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Impact of muons on the bulk viscosity of neutron star matter metamodels

José Luis Hernández1,2,3,*, Cristina Manuel1,2,†, and Laura Tolos1,2,‡

  • *Contact author: hernandez@ice.csic.es
  • †Contact author: cristina.manuel@csic.es
  • ‡Contact author: tolos@ice.csic.es
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Phys. Rev. D 114, 063045 – Published 23 September, 2026

DOI: https://doi.org/10.1103/9skl-rt58

Abstract

Recent studies invoke a unified description of different neutron star observables using metamodels, which parametrize the equation of state of neutron star matter close to nuclear saturation density in terms of few nuclear parameters. In this light, the bulk viscosity in the neutrino-transparent regime of dense nuclear matter composed of neutrons, protons and electrons has been recently shown to be mostly sensitive to the value of the nuclear symmetry energy. As muons are also present at densities around nuclear saturation, we further analyze in this manuscript their impact on this transport coefficient as a function of the slope L of the symmetry energy. We find that muons introduce both relevant qualitative and quantitative effects in the bulk viscous dissipation. Increasing L by a factor two has an effect of several orders of magnitude on the (frequency-independent) bulk viscosity. We also find that for all values of L the frequency-dependent bulk viscosity presents a double peak structure for some values of the density, absent without muons. This also represents changes in orders of magnitude of the viscosity in narrow windows of densities that could be attainable in a neutron star for enough high values of L. The double-peak structure is due to the fact that direct Urca processes open up at different densities for electrons and muons, the critical values depending on L. We present a systematic numerical analysis of both second-order transport coefficients, frequency-dependent bulk viscosity, and damping times of density oscillations as a function of the density and the slope, and find when these could be relevant for the dynamics of the merger of neutrons stars.

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Phys. Rev. D 114, 063045– Published 23 September, 2026
  • Received 8 April 2026
  • Accepted 10 August 2026
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