Cristina Manuel

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.

The dissipative properties of relativistic strongly interacting nuclear matter significantly influence the damping of stellar oscillations and density fluctuations during compact star mergers. In this work, we derive the evolution equation for the bulk viscous pressure in unpaired quark matter under small deviations from equilibrium. Our analysis reveals that it behaves like a two-component Burgers fluid. We identify four key transport coefficients—two relaxation times and two bulk viscosity coefficients—expressed in terms of equilibrium parameters and electroweak nonleptonic and semileptonic decay rates. The transport coefficients are evaluated for two distinct equations of state: one based on perturbative quantum chromodynamics and the other on a modified MIT bag model, valid in different density regimes. We also determine the temperature and density region where nonleptonic electroweak processes dominate the dissipation. Our formulation establishes a new way of describing bulk viscous effects in quark matter, applicable for numerical simulations of compact star mergers.

We develop a procedure to analytically calculate higher-order contributions to the high-temperature real-time static potential in QCD. It is based on the introduction of a semihard external scale, which lies between the hard scale (the temperature) and the soft scale (the screening mass), and the method of integration by regions. We calculate the leading and next-to-leading corrections in the region where bound states transit from narrow resonances to wide ones. The calculation involves both loop diagrams calculated in the hard thermal loop (HTL) effective theory and power corrections to the HTL Lagrangian calculated in QCD. We also calculate the thermal corrections to the heavy quarkonium spectrum, and estimate the dissociation temperatures. We compare our results with recent lattice data and discuss their usefulness to guide lattice inputs in inverse problems.

We calculate the leading and next-to-leading corrections to the real-time QCD static potential in a high-temperature medium in the region where bound states transit from narrow resonances to wide ones. We find sizable contributions to both the real and the imaginary part of the potential. The calculation involves both loop diagrams calculated in the hard thermal loop effective theory and power corrections to the hard thermal loop Lagrangian calculated in QCD. We compare our results with recent lattice data and check the consistency of different methods used in lattice calculations. We also discuss the usefulness of our results to guide lattice inputs. Published by the American Physical Society 2025

We develop an effective field theory for the description of high energetic or hard photons, the on-shell effective theory (OSEFT). The OSEFT describes the so-called eikonal or semiclassical optical limit, allowing for corrections organized in a systematic expansion on inverse powers of the photon energy. We derive the OSEFT from the Maxwell Lagrangian and study its different properties, such as the gauge symmetry and reparametrization invariance. The theory can be finally formulated in terms of a gauge invariant vector gauge field, without the need to introduce gauge-fixing. We then use the OSEFT to compute corrections to the Wigner photon function and derive its associated side jump effect from reparametrization invariance. Finally, we discuss how to properly define the Stokes parameters from transport theory once quantum effects are considered, so as to preserve their well-defined properties under Lorentz transformations. Published by the American Physical Society 2024

We develop an effective field theory for the description of high energetic or hard photons, the on-shell effective theory (OSEFT). The OSEFT describes the so called eikonal or semi-classical optical limit, allowing for corrections organized in a systematic expansion on inverse powers of the photon energy. We derive the OSEFT from the Maxwell Lagrangian, and study its different properties, such as the gauge symmetry and reparametrization invariance. The theory can be finally formulated in terms of a gauge invariant vector gauge field, without the need to introduce gauge-fixing. We then use the OSEFT to compute corrections to the Wigner photon function, and derive its associated side jump effect from reparametrization invariance. Finally, we discuss how to properly define the Stokes parameters from transport theory once quantum effects are considered, so as to preserve their well-defined properties under Lorentz transformations.

We study the damping of density oscillations in the quark matter phase that might occur in compact stars. To this end we compute the bulk viscosity and the associated damping time in three-flavor quark matter, considering both nonleptonic and semileptonic electroweak processes. We use two different equations of state of quark matter, more precisely, the MIT bag model and perturbative QCD, including the leading-order corrections in the strong coupling constant. We analyze the dependence of our results on the density, temperature and value of strange quark mass in each case. We then find that the maximum of the bulk viscosity is in the range of temperature from 0.01 to 0.1 MeV for frequencies around 1 kHz, while the associated minimal damping times of the density oscillations at those temperatures might be in the range of few to hundreds milliseconds. Our results suggest that bulk viscous damping might be relevant in the postmerger phase after the collision of two neutron stars if deconfined matter is achieved in the process. Published by the American Physical Society 2024