arXiv subjects

Bao-An Li

Publications and source records attributed to Bao-An Li.

At least 145 records · Page 8Linked to original sources

arXiv2012

Constraints on the symmetry energy from neutron star observations

The modeling of many neutron star observables incorporates the microphysics of both the stellar crust and core, which is tied intimately to the properties of the nuclear matter equation of state (EoS). We explore the predictions of such models over the range of experimentally constrained nuclear matter parameters, focusing on the slope of the symmetry energy at nuclear saturation density $L$. We use a consistent model of the composition and EoS of neutron star crust and core matter to model the binding energy of pulsar B of the double pulsar system J0737-3039, the frequencies of torsional oscillations of the neutron star crust and the instability region for r-modes in the neutron star core damped by electron-electron viscosity at the crust-core interface. By confronting these models with observations, we illustrate the potential of astrophysical observables to offer constraints on poorly known nuclear matter parameters complementary to terrestrial experiments, and demonstrate that our models consistently predict $L<70$ MeV.

astro-ph.HE↗

arXiv2012

Probing Nuclear Symmetry Energy and its Imprints on Properties of Nuclei, Nuclear Reactions, Neutron Stars and Gravitational Waves

Significant progress has been made in recent years in constraining nuclear symmetry energy at and below the saturation density of nuclear matter using data from both terrestrial nuclear experiments and astrophysical observations. However, many interesting questions remain to be studied especially at supra-saturation densities. In this lecture note, after a brief summary of the currently available constraints on nuclear symmetry energy near the saturation density we first discuss the relationship between the symmetry energy and the isopin and momentum dependence of the single-nucleon potential in isospin-asymmetric nuclear medium. We then discuss several open issues regarding effects of the tensor force induced neutron-proton short-range correlation (SRC) on nuclear symmetry energy. Finally, as an example of the impacts of nuclear symmetry energy on properties of neutron stars and gravitational waves, we illustrate effects of the high-density symmetry energy on the tidal polarizability of neutron stars in coalescing binaries.

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arXiv2012

Disentangling effects of collision geometry and symmetry energy in U+U collisions

Effects of the collision geometry on experimental observables that are known to be sensitive to the high-density behavior of nuclear symmetry energy are examined in U+U collisions at 0.52 GeV/nucleon using an isospin- and momentum-dependent interaction within the framework of IBUU transport model. It is found that the neutron-proton differential flow in tip-tip collisions and the difference of neutron and proton elliptic flow in body-body collisions are more sensitive to the symmetry energy at supra-saturation densities compared with collisions of spherical nuclei of the same masses. In addition, the n/p ratio of pre-equilibrium nucleons is found to be slightly more sensitive to the symmetry energy in tip-tip collisions, and the collision geometry affects the π^-/π^+ ratio significantly.

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arXiv2012

Constraining the High-Density Behavior of Nuclear Symmetry Energy with the Tidal Polarizability of Neutron Stars

Using a set of model equations of state satisfying the latest constraints from both terrestrial nuclear experiments and astrophysical observations as well as state-of-the-art nuclear many-body calculations of the pure neutron matter equation of state, the tidal polarizability of canonical neutron stars in coalescing binaries is found to be a very sensitive probe of the high-density behavior of nuclear symmetry energy which is among the most uncertain properties of dense neutron-rich nucleonic matter. Moreover, it changes less than $\pm 10%$ by varying various properties of symmetric nuclear matter and symmetry energy around the saturation density within their respective ranges of remaining uncertainty.

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arXiv2012

Pure Neutron Matter Constraints and Nuclear Symmetry Energy

In this review, we will discuss the results of our recent work to study the general optimization of the pure isovector parameters of the popular relativistic mean-field (RMF) and Skyrme-Hartree-Fock (SHF) nuclear energy-density functionals (EDFs), using constraints on the pure neutron matter (PNM) equation of state (EoS) from recent {\sl ab initio} calculations. By using RMF and SHF parameterizations that give equivalent predictions for ground-state properties of doubly magic nuclei and properties of symmetric nuclear matter (SNM) and PNM, we found that such optimization leads to broadly consistent symmetry energy $J$ and its slope parameter $L$ at saturation density within a tight range of $σ(J) < 2$ MeV and $σ(L) < 6$ MeV. We demonstrate that a clear model dependence shows up (a) in the curvature parameter of the symmetry energy $K_{\rm sym}$, (b) the symmetry energy at supra-saturation densities, and (c) the radius of neutron stars.

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arXiv2012

Non-Newtonian gravity in finite nuclei

In this talk, we report our recent study of constraining the non-Newtonian gravity at femtometer scale. We incorporate the Yukawa-type non-Newtonian gravitational potential consistently to the Skyrme functional form using the exact treatment for the direct contribution and density-matrix expansion method for the exchange contribution. The effects from the non-Newtonian potential on finite nuclei properties are then studied together with a well-tested Skyrme force. Assuming that the framework without non-Newtonian gravity can explain the binding energies and charge radii of medium to heavy nuclei within 2% error, we set an upper limit for the strength of the non-Newtonian gravitational potential at femtometer scale.

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arXiv2012

Generic Constraints on the Relativistic Mean-Field and Skyrme-Hartree-Fock Models from the Pure Neutron Matter Equation of State

We study the nuclear symmetry energy S(rho) and related quantities of nuclear physics and nuclear astrophysics predicted generically by relativistic mean-field (RMF) and Skyrme-Hartree-Fock (SHF) models. We establish a simple prescription for preparing equivalent RMF and SHF parametrizations starting from a minimal set of empirical constraints on symmetric nuclear matter, nuclear binding energy and charge radii, enforcing equivalence of their Lorenz effective masses, and then using the pure neutron matter (PNM) equation of state (EoS) obtained from ab-initio calculations to optimize the pure isovector parameters in the RMF and SHF models. We find the resulting RMF and SHF parametrizations give broadly consistent predictions of the symmetry energy J and its slope parameter L at saturation density within a tight range of <~2 MeV and <~6 MeV respectively, but that clear model dependence shows up in the predictions of higher-order symmetry energy parameters, leading to important differences in (a) the slope of the correlation between J and L from the confidence ellipse, (b) the isospin-dependent part of the incompressibility of nuclear matter K_tau, (c) the symmetry energy at supra-saturation densities, and (d) the predicted neutron star radii. The model dependence can lead to about 1-2 km difference in predictions of the neutron star radius given identical predicted values of J, L and symmetric nuclear matter (SNM) saturation properties. Allowing the full freedom in the effective masses in both models leads to constraints of 30<~J<~31.5 MeV, 35<~L<~60 MeV, -330<~K_tau<~-216 MeV for the RMF model as a whole and 30<~J<~33 MeV, 28<~L<~65 MeV, -420<~K_tau<~-325 MeV for the SHF model as a whole. Notably, given PNM constraints, these results place RMF and SHF models as a whole at odds with some constraints on K_tau inferred from giant monopole resonance and neutron skin experimental results.

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arXiv2012

Large-mass neutron stars with hyperonization

Within a density-dependent relativistic mean-field model using in-medium meson-hadron coupling constants and meson masses, we explore effects of in-medium hyperon interactions on properties of neutron stars. It is found that the hyperonic constituents in large-mass neutron stars can not be simply ruled out, while the recently measured mass of the millisecond pulsar J1614-2230 can constrain significantly the in-medium hyperon interactions. Moreover, effects of nuclear symmetry energy on hyperonization in neutron stars are also discussed.

astro-ph.SR↗

arXiv2012

Delineating effects of tensor force on the density dependence of nuclear symmetry energy

In this talk, we report results of our recent studies to delineate effects of the tensor force on the density dependence of nuclear symmetry energy within phenomenological models. The tensor force active in the isosinglet neutron-proton interaction channel leads to appreciable depletion/population of nucleons below/above the Fermi surface in the single-nucleon momentum distribution in cold symmetric nuclear matter (SNM). We found that as a consequence of the high momentum tail in SNM the kinetic part of the symmetry energy $E^{kin}_{sym}(ρ)$ is significantly below the well-known Fermi gas model prediction of approximately $12.5 (ρ/ρ_0)^{2/3}$. With about 15% nucleons in the high momentum tail as indicated by the recent experiments at J-Lab by the CLAS Collaboration, the $E^{kin}_{sym}(ρ)$ is negligibly small. It even becomes negative when more nucleons are in the high momentum tail in SNM. These features have recently been confirmed by three independent studies based on the state-of-the-art microscopic nuclear many-body theories. In addition, we also estimate the second-order tensor force contribution to the potential part of the symmetry energy. Implications of these findings in extracting information about nuclear symmetry energy from nuclear reactions are discussed briefly.

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arXiv2012

Single-nucleon potential decomposition of the nuclear symmetry energy

The nuclear symmetry energy $E_{sym}(ρ)$ and its density slope $L(ρ)$ can be decomposed analytically in terms of the single-nucleon potential in isospin asymmetric nuclear matter. Using three popular nuclear effective interaction models which have been extensively used in nuclear structure and reaction studies, namely, the isospin and momentum dependent MDI interaction model, the Skyrme Hartree-Fock approach and the Gogny Hartree-Fock approach, we analyze the contribution of different terms in the single-nucleon potential to the $E_{sym}(ρ)$ and $L(ρ)$. Our results show that the observed different density behaviors of $E_{sym}(ρ)$ for different interactions are essentially due to the variation of the symmetry potential $U_{sym,1}(ρ,k)$. Furthermore, we find that the contribution of the second-order symmetry potential $U_{sym,2}(ρ,k)$ to the $L(ρ)$ generally cannot be neglected. Moreover, our results demonstrate that the magnitude of the $U_{sym,2}(ρ,k)$ is generally comparable with that of $U_{sym,1}(ρ,k)$, indicating the second-order symmetry potential $U_{sym,2}(ρ,k)$ may have significant corrections to the widely used Lane approximation to the single-nucleon potential in extremely neutron(proton)-rich nuclear matter.

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arXiv2012

Sensitivity of the neutron star r-mode instability window to the density dependence of the nuclear symmetry energy

Using a simple model of a neutron star with a perfectly rigid crust constructed and a set of crust and core equations of state that span the range of nuclear experimental uncertainty in the density dependence of the symmetry energy from 25 MeV (soft EOS) to 115 MeV (stiff EOS), we calculate the instability window for the onset of the Chandrasekhar-Friedmann-Schutz (CFS) instability in r-mode oscillations for canonical neutron stars (1.4 M_{\odot}) and massive neutron stars (2.0 M_{\odot}). In these models the crust-core transition density, and thus crustal thickness, is calculated consistently with the core equation of state (EOS). For the canonical neutron star, the lower bound of the r-mode instability window is reduced in frequency by \approx150 Hz from the softest to the stiffest symmetry energy used, independent of mass and temperature. The instability window also drops by \approx 100 Hz independent of EOS when the mass is raised from 1.4 M_{\odot} to 2.0 M_{\odot}. Where temperature estimates are available, the observed neutron stars in low mass X-ray binaries (LMXBs) have frequencies below the instability window for the 1.4 M_{\odot} models, while some LMXBs fall within the instability window for 2.0 M_{\odot} stars if the symmetry energy is relatively stiff, indicating that a softer symmetry energy is more consistent with observations within this model. Thus we conclude that smaller values of L help stabilize neutron stars against runaway r-mode oscillations. The critical temperature, below which no star can reach the instability window without exceeding its Kepler frequency, varies by nearly an order of magnitude from soft to stiff symmetry energies. When the crust thickness and core EOS are treated consistently, a thicker crust corresponds to a lower critical temperature, the opposite result to previous studies in which the transition density was independent of the core EOS.

astro-ph.SR↗

arXiv2011

The nuclear symmetry energy, the inner crust, and global neutron star modeling

The structure and composition of the inner crust of neutron stars, as well as global stellar properties such as radius and moment of inertia, have been shown to correlate with parameters characterizing the symmetry energy of nuclear matter such as its magnitude J and density dependence L at saturation density. It is thus mutually beneficial to nuclear physicists and astrophysicists to examine the combined effects of such correlations on potential neutron star observables in the light of recent experimental and theoretical constraints on J, L, and relationships between them. We review some basic correlations between these nuclear and astrophysical observables, and illustrate the impact of recent progress in constraining the J-L parameter space on the composition of the inner crust, crust-core transition density and pressure, and extent of the hypothesized pasta region. We use a simple compressible liquid drop model in conjunction with a simple model of nuclear matter which allows for independent, smooth, variation of the J and L. We extend the model into the core using the same nuclear matter model to explore the effects on global crust and core properties, and on potential observables such as crust oscillation frequencies and mechanically supported crust deformation. Throughout we illustrate the importance of the relationship between J and L implicit in a particular model of nuclear matter to the predictions of neutron star properties.

astro-ph.SR↗

arXiv2011

Upper limits on the observational effects of nuclear pasta in neutron stars

The effects of the existence of exotic nuclear shapes at the bottom of the neutron star inner crust - nuclear `pasta' - on observational phenomena are estimated by comparing the limiting cases that those phases have a vanishing shear modulus and that they have the shear modulus of a crystalline solid . We estimate the effect on torsional crustal vibrations and on the maximum quadrupole ellipticity sustainable by the crust. The crust composition and transition densities are calculated consistently with the global properties, using a liquid drop model with a bulk nuclear equation of state (EoS) which allows a systematic variation of the nuclear symmetry energy. The symmetry energy J and its density dependence L at nuclear saturation density are the dominant nuclear inputs which determine the thickness of the crust, the range of densities at which pasta might appear, as well as global properties such as the radius and moment of inertia. We show the importance of calculating the global neutron star properties on the same footing as the crust EoS, and demonstrate that in the range of experimentally acceptable values of L, the pasta phase can alter the crust frequencies by up to a factor of three, exceeding the effects of superfluidity on the crust modes, and decrease the maximum quadrupole ellipticity sustainable by the crust by up to an order of magnitude. The signature of the pasta phases and the density dependence of the symmetry energy on the potential observables highlights the possibility of constraining the EoS of dense, neutron-rich matter and the properties of the pasta phases using astrophysical observations.

astro-ph.SR↗

arXiv2011

Magnetic effects in heavy-ion collisions at intermediate energies

The time-evolution and space-distribution of internal electromagnetic fields in heavy-ion reactions at beam energies between 200 and 2000 MeV/nucleon are studied within an Isospin-dependent Boltzmann-Uhling-Uhlenbeck transport model IBUU11. While the magnetic field can reach about $7\times 10^{16}$ G which is significantly higher than the estimated surface magnetic field ($\sim 10^{15}$ G) of magnetars, it has almost no effect on nucleon observables as the Lorentz force is normally much weaker than the nuclear force. Very interestingly, however, the magnetic field generated by the projectile-like (target-like) spectator has a strong focusing/diverging effect on positive/negative pions at forward (backward) rapidities. Consequently, the differential $π^-/π^+$ ratio as a function of rapidity is significantly altered by the magnetic field while the total multiplicities of both positive and negative pions remain about the same. At beam energies above about 1 GeV/nucleon, while the integrated ratio of total $π^-$ to $π^+$ multiplicities is not, the differential $π^-/π^+$ ratio is sensitive to the density dependence of nuclear symmetry energy $E_{\rm{sym}}(ρ)$. Our findings suggest that magnetic effects should be carefully considered in future studies of using the differential $π^-/π^+$ ratio as a probe of the $E_{\rm{sym}}(ρ)$ at supra-saturation densities.

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arXiv2011

Short-range tensor interaction and high-density nuclear symmetry energy

Effects of the short-range tensor interaction on the density-dependence of nuclear symmetry energy are examined by applying an approximate expression for the second-order tensor contribution to the symmetry energy derived earlier by G.E. Brown and R. Machleidt. It is found that the uncertainty in the short-range tensor force leads directly to a divergent high-density behavior of the nuclear symmetry energy.

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arXiv2011

Tensor force induced isospin-dependence of short-range nucleon-nucleon correlation and high-density behavior of nuclear symmetry energy

Quantitative information on the tensor force induced isospin-dependence of short-range nucleon-nucleon correlation (SRC) extracted from recent J-Lab experiments was used in constraining the high-momentum tails of single nucleon momentum distributions in both symmetric nuclear matter (SNM) and pure neutron matter (PNM). Its effects on the Equations of State of SNM and PNM as well as the nuclear symmetry energy are investigated. It is found that the tensor force induced isospin-dependence of SRC softens significantly the nuclear symmetry energy especially at supra-saturation densities.

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arXiv2011

Constraining the symmetry energy from the neutron skin thickness of Tin isotopes

We show in the Skyrme-Hartree-Fock approach that unambiguous correlations exist between observables of finite nuclei and nuclear matter properties. Using this correlation analysis to existing data on the neutron skin thickness of Sn isotopes, we find important constraints on the value E_{sym}(rho_0) and density slope L of the nuclear symmetry energy at saturation density. Combining these constraints with those from recent analyses of isospin diffusion and double neutron/proton ratio in heavy ion collisions leads to a value of L=58\pm 18 MeV approximately independent of E_{sym}(ρ_0).

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arXiv2011

Imprints of Nuclear Symmetry Energy on Properties of Neutron Stars

Significant progress has been made in recent years in constraining the density dependence of nuclear symmetry energy using terrestrial nuclear laboratory data. Around and below the nuclear matter saturation density, the experimental constraints start to merge in a relatively narrow region. At supra-saturation densities, there are, however, still large uncertainties. After summarizing the latest experimental constraints on the density dependence of nuclear symmetry energy, we highlight a few recent studies examining imprints of nuclear symmetry energy on the binding energy, energy release during hadron-quark phase transitions as well as the $w$-mode frequency and damping time of gravitational wave emission of neutron stars.

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