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Nuclear Theory·nucl-th

Friday·August 23, 2024

6 papers—2 primary·4 cross-listed

All6Primary2Cross-lists4
  1. 03

    Layers of electron captures in the crust of accreting neutron stars

    Lami Suleiman🇺🇸 · Julian-Leszek Zdunik🇵🇱 · Pawel Haensel🇵🇱

    The accumulation of accreted matter onto the neutron star surface triggers exothermic reactions in the crust. The heat released as a result influences the luminosity exhibited by the X-ray transient. The most common approach to the kinetics of exothermic reactions in the crust of accreting neutron stars is to consider an infinite reaction rate. Here, we investigate accretion-related heat release in the accreted outer crust of a neutron star by including a time-dependent accretion cycle and experimentally based reaction rates in the kinetics of electron captures above the reaction threshold. A simple model was used to compute the zero temperature equation of state of a crust in which two nuclei can coexist. We solved the abundance of parent nuclei as a function of the depth in the star and the time variable using astrophysically motivated features of the accreting system. We calculated the heat release and neutrino loss associated to reactions in the outer crust. We report the existence of layers in the outer crust, which contain both parent and grand-daughter nuclei of electron captures. The reactions can occur deeper in the shell than the reaction threshold, thus releasing more heat per accreted baryon for a given accretion rate. The electron capture layers continue to exist even when the accretion has stopped. The heat sources are time- and pressure-dependent in accreting crusts of neutron stars. The total heat released is a function of astrophysical (active and quiescent time) and microscopic (reaction rate) parameters Therefore, we conclude these parameters should be considered individually and carefully for a range of different sources.

    ↳ astro-ph.HEnucl-thAstron.Astrophys.(2024)·2 citations
  2. 04

    Color superconductivity and speed of sound in the two-flavor quark-meson diquark model

    Jens O. Andersen🇳🇴 · Mathias P. Nødtvedt🇳🇴

    We discuss the properties of the two-flavor quark-meson diquark (QMD) model as a renormalizable low-energy model for QCD in the 2SC phase of QCD. The effective degrees of freedom are the mesons (sigma and pions), quarks, and diquarks. Some of the parameters of the model can be determined by expressing them in terms of the vacuum meson masses and the pion decay constant using the on-shell renormalization scheme. The remaining parameters are considered free, although they in principle can be calculated from QCD. The thermodynamic potential is calculated in a mean-field approximation taking only quark loops into account. In this approximation, we derive a set of renormalization group equations for the running masses and couplings. The solutions to these equations are used to improve the thermodynamic potential and thereby thermodynamic quantities. Four parameter sets are chosen and the phase diagram in the -- plane is obtained (with ). We also calculate the speed of sound as a function of at vanishing temperature. For large values of , the speed of sound approaches the conformal limit from above, in disagreement with perturbative calculations, but agreement with hard-dense-loop resummed perturbation theory.

    ↳ hep-phnucl-thPRD(2025)·18 citations
  3. 05

    Interplay between the weak-coupling results and the lattice data in dense QCD

    Yuki Fujimoto🇺🇸

    We discuss the interplay between two first-principles calculations of QCD at high density: perturbative results in the weak-coupling regime and the recent lattice-QCD result at finite isospin density. By comparing these two results, we verify empirically that the weak-coupling calculations of the bulk thermodynamics and the gap parameter for Cooper pairing between quarks can be applicable down to the quark chemical potential GeV. Having verified the validity of the weak-coupling results in QCD at finite isospin density, we discuss possible effects on QCD at finite baryon density, which is relevant for the application to realistic environments such as neutron stars, by using the fact that QCD at finite baryon and isospin density have the common weak-coupling expansions. First, we show the size of the color-superconducting gap at finite baryon density is as small as a few MeV at GeV, which implies that the color-flavor locked phase may be unstable against unpairing up to GeV even in the weak-coupling regime. We also introduce a prescription to reduce the ambiguity arising from the undetermined renormalization scale in the weak-coupling calculation by matching with the lattice-QCD data. We demonstrate the effect of such reduction on neutron-star phenomenology by performing the Bayesian analysis.

    ↳ hep-phastro-ph.HEhep-latnucl-th15 citations
  4. 06

    Spectral eigenfunction decomposition of a Fokker-Planck operator for relativistic heavy-ion collisions

    A. Rizzi🇩🇪 · G. Wolschin🇩🇪

    A spectral solution method is proposed to solve a previuously developed non-equilibrium statistical model describing partial thermalization of produced charged hadrons in relativistic heavy-ion collisions, thus improving the accuracy of the numerical solution. The particle's phase-space trajectories are treated as drift-diffusion stochastic process, leading to a Fokker-Planck equation (FPE) for the single-particle probability distribution function. The drift and diffusion coefficients are derived from the expected asymptotic states via appropriate fluctuation-dissipation relations, and the resulting FPE is then solved numerically using a spectral eigenfunction decomposition. The calculated time-dependent particle distributions are compared to Pb-Pb data from the ATLAS and ALICE collaborations at the Large Hadron Collider.

    ↳ hep-phnucl-thEPJA(2024)·0 citations

Affiliations

first authorsco-authorsvia INSPIRE