arXiv subjects

Lie-Wen Chen

Publications and source records attributed to Lie-Wen Chen.

At least 109 records · Page 6Linked to original sources

arXiv2014

Old neutron stars as probes of isospin-violating dark matter

Isospin-violating dark matter (IVDM), which couples differently with protons and neutrons, provides a promising mechanism to ameliorate the tension among recent direct detection experiments. Assuming DM is non-interacting bosonic asymmetric IVDM, we investigate how the existence of old neutron stars limits the DM-proton scattering cross-section $σ_{\rm p}$, especially the effects of the isospin violating DM-nucleon interactions and the symmetry energy in the equation of state of isospin asymmetric nuclear matter. Our calculations are completely based on general relativity and especially the structure of neutron stars is obtained by solving the Tolman-Oppenheimer-Volkoff equations with nuclear matter equation of state constrained by terrestrial experiments. We find that, by considering the more realistic neutron star model rather than a simple uniform neutron sphere as usual, the $σ_{\rm p}$ bounds from old neutron stars can be varied by more than an order of magnitude depending on the specific values of the DM neutron-to-proton coupling ratio $f_{\rm n}/f_{\rm p}$, and they can be further varied by more than a factor of two depending on the density dependence of the symmetry energy. In particular, we demonstrate that the observed nearby isolated old neutron star PSR B1257+12 can set a very strong limit on $σ_{\rm p}$ for low-mass DM particles ($ \le 20 \, {\rm GeV}$) that reaches a sensitivity beyond the current best limits from direct detection experiments and disfavors the DM interpretation of previously-reported positive experimental results, including the IVDM.

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arXiv2014

Quark magnetar in three-flavor Nambu--Jona-Lasinio model with vector interaction and magnetized gluon potential

We investigate properties of strange quark matter in the framework of SU(3) Nambu--Jona-Lasinio(NJL) model with vector interaction under strong magnetic fields. The effects of vector-isoscalar and vector-isovector interaction on the equation of state of strange quark matter are investigated, and it is found that the equation of state is not sensitive to the vector-isovector interaction, however, a repulsive interaction in the vector-isoscalar channel gives a stiffer equation of state for cold dense quark matter. In the presence of magnetic field, gluons will be magnetized via quark loops, and the contribution from magnetized gluons to the equation of state is also estimated. The sound velocity square is a quantity to measure the hardness or softness of dense quark matter, and in the NJL model without vector interaction at zero magnetic field the sound velocity square is always less than 1/3. It is found that a repulsive vector-isoscalar interaction and a positive pressure contribution from magnetized gluons can enhance the sound velocity square, which can even reach 1. To construct quark magnetars under strong magnetic fields, we consider anisotropic pressures and use a density-dependent magnetic field profile to mimic the magnetic field distribution in a quark star. We also analyze the parameter region for the magnitude of vector-isoscalar interaction and the contribution from magnetized gluons in order to produce 2 solar mass quark magnetars.

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arXiv2014

Constraining the density slope of nuclear symmetry energy at subsaturation densities using electric dipole polarizability in $^{208}$Pb

Nuclear structure observables usually most effectively probe the properties of nuclear matter at subsaturation densities rather than at saturation density. We demonstrate that the electric dipole polarizibility $α_ {\text{D}}$ in $^{208}$Pb is sensitive to both the magnitude $E_{\text{sym}}(ρ_{\text{c}})$ and density slope $L(ρ_{\text{c}})$ of the symmetry energy at a subsaturation cross density $ρ_{\text{c}} = 0.11$ fm$^{-3}$. Using the experimental data of $α_ {\text{D}}$ in $^{208}$Pb from RCNP and the recent accurate constraint of $E_{\text{sym}}(ρ_{\text{c}})$ from the binding energy difference of heavy isotope pairs, we extract a value of $L(ρ_{\text{c}}) = 47.3 \pm 7.8$ MeV. The implication of the present constraint of $L(ρ_{\text{c}})$ to the symmetry energy at saturation density, the neutron skin thickness of $^{208}$Pb and the core-crust transition density in neutron stars is discussed.

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arXiv2014

Quark stars under strong magnetic fields

Within the confined isospin- and density-dependent mass model, we study the properties of strange quark matter (SQM) and quark stars (QSs) under strong magnetic fields. The equation of state of SQM under a constant magnetic field is obtained self-consistently and the pressure perpendicular to the magnetic field is shown to be larger than that parallel to the magnetic field, implying that the properties of magnetized QSs generally depend on both the strength and the orientation of the magnetic fields distributed inside the stars. Using a density-dependent magnetic field profile which is introduced to mimic the magnetic field strength distribution in a star, we study the properties of static spherical QSs by assuming two extreme cases for the magnetic field orientation in the stars, i.e., the radial orientation in which the local magnetic fields are along the radial direction and the transverse orientation in which the local magnetic fields are randomly oriented but perpendicular to the radial direction. Our results indicate that including the magnetic fields with radial (transverse) orientation can significantly decrease (increase) the maximum mass of QSs, demonstrating the importance of the magnetic field orientation inside the magnetized compact stars.

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arXiv2014

Form Factor Effects in the Direct Detection of Isospin-Violating Dark Matter

Isospin-violating dark matter (IVDM) provides a possible mechanism to ameliorate the tension among recent direct detection experiments. For IVDM, we demonstrate that the results of direct detection experiments based on neutron-rich target nuclei may depend strongly on the density dependence of the symmetry energy which is presently largely unknown and controls the neutron skin thickness that reflects the relative difference of neutron and proton form factors in the neutron-rich nuclei. In particular, using the neutron and proton form factors obtained from Skyrme-Hartree-Fock calculations by varying the symmetry energy within the uncertainty region set by the latest model-independent measurement of the neutron skin thickness of $^{208}$Pb from PREX experiment at JLab, we find that, for IVDM with neutron-to-proton coupling ratio fixed to $f_n/f_p=-0.7$, the form factor effect may enhance the sensitivity of Xe-based detectors (e.g., XENON100 and LUX) to the DM-proton cross section by a factor of $3$ in the DM mass region constrained by CMDS-II(Si) and even by more than an order of magnitude for heavy DM with mass larger than $80$ GeV, compared with the results using the empirical Helm form factor. Our results further indicate that the form factor effect can significantly modify the recoil spectrum of Xe-based detectors for heavy IVDM with $f_n/f_p=-0.7$.

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arXiv2014

Breaking the EOS-Gravity Degeneracy with Masses and Pulsating Frequencies of Neutron Stars

A thorough understanding of many astrophysical phenomena associated with compact objects requires reliable knowledge about both the equation of state (EOS)of super-dense nuclear matter and the theory of strong-field gravity simultaneously because of the EOS-gravity degeneracy. Currently, variations of the neutron star (NS) mass-radius correlation from using alternative gravity theories are much larger than those from changing the NS matter EOS within known constraints. At least two independent observables are required to break the EOS-gravity degeneracy. Using model EOSs for hybrid stars and a Yukawa-type non-Newtonian gravity, we investigate both the mass-radius correlation and pulsating frequencies of NSs. While the maximum mass of NSs increases, the frequencies of the $f$, $p_1$, $p_2$, and $w_I$ pulsating modes are found to decrease with the increasing strength of the Yukawa-type non-Newtonian gravity, providing a useful reference for future determination simultaneously of both the strong-field gravity and the supranuclear EOS by combining data of x-ray and gravitational wave emissions of neutron stars.

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arXiv2014

Probing isospin- and momentum-dependent nuclear effective interactions in neutron-rich matter

The single-particle potentials for nucleons and hyperons in neutron-rich matter generally depends on the density and isospin asymmetry of the medium as well as the momentum and isospin of the particle. It further depends on the temperature of the matter if the latter is in thermal equilibrium. We review here the extension of a Gogny-type isospin- and momentum-dependent interaction in several aspects made in recent years and their applications in studying intermediate-energy heavy ion collisions, thermal properties of asymmetric nuclear matter and properties of neutron stars. The importance of the isospin- and momentum-dependence of the single-particle potential, especially the momentum dependence of the isovector potential, is clearly revealed throughout these studies.

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arXiv2013

Probing nuclear symmetry energy at high densities using pion, kaon, eta and photon productions in heavy-ion collisions

The high-density behavior of nuclear symmetry energy is among the most uncertain properties of dense neutron-rich matter. Its accurate determination has significant ramifications in understanding not only the reaction dynamics of heavy-ion reactions especially those induced by radioactive beams but also many interesting phenomena in astrophysics, such as the explosion mechanism of supernova and the properties of neutron stars. The heavy-ion physics community has devoted much effort during the last few years to constrain the high-density symmetry using various probes. In particular, the pion-/pion+ ratio has been most extensively studied both theoretically and experimentally. All models have consistently predicted qualitatively that the pion-/pion+ ratio is a sensitive probe of the high-density symmetry energy especially with beam energies near the pion production threshold. However, the predicted values of the pion-/pion+ ratio are still quite model dependent mostly because of the complexity of modeling pion production and reabsorption dynamics in heavy-ion collisions, leading to currently still controversial conclusions regarding the high-density behavior of nuclear symmetry energy from comparing various model calculations with available experimental data. As more pion-/pion+ data become available and a deeper understanding about the pion dynamics in heavy-ion reactions is obtained, more penetrating probes, such as the kaon+/kaon0 ratio, eta meson and high energy photons are also being investigated or planned at several facilities. Here, we review some of our recent contributions to the community effort of constraining the high-density behavior of nuclear symmetry energy in heavy-ion collisions. In addition, the status of some worldwide experiments for studying the high-density symmetry energy, including the HIRFL-CSR external target experiment (CEE) are briefly introduced.

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arXiv2013

Quark matter symmetry energy and quark stars

We extend the confined-density-dependent-mass (CDDM) model to include isospin dependence of the equivalent quark mass. Within the confined-isospin-density-dependent-mass (CIDDM) model, we study the quark matter symmetry energy, the stability of strange quark matter, and the properties of quark stars. We find that including isospin dependence of the equivalent quark mass can significantly influence the quark matter symmetry energy as well as the properties of strange quark matter and quark stars. While the recently discovered large mass pulsars PSR J1614-2230 and PSR J0348+0432 with masses around $2M_{\odot}$ cannot be quark stars within the CDDM model, they can be well described by quark stars in the CIDDM model. In particular, our results indicate that the two-flavor $u$-$d$ quark matter symmetry energy should be at least about twice that of a free quark gas or normal quark matter within conventional Nambu-Jona-Lasinio model in order to describe the PSR J1614-2230 and PSR J0348+0432 as quark stars.

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arXiv2013

Shear viscosity of neutron-rich nucleonic matter near its liquid-gas phase transition

Within a relaxation time approach using free nucleon-nucleon cross sections modified by the in-medium nucleon masses that are determined from an isospin- and momentum-dependent effective nucleon-nucleon interaction, we investigate the specific shear viscosity ($η/s$) of neutron-rich nucleonic matter near its liquid-gas phase transition. It is found that as the nucleonic matter is heated at fixed pressure or compressed at fixed temperature, its specific shear viscosity shows a valley shape in the temperature or density dependence, with the minimum located at the boundary of the phase transition. Moreover, the value of $η/s$ drops suddenly at the first-order liquid-gas phase transition temperature, reaching as low as $4\sim5$ times the KSS bound of $\hbar/4π$. However, it varies smoothly for the second-order liquid-gas phase transition. Effects of the isospin degree of freedom and the nuclear symmetry energy on the value of $η/s$ are also discussed.

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arXiv2013

Relationship between the symmetry energy and the single-nucleon potential in isospin-asymmetric nucleonic matter

In this contribution, we review the most important physics presented originally in our recent publications. Some new analyses, insights and perspectives are also provided. We showed recently that the symmetry energy $E_{sym}(ρ)$ and its density slope $L(ρ)$ at an arbitrary density $ρ$ can be expressed analytically in terms of the magnitude and momentum dependence of the single-nucleon potentials using the Hugenholtz-Van Hove (HVH) theorem. These relationships provide new insights about the fundamental physics governing the density dependence of nuclear symmetry energy. Using the isospin and momentum (k) dependent MDI interaction as an example, the contribution of different terms in the single-nucleon potential to the $E_{sym}(ρ)$ and $L(ρ)$ are analyzed in detail at different densities. It is shown that the behavior of $E_{sym}(ρ)$ is mainly determined by the first-order symmetry potential $U_{sym,1}(ρ,k)$ of the single-nucleon potential. The density slope $L(ρ)$ depends not only on the first-order symmetry potential $U_{sym,1}(ρ,k)$ but also the second-order one $U_{sym,2}(ρ,k)$. Both the $U_{sym,1}(ρ,k)$ and $U_{sym,2}(ρ,k)$ at normal density $ρ_0$ are constrained by the isospin and momentum dependent nucleon optical potential extracted from the available nucleon-nucleus scattering data. The $U_{sym,2}(ρ,k)$ especially at high density and momentum affects significantly the $L(ρ)$, but it is theoretically poorly understood and currently there is almost no experimental constraints known.

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arXiv2013

Constraining the symmetry energy at subsaturation densities using isotope binding energy difference and neutron skin thickness

We show that the neutron skin thickness $Δr_{np}$ of heavy nuclei is uniquely fixed by the symmetry energy density slope $L(ρ)$ at a subsaturation cross density $ρ_c \approx 0.11$ fm$^{-3}$ rather than at saturation density $ρ_0$, while the binding energy difference $ΔE$ between a heavy isotope pair is essentially determined by the magnitude of the symmetry energy $E_{\text{sym}}(ρ)$ at the same $ρ_c$. Furthermore, we find a value of $L({ρ_c})$ leads to a negative $E_{\text{sym}}({ρ_{0}})$-$L({ρ_{0}})$ correlation while a value of $E_{\text{sym}}({ρ_{c})}$ leads to a positive one. Using data on $Δr_{np}$ of Sn isotopes and $ΔE$ of a number of heavy isotope pairs, we obtain simultaneously $E_{\text{sym}}({ρ_{c})}=26.65\pm0.20$ MeV and $L({ρ_c})= 46.0\pm4.5$ MeV at 95% confidence level, whose extrapolation gives $E_{\text{sym}}({ρ_{0}})=32.3\pm1.0$ MeV and $L({ρ_{0}})=45.2\pm10.0$ MeV. The implication of these new constraints on the $Δr_{np}$ of $^{208}$Pb and the core-crust transition density in neutron stars is discussed.

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arXiv2013

Energy dependence of pion in-medium effects on π^-/π^+ ratio in heavy-ion collisions

Within the framework of a thermal model with its parameters fitted to the results from an isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we study the pion in-medium effect on the charged-pion ratio in heavy-ion collisions at various energies. We find that due to the cancellation between the effects from pion-nucleon s-wave and p-wave interactions in nuclear medium, the π^-/π^+ ratio generally decreases after including the pion in-medium effect. The effect is larger at lower collision energies as a result of narrower pion spectral functions at lower temperatures.

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arXiv2013

Extracting the nuclear symmetry potential and energy from neutron-nucleus scattering data

According to the Hugenholtz-Van Hove theorem, the symmetry energy and its density slope can be decomposed uniquely in terms of the single-nucleon potential in asymmetric nuclear matter which, at normal density, can be constrained by the nucleon optical model potential extracted from analyzing the nucleon-nucleus scattering data. To more accurately extract information on the symmetry energy and its density slope at normal density from neutron-nucleus scattering data, going beyond the well-known Lane potential, we include consistently the second order terms in isospin asymmetry in both the optical model potential and the symmetry energy decomposition. We find that the strength of the second-order symmetry potential $U_{sym,2}$ in asymmetric nuclear matter is significant compared to the first-order one U_{sym,1}$, especially at high nucleon momentum. While the $U_{sym,1}$ at normal density decreases with nucleon momentum, the $U_{sym,2}$ is found to have the opposite behavior. Moreover, we discuss effects of the $U_{sym,1}$ and $U_{sym,2}$ on determining the density dependence of the symmetry energy, and we find that the available neutron-nucleus scattering data favor a softer density dependence of the symmetry energy at normal density with a negligible contribution from the $U_{sym,2}$.

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arXiv2013

Nuclear constraints on non-Newtonian gravity at femtometer scale

Effects of the non-Newtonian gravity on properties of finite nuclei are studied by consistently incorporating both the direct and exchange contribution of the Yukawa potential in the Hartree-Fock approach using a well-tested Skyrme force for the strong interaction. It is shown for the first time that the strength of the Yukawa term in the non-Newtonian gravity is limited to $\log(|α|)<1.75/[λ(\rm fm)]^{0.54} + 33.6$ within the length scale of $λ=1-10$ fm in order for the calculated properties of finite nuclei not to be in conflict with accurate experimental data available.

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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

Recent progress on the determination of the symmetry Energy

We summarize the current status on constraining the density dependence of the symmetry energy from terrestrial laboratory measurements and astrophysical observations. While the value $E_{sym}({ρ_{0}})$ and density slope $L$ of the symmetry energy at saturation density $ρ_{0}$ can vary largely depending on the data or methods, all the existing constraints are essentially consistent with $E_{sym}({ρ_{0}}) = 31 \pm 2$ MeV and $L = 50 \pm 20$ MeV. The determination of the supra-saturation density behavior of the symmetry energy remains a big challenge.

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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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