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Bao-An Li

Publications and source records attributed to Bao-An Li.

At least 181 records · Page 10Linked to original sources
arXiv2009

Effects of medium-induced $ρ-ω$ meson mixing on the equation of state in isospin-asymmetric nuclear matter

We reexamine effects of the $ρ-ω$ meson mixing mediated by nucleon polarizations on the symmetry energy in isospin-asymmetric nuclear matter. Taking into account the rearrangement term neglected in previous studies by others, we evaluate the $ρ-ω$ mixing angle in a novel way within the Relativistic Mean-Field Models with and without chiral limits. It is found that the symmetry energy is significantly softened at high densities contrary to the finding in earlier studies. As the first step of going beyond the lowest-order calculations, we also solve the RPA equation for the $ρ-ω$ mixing. In this case, it is found that the symmetry energy is not only significantly softened by the $ρ-ω$ mixing at supra-saturation densities, similar to the lowest-order $ρ-ω$ mixing, but interestingly also softened at subsaturation densities. In addition, the softening of the symmetry energy at subsaturation densities can be partly suppressed by the nonlinear self-interaction of the $σ$ meson.

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arXiv2009

A systematic study of the $π^-/π^+$ ratio in heavy-ion collisions with the same neutron/proton ratio but different masses

A systematic study of the $π^-/π^+$ ratio in heavy-ion collisions with the same neutron/proton ratio but different masses can help single out effects of the nuclear mean field on pion production. Based on simulations using the IBUU04 transport model, it is found that the $π^-$ / $π^+$ ratio in head-on collisions of $^{48}$Ca+$^{48}$Ca, $^{124}$Sn +$^{124}$Sn and $^{197}$Au+$^{197}$Au at beam energies from 0.25 to 0.6 \agev increases with increasing the system size or decreasing the beam energies. A comprehensive analysis of the dynamical isospin fractionation and the \rpi ratio as well as their time evolution and spatial distributions demonstrates clearly that the \rpi ratio is an effective probe of the high-density behavior of the nuclear symmetry energy.

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arXiv2009

Imprints of the nuclear symmetry energy on gravitational waves from the axial w-modes of neutron stars

The eigen-frequencies of the axial w-modes of oscillating neutron stars are studied using the continued fraction method with an Equation of State (EOS) partially constrained by the recent terrestrial nuclear laboratory data. It is shown that the density dependence of the nuclear symmetry energy $E_{sym}(ρ)$ affects significantly both the frequencies and the damping times of these modes. Besides confirming the previously found universal behavior of the mass-scaled eigen-frequencies as functions of the compactness of neutron stars, we explored several alternative universal scaling functions. Moreover, the $w_{II}$-mode is found to exist only for neutron stars having a compactness of $M/R\geq 0.1078$ independent of the EOS used.

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arXiv2009

Higher-order effects on the incompressibility of isospin asymmetric nuclear matter

Analytical expressions for the saturation density as well as the binding energy and incompressibility at the saturation density of asymmetric nuclear matter are given exactly up to 4th-order in the isospin asymmetry delta =(rho_n - rho_p)/rho using 11 characteristic parameters defined at the normal nuclear density rho_0. Using an isospin- and momentum-dependent modified Gogny (MDI) interaction and the SHF approach with 63 popular Skyrme interactions, we have systematically studied the isospin dependence of the saturation properties of asymmetric nuclear matter, particularly the incompressibility $K_{sat}(δ)=K_{0}+K_{sat,2}δ^{2}+K_{sat,4}δ^{4}+O(δ^{6})$ at the saturation density. Our results show that the magnitude of the high-order $K_{sat,4}$ parameter is generally small compared to that of the $K_{\sat,2}$ parameter. The latter essentially characterizes the isospin dependence of the incompressibility at the saturation density and can be expressed as $K_{sat,2}=K_{sym}-6L-\frac{J_{0}}{K_{0}}L$, Furthermore, we have constructed a phenomenological modified Skyrme-like (MSL) model which can reasonably describe the general properties of symmetric nuclear matter and the symmetry energy predicted by both the MDI model and the SHF approach. The results indicate that the high-order $J_{0}$ contribution to $K_{sat,2}$ generally cannot be neglected. In addition, it is found that there exists a nicely linear correlation between $K_{sym}$ and $L$ as well as between $J_{0}/K_{0}$ and $K_{0}$. These correlations together with the empirical constraints on $K_{0}$, $L$, $E_{sym}(ρ_{0})$ and the nucleon effective mass lead to an estimate of $K_{sat,2}=-370\pm 120$ MeV.

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arXiv2009

Nuclear constraints on properties of neutron star crusts

The transition density $ρ_{t}$ and pressure $P_{t}$ at the inner edge separating the liquid core from the solid crust of neutron stars are systematically studied using a modified Gogny (MDI) and 47 popular Skyrme interactions within well established dynamical and thermodynamical methods. It is shown that the widely used parabolic approximation to the full Equation of State (EOS) of isospin asymmetric nuclear matter may lead to huge errors in estimating the ρ_{t} and P_{t}, especially for stiffer symmetry energy functionals $E_{sym}(ρ)$. The ρ_{t} and P_{t} decrease roughly linearly with the increasing slope parameter $L$ of the $E_{sym}(ρ)$ using the full EOS within both methods. It is also shown that the thickness, fractional mass and moment of inertia of neutron star crust are all very sensitive to the parameter $L$ through the $ρ_{t}$. Moreover, it is shown that the $E_{sym}(ρ)$ constrained in the same sub-saturation density range as the neutron star crust by the isospin diffusion data in heavy-ion collisions at intermediate energies limits the transition density and pressure to 0.040$ fm^-3}< ρ_{t} < 0.065$ fm^-3 and 0.01 MeV/fm^3 < P_{t} < 0.26$ MeV/fm^3, respectively. These constrained values for the transition density and pressure are significantly lower than their fiducial values currently used in the literature. Furthermore, the mass-radius relation and several other properties closely related to the neutron star crust are studied by using the MDI interaction. It is found that the newly constrained $ρ_t$ and $P_t$ together with the earlier estimate of $ΔI/I>0.014$ for the crustal fraction of the moment of inertia of the Vela pulsar impose a stringent constraint of R>= 4.7+4.0M/M_sun km for the radius $R$ and mass $M$ of neutron stars.

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arXiv2009

Locating the inner edge of neutron star crust using terrestrial nuclear laboratory data

Within both dynamical and thermodynamical approaches using the equation of state for neutron-rich nuclear matter constrained by the recent isospin diffusion data from heavy-ion reactions in the same sub-saturation density range as the neutron star crust, the density and pressure at the inner edge separating the liquid core from the solid crust of neutron stars are determined to be 0.040 fm$^{-3}$ $\leq ρ_{t}\leq 0.065$ fm$^{-3}$ and 0.01 MeV/fm$^{3}$ $\leq P_{t}\leq 0.26$ MeV/fm$^{3}$, respectively. These together with the observed minimum crustal fraction of the total moment of inertia allow us to set a new limit for the radius of the Vela pulsar significantly different from the previous estimate. It is further shown that the widely used parabolic approximation to the equation of state of asymmetric nuclear matter leads systematically to significantly higher core-crust transition densities and pressures, especially with stiffer symmetry energy functionals.

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arXiv2009

Constraining the EOS of neutron-rich nuclear matter and properties of neutron stars with heavy-ion reactions

Heavy-ion reactions especially those induced by radioactive beams provide useful information about the density dependence of the nuclear symmetry energy, thus the Equation of State of neutron-rich nuclear matter, relevant for many astrophysical studies. The latest developments in constraining the symmetry energy at both sub- and supra-saturation densities from analyses of the isopsin diffusion and the $π^-/π^+$ ratio in heavy-ion collisions using the IBUU04 transport model are discussed. Astrophysical ramifications of the partially constrained symmetry energy on properties of neutron star crusts, gravitational waves emitted by deformed pulsars and the w-mode oscillations of neutron stars are presented briefly.

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arXiv2009

Circumstantial evidence for a soft nuclear symmetry energy at supra-saturation densities

Within an isospin- and momentum-dependent hadronic transport model it is shown that the recent FOPI data on the $π^-/π^+$ ratio in central heavy-ion collisions at SIS/GSI energies (Willy Reisdorf {\it et al.}, NPA {\bf 781}, 459 (2007)) provide circumstantial evidence suggesting a rather soft nuclear symmetry energy \esym at $ρ\geq 2ρ_0$ compared to the Akmal-Pandharipande-Ravenhall prediction. Some astrophysical implications and the need for further experimental confirmations are discussed.

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arXiv2008

Chiral condensate in nuclear matter with vacuum corrections

Within the relativistic Hartree approach using a Lagrangian with density-dependent parameters respecting the chiral limit, it is found that the vacuum corrections from the nucleon Dirac sea soften the equation of state and favor the chiral symmetry restoration at high densities.

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arXiv2008

Nuclear constraints on gravitational waves from rapidly rotating neutron stars

Gravitational waves are tiny disturbances in space-time and are a fundamental, although not yet directly confirmed, prediction of General Relativity. Rapidly rotating neutron stars are one of the possible sources of gravitational radiation dependent upon pulsar's rotational frequency, details of the equation of state of stellar matter, and distance to detector. Applying an equation of state with symmetry energy constrained by recent nuclear laboratory data, we set an upper limit on the strain-amplitude of gravitational waves emitted by rapidly rotating neutron stars.

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arXiv2008

Nuclear limits on gravitational waves from elliptically deformed pulsars

Gravitational radiation is a fundamental prediction of General Relativity. Elliptically deformed pulsars are among the possible sources emitting gravitational waves (GWs) with a strain-amplitude dependent upon the star's quadrupole moment, rotational frequency, and distance from the detector. We show that the gravitational wave strain amplitude $h_0$ depends strongly on the equation of state of neutron-rich stellar matter. Applying an equation of state with symmetry energy constrained by recent nuclear laboratory data, we set an upper limit on the strain-amplitude of GWs produced by elliptically deformed pulsars. Depending on details of the EOS, for several millisecond pulsars at distances $0.18kpc$ to $0.35kpc$ from Earth, the {\it maximal} $h_0$ is found to be in the range of $\sim[0.4-1.5]\times 10^{-24}$. This prediction serves as the first {\it direct} nuclear constraint on the gravitational radiation. Its implications are discussed.

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arXiv2008

Constraining the density dependence of nuclear symmetry energy with heavy-ion reactions and its astrophysical impact

Recent analyses of several isospin effects in heavy-ion reactions have allowed us to constrain the density dependence of nuclear symmetry energy at sub-saturation densities within a narrow range. Combined with constraints on the Equation of State (EOS) of symmetric nuclear matter obtained previously from analyzing the elliptic flow in relativistic heavy-ion collisions, the EOS of neutron-rich nuclear matter is thus partially constrained. Here we report effects of the partially constrained EOS of neutron-rich nuclear matter on the mass-radius correlation, moment of inertia, elliptical deformation and gravitational radiation of (rapidly) rotating neutron stars.

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arXiv2008

Nuclear constraints on the momenta of inertia of neutron stars

Properties and structure of neutron stars are determined by the equation of state (EOS) of neutron-rich stellar matter. While the collective flow and particle production in relativistic heavy-ion collisions have constrained tightly the EOS of symmetric nuclear matter up to about five times the normal nuclear matter density, the more recent experimental data on isospin-diffusion and isoscaling in heavy-ion collisions at intermediate energies have constrained considerably the density dependence of the nuclear symmetry energy at subsaturation densities. Although there are still many uncertainties and challenges to pin down completely the EOS of neutron-rich nuclear matter, the heavy-ion reaction experiments in terrestrial laboratories have limited the EOS of neutron-rich nuclear matter in a range much narrower than that spanned by various EOSs currently used in astrophysical studies in the literature. These nuclear physics constraints could thus provide more reliable information about properties of neutron stars. Within well established formalisms using the nuclear constrained EOSs we study the momenta of inertia of neutron stars. We put the special emphasis on the component A of the extremely relativistic double neutron star system PSR J0737-3039. Its moment of inertia is found to be between 1.30 and 1.63 $(\times10^{45}g$ $cm^2)$. Moreover, the transition density at the crust-core boundary is shown to be in the narrow range of $ρ_t=[0.091-0.093](fm^{-3})$.

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arXiv2008

Recent Progress and New Challenges in Isospin Physics with Heavy-Ion Reactions

The ultimate goal of studying isospin physics via heavy-ion reactions with neutron-rich, stable and/or radioactive nuclei is to explore the isospin dependence of in-medium nuclear effective interactions and the equation of state of neutron-rich nuclear matter, particularly the isospin-dependent term in the equation of state, i.e., the density dependence of the symmetry energy. Because of its great importance for understanding many phenomena in both nuclear physics and astrophysics, the study of the density dependence of the nuclear symmetry energy has been the main focus of the intermediate-energy heavy-ion physics community during the last decade, and significant progress has been achieved both experimentally and theoretically. In particular, a number of phenomena or observables have been identified as sensitive probes to the density dependence of the nuclear symmetry energy. Experimental studies have confirmed some of these interesting isospin-dependent effects and allowed us to constrain relatively stringently the symmetry energy at sub-saturation densities. The impacts of this constrained density dependence of the symmetry energy on the properties of neutron stars have also been studied, and they were found to be very useful for the astrophysical community. With new opportunities provided by the various radioactive beam facilities being constructed around the world, the study of isospin physics is expected to remain one of the forefront research areas in nuclear physics. In this report, we review the major progress achieved during the last decade in isospin physics with heavy ion reactions and discuss future challenges to the most important issues in this field.

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arXiv2008

Neutron-proton bremsstrahlung from intermediate energy heavy-ion reactions as a probe of the nuclear symmetry energy?

Hard photons from neutron-proton bremsstrahlung in intermediate energy heavy-ion reactions are examined as a potential probe of the nuclear symmetry energy within a transport model. Effects of the symmetry energy on the yields and spectra of hard photons are found to be generally smaller than those due to the currently existing uncertainties of both the in-medium nucleon-nucleon cross sections and the photon production probability in the elementary process $pn\to pnγ$. Very interestingly, nevertheless, the ratio of hard photon spectra $R_{1/2}(γ)$ from two reactions using isotopes of the same element is not only approximately independent of these uncertainties but also quite sensitive to the symmetry energy. For the head-on reactions of $^{132}Sn+^{124}Sn$ and $^{112}Sn+^{112}Sn$ at $E_{beam}/A=50$ MeV, for example, the $R_{1/2}(γ)$ displays a rise up to 15% when the symmetry energy is reduced by about 20% at $ρ=1.3ρ_0$ which is the maximum density reached in these reactions.

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arXiv2007

Constraining properties of rapidly rotating neutron stars using data from heavy-ion collisions

Properties, structure, and thermal evolution of neutron stars are determined by the equation of state of stellar matter. Recent data on isospin-diffusion and isoscaling in heavy-ion collisions at intermediate energies as well as the size of neutron skin in $^{208}Pb$ have constrained considerably the density dependence of the nuclear symmetry energy and, in turn, the equation of state of neutron-rich nucleonic matter. These constraints could provide useful information about the global properties of rapidly rotating neutron stars. Models of rapidly rotating neutron stars are constructed applying several nucleonic equations of state. Particular emphasis is placed on configurations rotating rigidly at $716$ and $1122Hz$. The range of allowed hydrostatic equilibrium solutions is determined and tested for stability. The effect of rotation on the internal composition and thermal properties of neutron stars is also examined. At a given rotational frequency, each equation of state yields a range of possible neutron stars configurations restricted by the Keplerian (mass-shedding) limit, corresponding to the maximal circumferential radius, and the limit due to the onset of instabilities with respect to axial-symmetric perturbations, corresponding to the minimal equatorial radius of a stable neutron star model. We show that the mass of a neutron star rotating uniformly at $1122Hz$ is between $1.7$ and $2.1M_{\sun}$. Central stellar density and proton fraction decrease with increasing rotational frequency with respect to static models, and depending on the exact stellar mass and angular velocity, can drop below the Direct Urca threshold thus closing the fast cooling channel.

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arXiv2007

Determining the density dependence of the nuclear symmetry energy using heavy-ion reactions

We review recent progress in the determination of the subsaturation density behavior of the nuclear symmetry energy from heavy-ion collisions as well as the theoretical progress in probing the high density behavior of the symmetry energy in heavy-ion reactions induced by high energy radioactive beams. We further discuss the implications of these results for the nuclear effective interactions and the neutron skin thickness of heavy nuclei.

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