}4.5{\ensuremath{\rho}}_{0}$.

Benchmark calculations of infinite neutron matter with realistic two- and three-nucleon potentials

Abstract: We present the equation of state of infinite neutron matter as obtained from highly-realistic Hamiltonians that include nucleon-nucleon and three-nucleon coordinate-space potentials. We benchmark three independent many-body methods: Brueckner-Bethe-Goldstone (BBG), Fermi hypernetted chain/single-operator chain (FHNC/SOC), and auxiliary-field diffusion Monte Carlo (AFDMC). We find them to provide similar equations of state when the Argonne $v_{18}$ and the Argonne $v_{6}^\prime$ nucleon-nucleon potentials are used in combination with the Urbana IX three-body force. Only at densities larger than about 1.5 the nuclear saturation density ($\rho_0 = 0.16\,\rm{fm}^{-3}$) the FHNC/SOC energies are appreciably lower than the other two approaches. The AFDMC calculations carried out with all of the Norfolk potentials fitted to reproduce the experimental trinucleon ground-state energies and $nd$ doublet scattering length yield unphysically bound neutron matter, associated with the formation of neutron droplets. Including tritium $\beta$-decay in the fitting procedure, as in the second family of Norfolk potentials, mitigates but does not completely resolve this problem. An excellent agreement between the BBG and AFDMC results is found for the subset of Norfolk interactions that do not make neutron-matter collapse, while the FHNC/SOC equations of state are moderately softer.

OUP accepted manuscript

Abstract: Abstract The determination of the equation of state (EOS) for nuclear matter has been one of the biggest problems in nuclear astrophysics, because the EOS is essential for determining the properties of neutron stars. To constrain the density dependence of the nuclear symmetry energy, several nuclear experiments, e.g., reported by the SπRIT and PREX-II Collaborations, have recently been performed. However, since their uncertainties are still large, additional constraints such as astronomical observations are crucial. In addition, it is interesting to see the effect of their reported values on neutron-star properties. In this study, focusing on a relatively lower-density region, we investigate the allowed area of the neutron-star mass and radius relation by assuming the constraints from SπRIT and PREX-II. Each region predicted by these experiments is still consistent with the allowed area constrained by the various astronomical observations. Our results show that terrestrial nuclear experiments must provide further constraints on the EOS for neutron stars, complementing astronomical observations.

Translating Neutron Star Observations to Nuclear Symmetry Energy via Deep Neural Networks

Abstract: One of the most significant challenges involved in efforts to understand the equation of state of dense neutron-rich matter is the uncertain density dependence of the nuclear symmetry energy. Because of its broad impact, pinning down the density dependence of the nuclear symmetry energy has been a longstanding goal of both nuclear physics and astrophysics. Recent observations of neutron stars, in both electromagnetic and gravitational-wave spectra, have already constrained significantly the nuclear symmetry energy at high densities. Training deep neural networks to learn a computationally efficient representation of the mapping between astrophysical observables of neutron stars, such as masses, radii, and tidal deformabilities, and the nuclear symmetry energy allows its density dependence to be determined reliably and accurately. In this work we use a deep learning approach to determine the nuclear symmetry energy as a function of density directly from observational neutron star data. We show for the first time that artificial neural networks can precisely reconstruct the nuclear symmetry energy from a set of available neutron star observables, such as, masses and radii as those measured by, e.g., the NICER mission, or masses and tidal deformabilities as measured by the LIGO/VIRGO/KAGRA gravitational-wave detectors. These results demonstrate the potential of artificial neural networks to reconstruct the symmetry energy, and the equation of state, directly from neutron star observational data, and emphasize the importance of the deep learning approach in the era of Multi-Messenger Astrophysics.

Cooper quartet correlations in infinite symmetric nuclear matter

Abstract: We investigate the quartet correlations in four-component fermionic systems at the thermodynamic limit within a variational many-body theory. The Bardeen-Cooper-Schrieffer (BCS)--type variational wave function is extended to the systems with the coexistence of pair and quartet correlations at zero temperature. Special attention is paid to the application of the present framework to an $\ensuremath{\alpha}$-particle condensation in symmetric nuclear matter, where the coexistence of deuteron and $\ensuremath{\alpha}$ condensations is anticipated. We also discuss how physical properties, such as quasiparticle dispersion, can be modified by the pair and quartet correlations and show a hierarchical structure of in-medium cluster formations in infinite nuclear matter. The present results may also contribute to the interdisciplinary understanding of fermionic condensations beyond the BCS paradigm in many-body systems.

Dark matter admixed neutron star properties in light of gravitational wave observations: A two fluid approach

Abstract: We consider the effect of density dependent dark matter on the neutron star mass, radius, and tidal deformability. Nuclear matter (normal matter) as well as the fermionic dark matter sector is considered in a mean field model. We adopt the two fluid formalism to investigate the effect of dark matter on the neutron star properties. In the two fluid picture, there is no direct interaction between the dark matter and the nuclear matter. Rather these two sectors interact only through gravitational interaction. The nuclear matter sector is described by the $\ensuremath{\sigma}\ensuremath{-}\ensuremath{\omega}\ensuremath{-}\ensuremath{\rho}$ meson interaction in the ``FSU2R'' parametrization. In the dark matter sector, we use the Bayesian parameter optimization technique to fix the unknown parameters in the dark matter equation of state. In the two fluid picture, we solve the coupled Tolman-Oppenheimer-Volkoff (TOV) equations to obtain the mass and radius of dark matter admixed neutron stars (DANSs). We also estimate the effect of the density dependent dark matter sector on the tidal deformability of dark matter admixed neutron stars (DANSs).

Symmetry energy effects on the properties of hybrid stars

Abstract: Symmetry energy is an important part of the equation of state of isospin asymmetry matter. However, the huge uncertainties of symmetry energy remain at suprasaturation densities, where the phase transitions of strongly interacting matter and the quark matter symmetry energy are likely to be taken into account. In this work, we investigate the properties of symmetry energy by using a hybrid star with the hadron-quark phase transition. The interaction among strange quark matter (SQM) in hybrid stars is based on a 3-flavor NJL model with different vector and isovector channels, while the equation of state (EOS) of the nuclear matter is obtained by considering the ImMDI-ST interaction by varying the parameters x, y, and z. Our results indicate that the various parameters and coupling constants of the interactions from the ImMDI-ST and NJL model can lead to widely different trends for the symmetry energy in the hadron-quark mixed phase and the different onsets of the hadron-quark phase transition. In addition, it has been found that the radii and tidal deformabilities of hybrid stars constrain mostly the density dependence of symmetry energy while the observed maximum masses of hybrid stars constrain mostly the EOS of symmetric nuclear and quark matter.

Properties of the neutron star crust: Quantifying and correlating uncertainties with improved nuclear physics

Abstract: A compressible liquid-drop model (CLDM) is used to correlate uncertainties associated with the properties of the neutron star (NS) crust with theoretical estimates of the uncertainties associated with the equation of state (EOS) of homogeneous neutron and nuclear matter. For the latter, we employ recent calculations based on Hamiltonians constructed using chiral effective-field theory ($\ensuremath{\chi}\mathrm{EFT}$). Fits to experimental nuclear masses are employed to constrain the CLDM further, and we find that they disfavor some of the $\ensuremath{\chi}\mathrm{EFT}$ Hamiltonians. The CLDM allows us to study the complex interplay between bulk, surface, curvature, and Coulomb contributions, and their impact on the NS crust. It also reveals how the curvature energy alters the correlation between the surface energy and the bulk symmetry energy. Our analysis quantifies how the uncertainties associated with the EOS of homogeneous matter implies significant uncertainties for the composition of the crust, its proton fraction, and the volume fraction occupied by nuclei. We find that the finite-size effects impact the crust composition but have a negligible effect on the net isospin asymmetry of matter. The isospin asymmetry is largely determined by the bulk properties and the isospin dependence of the surface energy. The most significant uncertainties associated with matter properties in the densest regions of the crust, the precise location of the crust-core transition, are found to be strongly correlated with uncertainties associated with the Hamiltonians. By adopting a unified model to describe the crust and the core of NSs, we tighten the correlation between their global properties such as their mass-radius relationship, moment of inertia, crust thickness, and tidal deformability with uncertainties associated with the nuclear Hamiltonians.

Asymmetric nuclear matter and neutron star properties in relativistic <i>ab initio</i> theory in the full Dirac space

Abstract: The long-standing controversy about the isospin dependence of the effective Dirac mass in ab initio calculations of asymmetric nuclear matter is clarified by solving the relativistic Brueckner-Hartree-Fock equations in the full Dirac space. The symmetry energy and its slope parameter at the saturation density are $E_{\text{sym}}(\rho_0)=33.1$ MeV and $L=65.2$ MeV, in agreement with empirical and experimental values. Further applications predict the neutron star radius $R_{1.4M_\odot}\approx 12$ km and the maximum mass of a neutron star $M_{\text{max}}\leq 2.4M_\odot$.

Pre-Equilibrium Clustering in Production of Spectator Fragments in Collisions of Relativistic Nuclei

Abstract: An algorithm of pre-equilibrium clustering of spectator matter based on the construction of the minimum spanning tree (MST) is presented. The algorithm was implemented in the Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model designed to study the characteristics of spectator matter in collisions of relativistic nuclei. Due to accounting for the pre-equilibrium clusters in modelling 208Pb–208Pb collisions at the LHC, the agreement of simulation results with experimental data on the average multiplicities of spectator nucleons was improved. The results of the AAMCC-MST were compared with experimental data on the interactions of 197Au nuclei in nuclear photoemulsion. Comparison of the yields of spectator nuclei calculated for 16O–16O collisions with the yields measured in interactions of 16O with light nuclei of photoemulsion made it possible to estimate the effect of MST-clustering in small nuclear systems.

Confronting a set of Skyrme and $$\chi _{EFT}$$ χ EFT predictions for the crust of ne

Abstract: With the improved accuracy of neutron star observational data, it is necessary to derive new equation of state where the crust and the core are consistently calculated within a unified approach. For this purpose we describe non-uniform matter in the crust of neutron stars employing a compressible liquid-drop model, where the bulk and the neutron fluid terms are given from the same model as the one describing uniform matter present in the core. We then generate a set of fifteen unified equations of state for cold catalyzed neutron stars built on realistic modelings of the nuclear interaction, which belongs to two main groups: the first one derives from the phenomenological Skyrme interaction and the second one from $\chi_{EFT}$ Hamiltonians. The confrontation of these model predictions allows us to investigate the model dependence for the crust properties, and in particular the effect of neutron matter at low density. The new set of unified equations of state is available at the CompOSE repository.

Exploring terra incognita in the phase diagram of strongly interacting matter—experiments at FAIR and NICA

Abstract: The fundamental properties of dense nuclear matter, as it exists in the core of massive stellar objects, are still largely unknown. The investigation of the high-density equation of state (EOS), which determines mass and radii of neutron stars and the dynamics of neutron star mergers, is in the focus of astronomical observations and of laboratory experiments with heavy-ion collisions. Moreover, the microscopic degrees-of-freedom of strongly interacting matter at high baryon densities are also unknown. While Quantum-Chromo-Dynamics (QCD) calculations on the lattice find a smooth chiral crossover between hadronic matter and the quark-gluon plasma for high temperatures at zero baryon chemical potential, effective models predict a 1st order chiral transition with a critical endpoint for matter at large baryon chemical potentials. Up to date, experimental data both on the high-density EOS and on a possible phase transition in dense baryonic matter are very scarce. In order to explore this terra incognita, dedicated experimental programs are planned at future heavy-ion research centres: the CBM experiment at FAIR, and the MPD and BM@N experiments at NICA. The research programs and the layout of these experiments will be presented. The future results of these laboratory experiments will complement astronomical observations concerning the EOS, and, in addition, will shed light on the microscopic degrees of freedom of QCD matter at neutron star core densities.

Impact of fragment formation on shear viscosity in the nuclear liquid-gas phase transition region

Abstract: Within the improved quantum molecular dynamic (ImQMD) model we follow the evolution of nuclear matter for planar Couette flow in a periodic box. We focus on the region of liquid-gas phase transition and extract the shear viscosity coefficient from the local stress tensor, directly following viscosity definition. By switching on and off the mean field and thus inducing the phase transition, we are able to observe the impact of clumping in the phase-transition region onto the viscosity.