Kaon properties in (proto-)neutron star matter

The modification of kaon and antikaon properties in the interior of (proto-)neutron stars is investigated using a chiral SU(3) model. The parameters of the model are fitted to nuclear-matter saturation properties, baryon octet vacuum masses, hyperon optical potentials and low-energy kaon-nucleon scattering lengths. We study the kaon/antikaon medium modification and explore the possibility of antikaon condensation in (proto-)neutron star matter at zero as well as finite temperature/entropy and neutrino content. The effect of hyperons on kaon and antikaon optical potentials is also investigated at different stages of the neutron star evolution.     

Emission of droplets of strange quark matter in RHIC

We demonstrate that strangeness separates in the quark-hadron coexistence (even atT=0) and prompt kaon emission results in a strong enhancement of thes-quark abundance in the quark phase during the phase transition to hadron matter. Condensation into stabilized droplets of strange quark matter (“strangelets”) does occur during the phase transition. The so formed cool, compact, long-lived clusters could be experimentally observed by their smallZ/A-ratio. If the late quark matter phase is unstable, it should be observable by the delayed, correlated emission of several hyperons.     

Are strange baryons non-spherical?

We calculate the distribution of baryon number for the ground state hyperons. Strange baryons are described as bound kaon—skyrmion systems. A striking consequence of the model is the non-sphericity of baryonic density for strange baryons.     

On the radiative Kaon capture by nuclei

The process of the radiative kaon capture by nuclei is considered. The total yields of γ-quanta corresponding to the production of different hyperons are calculated.     

高密核物质中核子的运动对于K介子的等效质量和能量的影响

本文对比分析了目前用于研究K介子产生与演化的不同动力学模型中,高温高密核物质中K⁺介子和K^-介子的等效质量和它所感受到的平均场.所得结果显示:在关于K介子的一般的平均场理论中,所定义的K⁺介子的等效质量随着核物质密度的增大而增加, K^-介子的等效质量随着核物质密度的增大而减小;但在K介子演化的协变动力学模型中,所采用的K⁺介子和K^-介子等效质量是随着核物质密度的增大都是减小的 关键词： 高密核物质 K介子 等效质量 核子运动     

Phase transition to hyperon matter in neutron stars

Recent progress in the understanding of the high density phase of neutron stars advances the view that a substantial fraction of the matter consists of hyperons. The possible impacts of a highly attractive interaction between hyperons on the properties of compact stars are investigated. We find that a hadronic equation of state with hyperons allows for a first order phase transition to hyperonic matter. The corresponding hyperon stars can have rather small radii of R approximately equal 8 km.     

Meson-hyperon couplings in the bound-state approach to the Skyrme model

Kaon and pion coupling constants to hyperons are calculated in the bound-state approach to strangeness in the Skyrme-soliton model. The pion and kaon coupling constants are properly defined as matrix elements of source terms of the mesons sandwiched between two single-baryon states. Numerical calculation of the coupling constants shows that the bound-state approach well reproduces the empirical values.     

Strangeness in stellar matter

A protoneutron star is formed immediately after the gravitational collapse of the core of a massive star. At birth, the hot and high density matter in such a star contains a large number of neutrinos trapped during collapse. Trapped neutrinos generally inhibit the presence of exotic matter — hyperons, a kaon condensate, or quarks. However, as the neutrinos diffuse out in about 10–15 s, the threshold for the appearance of strangeness is reduced; hence, the composition and the structure of the star can change significantly. The effect of exotic, negatively-charged, strangeness-bearing components is always to soften the equation of state, and the possibility exists that the star collapses to a black hole at this time. This could explain why no neutron star has yet been seen in the remnant of supernova SN1987A, even though one certainly existed when neutrinos were detected on Feb. 23, 1987. With new generation neutrino detectors it is feasible to test different theoretical scenarios observationally.     

Magnetic moment of hyperons in nuclear matter by using quark–meson coupling models

We calculate the magnetic moments of hyperons in dense nuclear matter by using relativistic quark models. Hyperons are treated as MIT bags, and the interactions are considered to be mediated by the exchange of scalar and vector mesons which are approximated as mean fields. Model dependence is investigated by using the quark–meson coupling model and the modified quark–meson coupling model; in the former the bag constant is independent of density and in the latter it depends on density. Both models give us the magnitudes of the magnetic moments increasing with density for most octet baryons. But there is a considerable model dependence in the values of the magnetic moments in dense medium. The magnetic moments at the nuclear saturation density calculated by the quark–meson coupling model are only a few percents larger than those in free space, but the magnetic moments from the modified quark–meson coupling model increase more than 10% for most hyperons. The correlations between the bag radius of hyperons and the magnetic moments of hyperons in dense matter are discussed.     

(K)-nucleus dynamics: from quasibound states to kaon condensation

Coupled-channel (K)N dynamics near threshold and its repercussions in few-body (K)-nuclear systems are briefly reviewed, highlighting studies of a K-pp quasibound state. In heavier nuclei, the extension of mean-field calculations to multi-(K) nuclear and hypernuclear quasibound states is discussed. It is concluded that strangeness in finite self-bound systems is realized through hyperons, with no room for kaon condensation.     

Kaon-condensed hypernuclei as highly dense self-bound objects

The structure of K⁻-condensed hypernuclei, which may be produced in the laboratory in strangeness-conserving processes, is investigated using an effective chiral Lagrangian for the kaon–baryon interaction, combined with a nonrelativistic baryon–baryon interaction model. It is shown that a large value of negative strangeness is needed for the formation of highly dense and deeply bound state with kaon condensates and that part of the strangeness should be carried by hyperons mixed in the nucleus. The properties of kaon-condensed hypernuclei such as the ground state energy and particle composition are discussed. Such a self-bound object has a long lifetime and may decay only through weak interaction processes. Comparison with other possible nuclear states is also made, such as kaon-condensed nuclei without mixing of hyperons and noncondensed multistrange hypernuclei. Implications of kaon-condensed hypernuclei for experiments are mentioned.     

Supercurrents in color-superconducting quark matter

We review the basic properties of the currCFL-K⁰ phase in dense quark matter. At asymptotically large densities, three-flavor quark matter is in the color-flavor locked (CFL) state. The currCFL-K⁰ state is a way to respond to “stress” on the quark Cooper pairing, imposed at more moderate densities by the strange quark mass and the conditions of electric and color neutrality. We show how a kaon supercurrent is incorporated in a purely fermionic formalism, and show that the net current vanishes due to cancellation of fermion and charge-conjugate fermion contributions.     

Charged kaon condensation in high density quark matter

We show that at asymptotically high densities the “color-flavor-locked+neutral kaon condensate” phase of QCD develops a charged kaon condensate through the Coleman–Weinberg mechanism. At densities achievable in neutron stars a charged kaon condensate forms only for some (natural) values of the low energy constants describing the low-lying excitations of the ground state.     

致密物质性质和适用于超新星及中子星研究的状态方程(英文)

采用相对论平均场方法研究了致密物质的性质, 构造了包括较宽温度、 同位旋不对称度和密度范围的适用于超新星模拟研究的状态方程, 均匀物质由相对论平均场理论描述, 非均匀物质由托马斯 费米近似给出。讨论了包含超子自由度的中子星物质的状态方程。 计算结果表明, 包含超子可以有效地软化高密度区的状态方程, Λ超子的超流态有可能存在于大质量中子星内部。The properties of dense matter are studied within the relativistic mean field theory. The equation of state (EOS) of dense matter are constructed covering a wide range of temperature, proton fraction, and density for the use of supernova simulations. The relativistic mean field theory is employed to describe the uniform matter, while the Thomas Fermi approximation is adopted to describe the non uniform matter. The EOS of neutron star matter is discussed with the inclusion of hyperons. It is found that the EOS at high density can be significantly softened by the inclusion of hyperons. The 1S₀ superfluidity of Λ hyperons may exist in massive neutron stars.     

Evolution of proto-neutron stars with quarks

Neutrino fluxes from proto-neutron stars with and without quarks are studied. Observable differences become apparent after 10-20 s of evolution. Sufficiently massive stars containing negatively charged, strongly interacting, particles collapse to black holes during the first minute of evolution. Since the neutrino flux vanishes when a black hole forms, this is the most obvious signal that quarks (or other types of strange matter) have appeared. The metastability time scales for stars with quarks are intermediate between those containing hyperons and kaon condensates.     

Spin polarizability of hyperons

We review the recent progress of the theoretical understanding of spin polarizabilities of the hyperon in the framework of SU(3) heavy baryon chiral perturbation theory (HBChPT). We present the results of a systematic leading-order calculation of hyperon Compton scattering and extract the forward spin polarizability (γ ₀) of hyperons. The results obtained for γ ₀ in the case of nucleons agree with the known results of SU(2) HBChPT when kaon loops are not considered.     

Antikaon Condensation and In-medium Kaon and Antikaon Production in Protoneutron Stars

Antikaon condensation and kaon and antikaon production in protoneutron stars are investigated in a chiral hadronic model （also referred to as the FST model in this paper）. The effects of neutrino trapping on protoneutron stars are analyzed systematically. It is shown that neutrino trapping makes the critical density of K^- condensation delay to higher density and fifo condensation not occur. The equation of state （EOS） of （proto）neutron star matter with neutrino trapping is stiffer than that without neutrino trapping. As a result, the maximum masses of （proto）neutron stars with neutrino trapping are larger than those without neutrino trapping. If hyperons are taken into account, antikaon does not form a condensate in （proto）neutron stars. Meanwhile, the corresponding EOS becomes much softer, and the maximum masses of （proto）neutron stars are smaller than those without hyprons. Finally, our results illustrate that the Q values for K^＋ and K^- production in （proto）neutron stars are not sensitive to neutrino trapping and inclusion of hyperons.     

Phase transition of the baryon-antibaryon plasma in hot and dense nuclear matter

We investigate the presence of thermodynamic instabilities in a hot and dense nuclear medium where a phase transition from a gas of massive hadrons to a nearly massless baryon, antibaryon plasma can take place. The analysis is performed by requiring the global conservation of baryon number and zero net strangeness in the framework of an effective relativistic mean field theory with the inclusion of the Δ(1232)-isobars, hyperons and the lightest pseudoscalar and vector meson degrees of freedom. Similarly to the low density nuclear liquid-gas phase transition, we show that such a phase transition is characterized by both mechanical instability (fluctuations on the baryon density) that by chemical- diffusive instability (fluctuations on the strangeness concentration). It turns out that, in this situation, phases with different values of antibaryon-baryon ratios and strangeness content may coexist.     

In-Medium Kaon Potential and Nuclear Equation of State Measured in Nucleus–Nucleus Collisions

The kaon production in heavy ion collisions at intermediate energies provides a sensitive probe to study the in-medium properties and nuclear equation of state of hadrons. Properties of kaons in dense hadronic matter are important for a better understanding of both, the possible restoration of chiral symmetry in dense hadronic matter and the properties of nuclear matter at high densities. We investigated the in-medium kaon potential and nuclear equation of state by transverse mass spectra of K ⁺ mesons in heavy ion collisions. We use quantum molecular dynamics (QMD) models based on covariant kaon dynamics to simulate ${_{28}^{58}Ni +_{28}^{58}Ni}$ collisions at 1.93 A GeV, to analyze the transverse mass spectra of K ⁺. Calculated results with a repulsive in-medium K ⁺N potential can reasonably describe the features of KaoS data. They also shown that the transverse mass spectrum of K ⁺ mesons is a sensitive observable to probe the kaon in-medium potential in dense nuclear matter.     

Constant-cutoff approach to meson-hyperon couplings

The kaon coupling constants at hyperon-nucleon vertices and the pion coupling constants at hyperon-hyperon vertices are calculated in the framework of the constant-cutoff approach to the CHK bound-state model of hyperons, where the postive-parity hyperons such as Λ, Σ, and ∑^*=∑(1385) are theP-wave bound states of an antikaon and theSU(2) Skyrme soliton, while Λ^* is theS-wave bound state. Meson coupling constants are defined as matrix elements of the meson-source terms between two single-baryon states following the method developed for resolving the Yukawa coupling problem in theSU(2) Skyrme soliton model. The magnitudes of the meson coupling constants are found to be close to those obtained using the complete Skyrme model and the phenomenological values.