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Register a new userChiral symmetry restoration by parity doubling and the structure of neutron stars
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Added by
Micha{\\l} Marczenko
Publication date
2018
fields
Physics
and research's language is
English
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We investigate the equation of state for a recently developed hybrid quark-meson-nucleon model under neutron star conditions of $beta-$equilibrium and charge neutrality. The model has the characteristic feature that at increasing baryon density chiral symmetry is restored in a first order transition within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. Most important for this study are the nucleon (neutron, proton) and $N(1535)$ states. We present three sets for the two free parameters which result in compact star mass-radius relations in accordance with modern constraints on the mass from PSR~J0437-4715 and on the compactness from GW170817. We also consider the threshold for the direct URCA process for which a new relationship is given and suggest as an additional constraint on the parameter choice of the model that this process shall become operative at best for stars with masses above the range for binary radio pulsars, $M>1.4~M_odot$.
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Recent lattice QCD studies at vanishing density exhibit the parity-doubling structure for the low-lying baryons around the chiral crossover temperature. This finding is likely an imprint of the chiral symmetry restoration in the baryonic sector of QCD, and is expected to occur also in cold dense matter, which makes it of major relevance for compact stars. By contrast, typical effective models for compact star matter embody chiral physics solely in the deconfined sector, with quarks as degrees of freedom. In this contribution, we present a description of QCD matter based on the effective hybrid quark-meson-nucleon model. Its characteristic feature is that, under neutron-star conditions, the chiral symmetry is restored in a first-order phase transition deep in the hadronic phase, before the deconfinement of quarks takes place. We discuss the implications of the parity doubling of baryons on the mass-radius relation for compact stars obtained in accordance with the modern constraints on the mass from PSR J0348+0432, the compactness from GW170817, as well as the direct URCA process threshold. We show that the existence of high-mass stars might not necessarily signal the deconfinement of quarks.
Based on an equivparticle model, we investigate the in-medium quark condensate in neutron stars. Carrying out a Taylor expansion of the nuclear binding energy to the order of $rho^3$, we obtain a series of EOSs for neutron star matter, which are confronted with the latest nuclear and astrophysical constraints. The in-medium quark condensate is then extracted from the constrained properties of neutron star matter, which decreases non-linearly with density. However, the chiral symmetry is only partially restored with non-vanishing quark condensates, which may vanish at a density that is out of reach for neutron stars.
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 nuclear EOSs at low density, and has strong correlations with the radii of neutron stars. Using the constraint to the radius obtained by LIGO-Virgo and NICER, we find that $m_0$ is restricted as $600,mathrm{MeV}lesssim m_0 lesssim 900,mathrm{MeV}$.
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 invariant mass ($m_0 gtrsim 500$ MeV, which is favored by the neutron star observations) predicts the mild chiral restoration, and the significant chiral condensate remains to baryon density $n_B sim 2-3n_0$ ($n_0simeq 0.16,{rm fm}^{-3}$: nuclear saturation density), smoothly approaching the NJL predictions for the color-flavor-locked phase at $n_B gtrsim 5n_0$. The same method is applied to estimate diquark condensates, number densities of up-, down- and strange-quarks, and the lepton fraction. In our descriptions the chiral restoration in the interpolated domain proceeds with two conceptually distinct chiral restoration effects; the first is associated with the positive scalar density in a nucleon, relevant in dilute regime, and the other primarily arises from the modification of the quark Dirac sea, which is triggered by the growth of the quark Fermi sea. We discuss several qualitative conjectures to interpolate the microphysics in nuclear and quark matter.
We study the phase structure of dense hadronic matter including $Delta(1232)$ as well as N(939) based on the parity partner structure, where the baryons have their chiral partners with a certain amount of chiral invariant masses. We show that, in symmetric matter, $Delta$ enters into matter in the density region of about one to four times of normal nuclear matter density, $rho_B sim 1 - 4rho_0$. The onset density of $Delta$ matter depends on the chiral invariant mass of $Delta$, $m_{Delta0}$: The lager $m_{Delta0}$, the bigger the onset density. The $Delta$ matter of $rho_B sim 1 - 4rho_0$ is unstable due to the existence of $Delta$, and the stable $Delta$-nucleon matter is realized at about $rho_B sim 4rho_0$, i.e., the phase transition from nuclear matter to $Delta$-nucleon matter is of first order for small $m_{Delta0}$, and it is of second order for large $m_{Delta0}$. We find that, associated with the phase transition, the chiral condensate changes very rapidly, i.e., the chiral symmetry restoration is accelerated by Delta matter. As a result of the accelerations, there appear $N^*$(1535) and $Delta$(1700), which are the chiral partners to N(939) and ${Delta}$(1232), in high density matter, signaling the partial chiral symmetry restoration. Furthermore, we find that complete chiral symmetry restoration itself is delayed by $Delta$ matter. We also calculate the effective masses, pressure and symmetry energy to study how the transition to $Delta$ matter affects such physical quantities. We observe that the physical quantities change drastically at the transition density.
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