The question of whether quark matter exists in neutron stars is a long standing one. Generally one finds that a first order phase transition from baryons to quarks softens the equation of state so much that the star would collapse into a black hole. We consider a crossover equation of state, similar to the crossover that is found in lattice QCD studies at finite temperature and zero or small baryon chemical potentials. We find that with reasonable parameters it may be possible to support neutron stars up to about 2.2 solar masses. In that case 1 to 10% of the pressure would be contributed by quark matter in the central core of the highest mass stars.
The equations of state of the neutron star at the hadron-quark crossover region are interpolated with the Gaussian process regression (GPR) method, which can reduce the randomness of present interpolation schemes. The relativistic mean-field (RMF) model and Nambu-Jona-Lasinio (NJL) model are employed to describe the hadronic phase and quark phase, respectively. In the RMF model, the coupling term between $ω$ and $ρ$ mesons is considered to control the density-dependent behaviors of symmetry energy, i.e. the slope of symmetry energy, $L$. Furthermore, the vector interaction between quarks is in…
BCS-BEC crossover of neutron pairs in symmetric and asymmetric nuclear matter
我们提出了新的类型的密度依赖的接触配对相互作用，再现的配对间隙的对称和中子物质的微观处理的基础上的核子-核子相互作用。这些相互作用能够模拟裸相互作用或被介质极化效应屏蔽的相互作用的配对间隙。结果表明，介质的极化效应不能被转换成密度幂律函数，通常与接触相互作用一起引入，并需要引入另一个等密度项。利用这些接触相互作用研究了对称和对称核物质中中子对的BCS-BEC渡越。结果表明，在对称核物质中，裸对相互作用和屏蔽对相互作用导致BCS-BEC交叉的不同特征。对于屏蔽对相互作用，在对称物质中形成双中子BEC态，其密度为$k_{Fn}\sim 0.2$ fm $^{-1}$（中子密度$ρ_n/ρ_0\sim 10^{-3}$）。相反，裸相互作用在任何中子密度下都不形成BEC态。
Due to the large neutron-neutron scattering length, dilute neutron matter resembles the unitary Fermi gas, which lies half-way in the crossover from the BCS phase of weakly coupled Cooper pairs to the Bose-Einstein condensate of dimers. We discuss crossover effects in analogy with the T-matrix theory used in the physics of ultracold atoms, which we generalize to the case of a non-separable finite-range interaction. A problem of the standard Nozieres-Schmitt-Rink approach and different ways to solve it are discussed. It is shown that in the strong-coupling regime, the spectral function exhibits…
In this contribution, we summarize our recent studies on the chiral invariant mass and the chiral condensates in neutron star matter. We construct a unified equations of state assuming the crossover phase transition from hadronic matter described by a parity doublet model to quark matter by an Nambu--Jona-Lasinio type quark model. We first show that the chiral invariant mass is constrained to be 600 MeV $\lesssim m_0 \lesssim$ 900 MeV from recent observations of neutron stars. We then determine the density dependence of the chiral condensate in the crossover description, and show that the chir…
We follow the idea that the QCD phase diagram may be described by a crossover from a hadron resonance gas to perturbative QCD using the switch function ansatz of Albright, Kapusta and Young [1]. While the switch function could be calibrated at vanishing baryon chemical potential with data from lattice QCD simulations, it has been suggested recently by Kapusta and Welle [2] that in the zero temperature limit, the switch function parameter $μ_0$ could be constrained by neutron star phenomenology, in particular by massive pulsars like PSR J0740+6620 with a mass exceeding $2~M_\odot$. In this work…
The speed of sound of the matter within neutron stars may contain non-smooth structure related to first-order phase transitions or or crossovers. Here we investigate what are the observable consequences of structure, such as bumps, spikes, step functions, plateaus, and kinks. One of the main consequences is the possibility of ultra-heavy neutron stars, i.e.~stars with masses significantly heavier than two solar masses. These stars pass all observational and theoretical constraints, including those imposed by recent LIGO/Virgo gravitational-wave observations and NICER X-ray observations. We tho…
We construct an equation of state (EOS) for neutron stars by interpolating hadronic EOS at low density and quark EOS at high density. A hadronic model based on the parity doublet structure is used for hadronic matter and a quark model of Nambu--Jona-Lasinio type is for quark matter. We assume crossover between hadronic matter and quark matter in the the color-flavor locked phase. The nucleon mass of the parity doublet model has a mass associated with the chiral symmetry breaking, and a chiral invariant mass $m_0$ which is insensitive to the chiral condensate. The value of $m_0$ affects the nuc…
We perform the first study of the principal core $g$-mode oscillation in hybrid stars containing quark matter, utilizing a crossover model for the hadron-to-quark transition inspired by lattice QCD. The ensuing results are compared with our recent findings of $g$-mode frequencies in hybrid stars with a first-order phase transition using Gibbs constructions. We find that models using Gibbs construction yield $g$-mode amplitudes and the associated gravitational energy radiated that dominate over those of the chosen crossover model owing to the distinct behaviors of the equilibrium and adiabatic …
We study the chiral condensates in neutron star matter from nuclear to quark matter domain. We describe nuclear matter with a parity doublet model (PDM), quark matter with the Nambu--Jona-Lasino (NJL) model, and a matter at the intermediate density by interpolating nuclear and quark matter equations of state. The model parameters are constrained by nuclear physics and neutron star observations. Various condensates in the interpolated domain are estimated from the chemical potential dependence of the condensates at the boundaries of the interpolation. The use of the PDM with substantial chiral …
Fully general-relativistic binary-neutron-star (BNS) merger simulations with quark-hadron crossover (QHC) equations of state (EOSs) are studied for the first time. In contrast to EOSs with purely hadronic matter or with a first-order quark-hadron phase transition (1PT), in the transition region QHC EOSs show a peak in sound speed, and thus a stiffening. We study the effects of such stiffening in the merger and post-merger gravitational (GW) signals. Through simulations in the binary-mass range $2.5 < M/M_{\odot} < 2.75$, characteristic differences due to different EOSs appear in the frequency …
Finite-Temperature Axion-Admixed Neutron Stars in a Quarkyonic Crossover Framework
Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, India（印度布巴内斯瓦尔物理研究所）；Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India（印度孟买霍米巴巴哈国立研究所）；Department of Physics, Indira Gandhi Institute of Technology, Sarang, Dhenkanal, Odisha-759146, India（印度奥迪沙邦德赫卡纳尔萨拉格英迪拉甘地理工学院物理系）
The phase transition calculations are utilized for an in-depth understanding of the thermodynamics of the deconfinement transition to perform the best analysis of the QCD phase diagram. The phenomenological justifications for a mathematical approach to constructing the phase transition are found based on the properties of hadronic matter at low densities as well as quark matter at high densities. We modify both the quark matter equation of state by confining effects at low densities and the hadronic matter equation of state by excluded volume effects at higher densities. Over the intermediate …
We review the long standing problem of superfluid pairing in pure neutron matter. For the $s$-wave pairing, we summarize the state of the art of many-body approaches including different $nn$ interactions, medium polarization, short-range correlations and BCS-BEC crossover effects, and compare them with quantum Monte Carlo results at low-densities. We also address pairing in the $p$-wave, which appears at higher densities and hence has large uncertainties due to the poorly constrained interactions, medium effects and many-body forces.
The Non-parametric Equation of State Realizes a Generalized Quark-Hadron Crossover
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science（中国科学院紫金山天文台暗物质与空间天文重点实验室）；RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS), RIKEN（理化学研究所交叉理论数学科学中心(iTHEMS)）；School of Astronomy and Space Sciences, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
Comment on "Three-dimensional hydrodynamic simulations of the combustion of a neutron star into a quark star"
如果奇异物质是绝对稳定的，那么普通的原子核就会衰变为奇异核，而中子星则会转化为奇异星。普通核的寿命在实验上被限制在$\sim 10^{33}$年以上，而亚稳态中子星的寿命取决于中子星的质量，可以超过宇宙的年龄。因此，中子星和奇异星可以在宇宙中共存。我们指出，M。Herzog和F. K. Roepke在Phys. Rev. D 84，083002（2011）[arXiv：1109.0539]中的研究集中在奇异物质参数空间中的一个区域，在该区域中，低质量中子星和奇异星共存，而大质量中子星在天文时间尺度上是不稳定的，寿命很短。
The hypothesis that neutrons might decay into dark matter is explored using neutron stars as a testing ground. It is found that in order to obtain stars with masses at the upper end of those observed, the dark matter must experience a relatively strong self-interaction. Conservation of baryon number and energy then require that the star must undergo some heating, with a decrease in radius, leading to an increase in speed of rotation over a period of days.
