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2nd Workshop on modern equations of state and spectroscopy in neutron-star matter
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Europe/Madrid
Universidad de Alcalá
Universidad de Alcalá
Colegio de San Ildefonso, Plaza de San Diego, s/n, 28801 Alcalá de Henares, Madrid.
Jose Manuel Alarcón (chair at Univ. Alcalá)
(Univ. Alcalá de Henares),
Felipe J. Llanes-Estrada (cochair at Univ. Complutense)
(Universidad Complutense de Madrid)
Description
Jointly organized by the Universidad Complutense de Madrid and the Universidad de Alcalá, that will be physically hosting it.
Scientific Committee
Jose Manuel Alarcón (Univ. Alcalá)
Mark Alford (Washington Univ. St. Louis)
Norbert Kaiser (Technische Univ. München)
Felipe J. Llanes-Estrada (Univ. Complutense Madrid)
Jose Antonio Oller (Univ. Murcia)
Laura Tolós (Instituto de Ciencias del Espacio, Barcelona)
Participants
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09:00
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25+5 minute talks: 1.1Convener: José Manuel Alarcón (Universidad de Alcalá)
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09:30
Einstein Telescope: expanding the horizons of Gravitational Wave Astronomy 30m
Second-generation gravitational-wave (GW) detectors, Advanced LIGO and Advanced Virgo, have opened a new window on the Universe. The first detection of GWs from a binary black hole merger in 2015 marked a major scientific breakthrough, followed by the observation of a neutron star binary coalescence accompanied by electromagnetic counterparts.
Since then, regular detections—particularly of black hole binaries—have become routine, providing valuable insights into compact object populations, stellar evolution, and strong-field gravity. These results, while groundbreaking, represent only the beginning of GW astrophysics.
Third-generation (3G) detectors like the Einstein Telescope (ET) and Cosmic Explorer (CE) aim to revolutionize the field further. With tenfold improved sensitivity and an extended frequency range, ET will access a vastly larger volume of the Universe and probe phenomena inaccessible to current detectors.
The talk will mainly focus on the ET detector, giving an overview of the scientific objectives, the status of design and of the site identification, the technological challenges and the expected implications to the GW Astronomy progress.Speaker: Dr Annalisa Allocca (Università Federico II di Napoli - INFN, sez. Napoli) -
10:00
Neutron Star Core Matter: Constraints on the Equation of State 30m
Observations of the heaviest neutron stars, together with mass and radius
measurements, and gravitational wave signals from binary neutron neutron star mergers, progressively tighten the constraints on the equation-of-state of dense baryonic matter. Using the presently available data base, results are presented of detailed Bayesian inference analyses. A focus is on prerequisites and limitations for hypothetical phase transitions at the baryon densities realized in neutron star cores. Consequences for the possible structure and composition of matter under such conditions are discussed.Speaker: Wolfram Weise (TU Munich) -
10:30
Dense matter EoS with BSM ingredients 30m
In this talk, I will provide a brief overview of some interesting aspects that the study of dense matter EoS may provide when combined with Beyond Standard Model ingredients. I will discuss some relevant features in the mechanisms and impact on the emission of multimessengers from dense stars. Additionally, I will highlight open questions that remain unresolved, yet are crucial for achieving a comprehensive understanding of transient stellar phenomenology and on exploring new avenues involving these invisible sectors.
Speaker: M. Angeles Perez-Garcia (Universidad de Salamanca, Spain)
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11:00
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11:30
Coffee Break
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11:30
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13:00
25+5 minute talks: 1.2
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11:30
Binary Neutron Stars: from macroscopic collisions to microphysics 30m
I will discuss the rapid recent progress made in modelling these systems and show how the gravitational signal can provide tight constraints on the equation of state, sound speed, and the occurrence of phase transitions. Finally, I will discuss a novel and tight correlation between the ratio of the energy and angular-momentum losses in the late-time portion of the post-merger signal, i.e., the "long ringdown", and the properties of the EOS at the highest pressures and densities in neutron-star cores.
Speaker: Prof. Luciano Rezzolla (Institute for Theoretical Physics, Frankfurt, Germany) -
12:00
Neutron Star Properties and Femtoscopic Constraints 30m
We construct the equation of state of hypernuclear matter and study the structure of neutron stars employing
a chiral hyperon-nucleon interaction of the Julich–Bonn group tuned to femtoscopic ¨ Λp data of the ALICE Collaboration, and ΛΛ and ΞN interactions determined from lattice QCD calculations by the HAL QCD Collaboration that
reproduce the femtoscopic ΛΛ and Ξ
− p data. We employ the ab-initio microscopic Brueckner–Hartree–Fock theory
extended to the strange baryon sector. A special focus is put on the uncertainties of the hyperon interactions and how
they are effectively propagated to the composition, equation of state, mass-radius relation and tidal deformability of
neutron stars. To such end, we consider the uncertainty due to the experimental error of the femtoscopic Λp data used
to fix the chiral hyperon-nucleon interaction and the theoretical uncertainty, estimated from the residual cut-off dependence of this interaction. We find that the final maximum mass of a neutron star with hyperons is in the range 1.3 − 1.4
M⊙, in agreement with previous works. The hyperon puzzle, therefore, remains still an open issue if only two-body
hyperon-nucleon and hyperon-hyperon interactions are considered. Predictions for the tidal deformability of neutron
stars with hyperons are found to be in agreement with the observational constraints from the gravitational wave event
GW170817 in the mass range 1.1 − 1.3 M⊙.Speaker: Isaac Vidaña (Istituto Nazionale di Fisica Nucleare) -
12:30
Mysterious Neutron Star at the Center of Supernova Remnant HESS J1731-347 30m
Neutron star masses and radii are observational tracers of the equation of state of cold dense matter.
One way to determine them is to model the thermal X-ray spectra of neutron stars. Here we present the results obtained by modeling the X-ray spectrum of a neutron star at the center of supernova remnant HESS J1731-347, the distance to which is 2.5(3)\,kpc, known from measurements by the Gaia astrometric observatory. Pulsations of the X-ray flux of the source were not detected at a level of 8\%. Therefore, for the theoretical description of the X-ray spectrum, it was assumed that the magnetic field of the star is absent, the surface of the star is homogeneous, covered with a carbon envelope, and its spectrum is described by the spectrum of a purely carbon model atmosphere. As a result, it was found that the mass and radius of the neutron star are unexpectedly small, 0.77$^{+0.20}_{-0.17}$ solar masses, and 10.4$^{+0.86}_{-0.74}$ km, respectively. Descriptions of the observed spectrum by models with a non-uniform surface and parameters typical for neutron stars, possible due to the not very strong constraint on the flow pulsations, are also discussed.Speaker: Valery Suleimanov (University of Tuebingen)
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Lunch
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15+5 minute talks: 2.1
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14:50
Analyzing Fermionic Dark Matter scenarios with the HESS J1731-347 Compact Object 20m
In this presentation I will show how we can constrain Dark Matter (DM) scenarios with the supernova remnant HESS J1731-347. We assume the compact object to be an admixture of DM and Neutron Star, and presume the former to behave as a free Fermi gas. For the Neutron Star we use recently calculated regulator-independent equations of state for neutron stars obtained from first principles. Using the two-fluid formalism we analyze the impact of the DM contribution to the mass and radius of the compact object in terms of the DM particle mass and the DM fraction. This allows us to constrain different scenarios for fermonic DM behaving as a free Fermi gas.
Speaker: Yaiza Cano (Universidad de Alcalá) -
15:10
Magnetized Neutron Stars in the Presence of Dark Matter: A Two-Fluid Perspective 20m
The impact of dark matter (DM) on highly magnetized neutron stars (NSs) is explored using a two-fluid approach. Our model considers self-interacting, non-annihilating, asymmetric fermionic DM coupled to baryonic matter via gravity. Using the relativistic mean-field model with density-dependent magnetic fields, we examine how DM particle mass, mass fraction, and magnetic field strength affect the NS equation of state, structure, and stability. Our analysis shows that increasing the DM fraction reduces the maximum gravitational mass of NSs, especially for heavier DM particles. In contrast, lighter DM particles can induce a transition from a DM core to a halo structure, enhancing the maximum mass. Strong magnetic fields soften the equation of state and lower the DM content sustainable in the NS core before halo formation. Comparing our results with NICER and GW170817 observations, we constrain the range of viable DM parameters. Incorporating magnetic fields slightly shifts these limits, mainly affecting the maximum mass and tidal deformability. These findings provide new insights into the interplay between DM and magnetic fields in shaping the properties of neutron stars.
Speaker: Dr Harish Chandra Das (INFN Catania, Italy) -
15:30
Role of the δ Meson in the Equation of State an Direct Urca Cooling of Neutron Stars 20m
The direct Urca (dUrca) process is a key mechanism driving rapid neutrino cooling in neutron stars, with its baryon density activation threshold determined by the microscopic model for nuclear matter. Understanding how nuclear interactions shape the dUrca threshold is essential for interpreting neutron star thermal evolution, particularly in light of recent studies on exceptionally cold objects. We investigate the impact of incorporating the scalar isovector δ meson into the neutron star equation of state, which alters the internal proton fraction and consequently affects the dUrca cooling threshold. Since proton superfluidity is known to suppress dUrca rates, we also examine the interplay between the nuclear interaction mediated by the δ meson and the 1S0 proton pairing gap. We perform a Bayesian analysis using models built within a relativistic mean-field approximation, incorporating constraints from astrophysical observations, nuclear experiments, and known results of \textit{ab initio} calculations of pure neutron matter. We then impose a constraint on the dUrca threshold based on studies of fast-cooling neutron stars. The inclusion of δ meson expands the range of possible internal compositions, directly influencing the stellar mass required for the central density to reach the dUrca threshold. Furthermore, we observe that the observation of relatively young and cold neutron stars provides insights into 1S0 proton superfluidity in the core of neutron stars.
Speaker: Luigi Scurto (University of Coimbra, CFISUC) -
15:50
Testing gravity with the latent heat of neutron star matter 20m
We explore "ridges" in the macroscopic properties of rotating neutron stars as potential indicators of first-order phase transitions in their matter. These phase transitions induce non-analytic behavior in observables like angular momentum, moment of inertia, mass, and radii, with the intensity of this behavior directly tied to the latent heat of the transition. Notably, the Seidov limit sets a bound on the maximum latent heat a phase transition can produce before its excess energy density, not compensated by additional pressure, results in gravitational collapse.
Additionally, we investigate how modified gravity theories, such as quadratic f(R) gravity, affect these phenomena. In this context, we find that the Seidov limit undergoes substantial modification compared to General Relativity. Breaching the Seidov limit would lead to two significant discoveries: evidence of a first-order phase transition in neutron star matter and a deviation from General Relativity.
Based on Annals Phys. 459 (2023) 169487, arXiv:2307.15366 [nucl-th] and JCAP 01 (2025) 015, arXiv:2409.16201v2 [gr-qc].
Speaker: Pablo Navarro Moreno (Universidad Complutense de Madrid) -
16:10
The thermal index of neutron-star matter in the virial approximation 20m
We study the thermal index of low-density, high-temperature dense matter. We use the virial expansion to account for nuclear interaction effects. We focus on the region of validity of the expansion, which reaches $10^{-3}$ fm$^{-3}$ at $T=5$ MeV up to almost saturation density at $T=50$ MeV.
In pure neutron matter, we find an analytical expression for the thermal index, and show that it is nearly density- and temperature-independent,
within a fraction of a percent of the non-interacting, non-relativistic value of $\Gamma_\text{th} \approx 5/3$.
When we incorporate protons, electrons and photons, we find that the density and temperature dependence of the thermal index changes significantly.
We predict a smooth transition between an electron-dominated regime with
$\Gamma_\text{th} \approx 4/3$ at low densities to a neutron-dominated region with $\Gamma_\text{th} \approx 5/3$ at high densities. This behavior is by and large independent of proton fraction and is not affected by nuclear interactions in the region where the virial expansion converges. We model this smooth transition analytically and provide a simple but accurate parametrization of the inflection point between these regimes. When compared to tabulated realistic models of the thermal index, we find an overall agreement at high temperatures that weakens for colder matter. The discrepancies can be attributed to the missing contributions of nuclear clusters. The virial approximation provides a clear and physically intuitive framework for understanding the thermal properties of dense matter, offering a computationally efficient solution that makes it particularly well-suited for the regimes relevant to neutron star binary remnants.Speaker: Giuseppe Rivieccio (Universitat de Valencia)
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Coffee Break
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25+5 minute talks: 1.3
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17:00
Equation of State at finite temperature in the era of new nuclear physics and multimessenger constraints 30m
In this contribution, I will start with an overview of different types of equation of state modelling in the Bayesian formalism, to demonstrate the িmpact of different experimental and observational constraints. Further, I will present equations of state at finite temperature obtained with Brussels-Skyrme-on-a-Grid (BSkG) energy density functionals developed at Brussels, which are unified across the crust and core of the neutron star environment. These models have demonstrated remarkable accuracy over the whole nuclear chart on the masses, and fission barriers of nuclei, but at the same time they also satisfy recent astrophysical constraints. I will also outline the impact of our calculations at finite temperatures on the composition of the crust in the neutron stars. Our next goal is to apply these equations of state in the end-to-end simulation of binary neutron star mergers.
Speaker: Chiranjib Mondal (Universite Libre de Bruxelles) -
17:30
Combined nuclear physics and multimessenger inference of neutron star equation of state 30m
With ongoing advancements in nuclear theory and experimentation, coupled with a growing body of neutron star (NS) observations, a wealth of information concerning the equation of state (EOS) for matter at extreme densities has become accessible. In this talk, I will discuss the present status of NS EOS by combining this information using a comprehensive Bayesian statistics. We use a hybrid EOS formulation that connects the information obtained from nuclear theory at the lower densities which is then augmented by a generic 3-segment piecewise polytrope model at higher densities. Our approach integrates theoretical data from nuclear physics, including chiral effective field theory (χEFT) and perturbative quantum chromodynamics (pQCD), alongside observational data from NS mass measurements, X-ray observations, and gravitational wave detections from binary NS mergers. Our findings highlight that χEFT data significantly improve constraints on the EOS near nuclear saturation density along with astrophysical observations, while pQCD and nuclear experiments have less impact. Key parameters, such as the slope and curvature of the symmetry energy, the radius, and tidal deformability of a typical NS, and the maximum mass of non-rotating NSs, are estimated with high credibility.
Speaker: Bhaskar Biswas (Universität Hamburg)
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20:15
Social programme: City / old campus tour
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25+5 minute talks: 1.4
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09:00
Hot and dense baryon-rich QCD matter 30m
Properties of high-density strong-interaction matter of relevance for astrophysical scenarios that involve neutron stars are discussed. It is argued that theoretical insights from recent lattice comparisons of effective QCD models for vanishing baryo-chemical potential but finite isospin can guide realistic model building at high baryon density, as this regime is currently not accessible to first-principles numerical calculations of the QCD partition function. Special attention is payed to the chiral properties of high-density matter and the nature of a possible first-order chiral phase transition. In this transition hadronic parity-partners, in particular baryons, become spectrally degenerate with finite (pole) masses, as expected from general insight into the mass generation in QCD. Using a parity-doublet effective model in a RG invariant mean-field treatment, implications for cold and hot neutron-star matter are presented.
Speaker: Jochen Wambach (TU Darmstadt) -
09:30
Origin of Magnetar Magnetic Fields: Insights from 3D Magneto-Thermal Simulations 30m
The long-term evolution of neutron stars' strong internal magnetic fields requires advanced numerical modeling. Observations suggest a highly non-uniform magnetic structure, underscoring the need for three-dimensional simulations. In this talk, I will introduce MATINS, a novel three-dimensional numerical code based on a finite-volume scheme, specifically designed to simulate magneto-thermal evolution in neutron star crusts. I will present results from the first fully coupled three-dimensional magneto-thermal simulations incorporating realistic background structures and microphysics. Our simulations, initialized with intricate field configurations from proto-neutron star dynamo models, show that the surface dipolar component remains weak over time. This raises fundamental questions about the mechanisms driving the large-scale magnetic fields observed in neutron stars. To explore this, we examine the inverse cascade phenomenon, analyzing the influence of magnetic helicity, crustal geometry, and boundary conditions on field evolution. Additionally, we investigate the chiral magnetic instability, a microphysical process that can amplify large-scale magnetic fields in the crust. Driven by deviations from chemical equilibrium caused by spin-down evolution, this instability naturally generates magnetic helicity and facilitates the formation of strong toroidal fields, offering an alternative to classical dynamo models. These findings provide fresh insight into the origin and dynamics of neutron star magnetic fields, particularly in magnetars.
Speaker: Clara Dehman (University of Alicante) -
10:00
Post-merger gravitational wave signals from binary neutron stars: Effect of the magnetic field 30m
The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz postmerger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct full general-relativistic magnetohydrodynamics simulations of NSNS mergers with two different masses and two different EOSs (SLy and ALF2) with three different initial magnetic field topologies (poloidal and toroidal only, confined to the interior, and “pulsar-like”: dipolar poloidal extending from the interior to the exterior), with four different initial magnetic field strengths. We find that the magnetic braking and magnetic effective turbulent viscosity drives the merger remnants towards uniform rotation and increases their overall angular momentum loss. This causes the remnant to contract and the angular velocity of the quadrupole density oscillation to increase in such a way that it rotates faster than the fluid itself, in stark contrast with nonmagnetized simulations. As a result, the f2 frequency of the dominant postmerger GW mode shifts upwards over time. The overall shift is up to ∼ 200 Hz for the strongest magnetic field we consider and ∼ 50Hz for the median case and is therefore detectable in principle by future GW observatories, which should include the magnetic field in their analyses.
Speaker: Milton Ruiz (University of Valencia) -
10:30
Hyperons in dense and hot matter and their influence on neutron star mergers 30m
We study the influence of hyperons in binary neutron star (NS) mergers, considering equations of state (EoSs) models which include hyperonic degrees of freedom and partly delta resonances. Thermally produced hyperons induce a higher heat capacity and a lower thermal index, i.e. a reduced thermal pressure for a given amount of thermal energy, compared to purely nucleonic models. These EoSs are inserted in simulations of NS mergers to explore the impact on observables of these events. We find a characteristic increase of the dominant postmerger gravitational-wave (GW) frequency by a few per cent, which is specifically linked to the occurrence of hyperons and can thus be potentially used as a discriminator between purely nucleonic and hyperonic systems. Also, the threshold mass for prompt black hole formation is reduced by about 0.05M⊙ for hyperonic EoSs in comparison to nucleonic ones with the same stellar parameters of cold NSs.
Speaker: Angels Ramos (University of Barcelona)
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09:00
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11:00
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11:30
Coffee Break
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25+5 minute talks: 1.5
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11:30
Hybrid stars: equilibrium structure and bulk viscosity 30m
I will discuss the recent work on the construction of equilibrium hybrid stars, the emergence of twins and triplets and their signatures, such as mass, radius and tidal deformability. I will further discuss the bulk viscosity of hybrid star matter which is relevant for the studies of binary neutron star mergers and their gravitational wave signals.
Speaker: Prof. Armen Sedrakian (Frankfurt Inst. for Advanced Studies) -
12:00
Fastest spinning millisecond pulsars as indicators of deconfined quark matter in neutron stars 30m
We study rotating hybrid stars with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson and diquark couplings in the quark matter phase, we obtain different hybrid star sequences with varying extensions of the quark matter core, ensuring consistency with astrophysical constraints from mass, radius, and tidal deformability measurements. We test whether the early deconfinement phase transition is consistent with the present observational data. We show how the fastest spinning pulsars, the appearance of the quasi-radial oscillations, and non-axisymmetric instabilities constrain the strongly interacting matter equation of state at zero temperature. Our findings reveal that incorporating the hybrid equation of state into the analysis of pulsars has significant implications for the constraints on the properties of strongly interacting matter and neutron stars.
Speaker: Dr Violetta Sagun (University of Southampton) -
12:30
Quark-meson diquark model and dense QCD 30m
In this talk, I will discuss the properties of the two-flavor quark-meson diquark model as a low-energy effective model for QCD at finite density.
I will map out the phase diagram in the mu_I-mu_B plane, where mu_I and mu_B are the isospin and baryon chemical potentials, respectively.
I will present results for the speed of sound in two cases a) finite mu_I and mu_B=0, and b) finite mu_B and mu_I=0. In case a), the results compare
favorably to lattice simulations and chiral perturbation theory.
In case b) the speed of sound approaches the conformal limit from above at odds with perturbative calculations. I will discuss the 2SC phase of
QCD and the effects of imposing charge neutrality. Finally, I will show some preliminary results for the mass-radius relations of non-strange
pure quark stars.Speaker: Jens Oluf Andersen (NTNU)
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11:30
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Lunch
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15+5 minute talks: 2.2
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14:30
Imprints of Phase Transition from Neutron Star Mergers 20m
The core of a neutron star is an entity of enigma due to the mystery surrounding its composition. Insights from nuclear theory and perturbative QCD suggest the possible existence of hybrid stars having a phase transition from hadronic matter to quark matter. This study presents how different types of phase transition (namely, Maxwell and Gibbs) can leave imprints on gravitational waves from binary neutron star mergers and how this helps us to differentiate types of phase transition from post-merger signals. We also comment on the type of phase transition for the merger event of GW170817.
Speaker: Dr SAGNIK CHATTERJEE (IFIN-HH ; IISER BHOPAL) -
14:50
Nuclear Matter Properties and Neutron Star Structures from an Extended Linear Sigma Model 20m
Unlike the widely used Walecka-type models for studying nuclear matter properties and neutron star structures, we extended the linear sigma model, originally developed by Schechter and his colleagues, to the baryonic sector to explore dense nucleon systems. This extended framework is termed the baryonic extended linear sigma model (bELSM).
The bELSM incorporates 2-quark and 4-quark configurations to address the P-wave problem of the lowest-lying scalar meson, sigma. Beyond its original purpose, the model offers a systematic way to include delta mesons and hyperons, guided by the chiral symmetry pattern. By employing the relativistic mean field theory, we introduced density effects, revealing a plateau-like behavior of symmetry energy at intermediate densities. This outcome aligns with the FSU-delta model results, where delta mesons were included to resolve inconsistencies between the properties of (^{208})Pb and the tidal deformation of neutron stars observed in GW170817. Notably, the delta meson couplings, including (g_{\delta NN}) and its interactions with sigma, differ significantly from those in Walecka-type models that include delta meson dynamics.
An intriguing discovery is that, when incorporating all possible leading-order terms, the squared sound velocity approaches (1/3) at high densities, following parameters determined by empirical data. As the bELSM encodes the chiral symmetry pattern of QCD, it offers valuable insights into how QCD phenomena impact macroscopic observations.
Additionally, I aim to present some preliminary results for the SU(3) case, as well as introduce an AI-driven platform designed to navigate the complex parameter space of low-energy effective theories and models.
Speaker: Yao Ma (Nanjing University) -
15:10
Absolutely stable color-superconducting strange quark matter formation in compact stars 20m
An intriguing hypothesis states that ordinary hadronic matter in bulk is a metastable state (i.e., a local minimum) of strongly interacting matter, while strange quark matter (SQM) is absolutely stable (i.e., the global minimum). These two phases would be separated by a potential barrier that prevents a spontaneous deconfinement transition of ordinary hadronic matter into SQM.
Our work aims to determine the conditions under which this barrier can be overcome and to assess whether such conditions may naturally occur in high-energy astrophysical environments, such as core-collapse supernovae and binary compact star mergers. In these scenarios, the formation of a critical SQM seed could ultimately lead to the conversion of a hadronic star into a strange quark star.
It is usually assumed that the local flavor composition remains fixed during the initial formation of the SQM seed, given that the weak interactions are too slow to change it significantly. However, it has been suggested that the composition fluctuates around its average equilibrium values at the typical temperatures of high-energy astrophysical processes. I will address this effect by considering the local thermal fluctuations of the hadronic composition, showing that they make the formation of SQM much easier. Moreover, I will discuss the role of color-superconductivity in such a phenomenon.Speaker: Mirco Guerrini (University of Ferrara) -
15:30
Pulse Modulation due to Superfluid transition in Neutron Star 20m
Matter with extreme baryon density is believed to occur in the core of a neutron star, allowing for various novel phases, from conventional nucleon superfluid phase to exotic high baryon density QCD phases. Here, we point out a unique phenomenon associated with phase transitions to a superfluid phase, which may be the nucleon superfluid phase, or a phase like CFL phase, allowing for superfluid vortices. A random network of vortices forms via the Kibble-Zurek mechanism in any superfluid phase transition. The angular momentum carried by the random superfluid vortex network has to be balanced by an equal and opposite angular momentum in the normal fluid due to the conservation of angular momentum, thereby imparting an arbitrarily oriented angular momentum component to the outer shell in the case of a neutron star. This will induce a wobbling
of the neutron star and affect its spin, which can be detected for pulsars with high-precision measurements of the pulse timings and the pulse profile changes.Speaker: Deepthi Godaba Venkata (Birla Institute of Technology and Science Pilani, India) -
15:50
Role of isospin asymmetry in the onset of quark matter in neutron stars 20m
While symmetric nuclear matter has been studied in laboratories, neutron star matter is characterized by high asymmetry. Therefore, by examining the strongly interacting matter properties in a wide range of densities and isospin asymmetry we confront two regimes to understand how the enforced electric neutrality and beta equilibrium alter the onset density of quark matter. Particularly, we demonstrate the dependence of the onset density of deconfined quarks in the electrically neutral beta-equilibrated matter on the onset density for symmetric matter. This allows us to map the phase diagram of cold strongly interacting matter in the plane of baryon density vs isospin asymmetry which is important in modeling hybrid stars based on the nuclear and low-energy heavy-ion collision experiments.
Speaker: Pavlo Panasiuk (University of Coimbra) -
16:10
Quantum Simulations of the QCD EoS 20m
The influence of modified gravity on the bulk properties of neutron stars can be constrained through integration of the Tolman–Oppenheimer–Volkoff (TOV) equations, provided the input equation of state (EoS) is derived solely from microscopic physics. This becomes particularly challenging in the intermediate to high baryon-density regime, where neither nuclear nor chiral effective field theories are reliable, perturbative QCD breaks down, and the sign problem prohibits Monte Carlo methods.
Quantum computing offers promising new avenues to address these difficulties — if scalable architectures become available — but they face intrinsic complexities such us the trade-off between a dense encoding of the relevant degrees of freedom and an efficient decomposition of the resulting unitary transformations. In this talk, we present a novel register-based encoding of canonically quantized QCD in the time-axial / Weyl gauge. We detail the implementation of key Hamiltonian terms and demonstrate small-scale simulations that provide a preliminary assessment of the computational time and memory resources required.Speaker: Juan José Gálvez Viruet (Univ. Complutense de Madrid)
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Coffee Break
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25+5 minute talks
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17:00
Nuclear response functions with quantum (inspired) algorithms 30m
The ab-initio determination of the nuclear response to small perturbation is of fundamental importance to predict transport properties of neutron star matter and cross sections for electro-weak processes in neutron stars. The direct calculation of dynamical properties however poses serious challenges to traditional many-body methods. In this talk I will discuss how quantum computing could help in solving these problems and present some of the quantum algorithms that have been proposed to calculate nuclear responses. A variant of these schemes can actually be adapted to classical Coupled-Cluster methods and I will present recent results for the spin response of neutron matter obtained in this way.
Speaker: Alessandro Roggero (University of Trento) -
17:30
Inferring three-nucleon couplings from multi-messenger neutron-star observations 30m
Abstract: Understanding the interactions between nucleons in dense matter is one of the outstanding challenges of theoretical physics. Effective field theories have emerged as the dominant approach to address this problem at low energies, with many successful applications to the structure of nuclei and the properties of dense nucleonic matter. However, how far into the interior of neutron stars these interactions can describe dense matter is an open question. In this talk I will discuss how we developed a framework that enables the inference of three-nucleon couplings in dense matter directly from astrophysical neutron-star observations. We apply this formalism to the LIGO/Virgo gravitational-wave event GW170817 and the X-ray measurements from NASA's Neutron-Star Interior Composition Explorer, and establish direct constraints for the couplings that govern three-nucleon interactions in chiral effective field theory. Furthermore, we demonstrate how next-generation observations of a population of neutron-star mergers can offer stringent constraints on three-nucleon couplings, potentially at a level comparable to those from laboratory data. This work directly connects the microscopic couplings in quantum field theories to macroscopic observations of neutron stars, providing a way to test the consistency between low-energy couplings inferred from terrestrial and astrophysical data.
Speaker: Isak Svensson (TU Darmstadt) -
18:00
Manifestation of scale symmetry in dense nuclear matter 30m
The scale invariance of QCD at classical level in chiral limit is broken by quantum effect. When matching trace anomaly of QCD to the effective theory of strong interaction at low energy, one can setup a framework including the scalar meson field in chiral effective theory---scale-chiral EFT. In this talk, I will present the essence of the scale-chiral EFT and discuss the behavior of the trace anomaly in dense nuclear matter.
Speaker: Prof. Yong-Liang MA (Nanjing University)
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17:00
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22:30
Social programme: Dinner
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25+5 minute talks: 1.7
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Spanning neutron star properties with microscopic descriptions: can we learn about composition? 30m
The composition of the core of neutron stars is still under debate. One possibility is that matter could be deconfined into quark matter due to the high densities reached in their cores. The possible existence of hadronic and hybrid stars is studied using microscopic models to describe the different phases of matter, including RMF models, chiral symmetric models, and quark models. Within these microscopic models, we aim to calculate the properties of neutron stars and nuclear matter and discuss the effect of different compositions of matter. The model parameters will be determined by Bayesian inference from neutron star observations, nuclear matter properties at saturation, and constraints from chiral effective field theory calculations of neutron matter and from pQCD calculations at very high densities. Several properties of neutron stars will be discussed: the maximum mass, the radius of the canonical NS, the speed of sound in the star, the proton fraction and the onset of the pn direct Urca processes, the transition to quark matter, the presence of hyperons. It will be shown that different compositions of neutron stars are compatible with current observational data. We will also discuss some effects on the neutron star properties due the presence of dark matter. The implications of possible information on the local derivatives from the mass-radius diagram in neutron star matter will be discussed. It is expected that the next generation of gravitational wave and electromagnetic detectors will allow the determination of the neutron star radius and mass with a small uncertainty, which will have an important impact on the information that can be extracted about the high density equation of state of baryonic matter.
Speaker: Constança Providência (University of Coimbra) -
09:30
Neutron Star Masses and Radii from NICER Data 30m
Precise and reliable measurements of neutron star radii are essential to our understanding of cold, catalyzed matter beyond nuclear saturation density. After reviewing the methodology used to measure these quantities from NICER X-ray, I will present our current mass and radius measurements and discuss their implications for the matter in the cores of neutron stars.
Speaker: Alexander Dittmann (Institute for Advanced Study) -
10:00
The warm low-density EoS calibrated to HIC data: can exotic clusters also play a role? 30m
Light nuclear clusters, such as H and He isotopes, are expected to be present in astrophysical environments and play an important role in different astrophysical phenomena involving ultra-dense baryonic matter: in binary neutron star mergers, the abundance of light clusters has a direct influence on the fraction of the ejecta, or on the viscous evolution of the accretion disk after the merger. However, the estimation of their abundance demands to correctly estimate the in-medium modification of their binding energy. This can be achieved in a phenomenological way, if theoretical models are calibrated to experimental data from heavy-ion collisions (HIC), where these same clusters are produced in comparable density and temperature conditions.
Useful observables to pin down the light nuclei effects with the medium have been extracted from HIC by the NIMROD and the INDRA collaborations. Recently, the INDRA data has been used to constraint a phenomenological model using Bayesian inference.In this talk, we will address the low-density equation of state with the inclusion of light clusters. We will consider not only from the theoretical point of view how these light clusters are calculated for warm nuclear matter in the framework of relativistic mean-field models with in-medium effects, but also how these models were calibrated to experimental data from heavy-ion collisions, measured by the INDRA Collaboration. The in-medium effects are included in a two-fold way: via the couplings of the clusters to the mesons, that were calibrated to the experimental data, and via a binding energy shift.
We will also analyze the effect of including an exotic state state, the tetraneutron, that was reported in Duer et al, Nature 606, 678 (2022) as a resonant state, on the yields of the other light clusters. We calculate the abundances of the light clusters and chemical equilibrium constants with and without this exotic cluster. We also analyze how the associated energy of the tetraneutron would influence such results.
We find that the low-temperature, neutron-rich systems are the ones most affected by the presence of the tetraneutron, making neutron stars excellent environments for their formation. Moreover, its presence in strongly asymmetric matter may increase the proton and $\alpha-$particle fractions considerably. This may have an influence on the dissolution of the accretion disk of the merger of two neutron stars.Speaker: Helena Pais (University of Coimbra) -
10:30
Exploring Composition of Neutron Star Matter with a Relativistic Density Functional 30m
In this work, we perform a Bayesian analysis putting together the available knowledge from the nuclear physics experiments and astrophysical observations to explore the equation of state of supranuclear matter. In particular, we employ a relativistic metamodeling technique to nuclear matter to cover the uncertainties in the parameter space of the saturation properties of nuclear matter, both in the isoscalar and isovector sectors. Then, we investigate if it is possible to reconcile the inferred values of those quantities from observational data with the values obtained from nuclear experiments and compute a joint posterior of these quantities, incorporating all the available knowledge. We further probe the fractions of different particle species that the interior of a neutron star may contain, particularly the proton fraction in the core and the consequences of the allowed compositions within our metamodel. We also incorporate the possible emergence of hyperons in the system and the number of ways that the nucleonic metamodel can accommodate hyperons in the neutron star matter. Finally, we calculate the strangeness content in the star and discuss its observational implications.
Speaker: Prasanta Char (Universidad de Salamanca)
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Coffee Break
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25+5 minute talks: 1.8
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11:30
Unified equations of state for neutron stars based on the nuclear energy density functional theory 30m
Formed in the aftermath of gravitational core-collapse supernova explosions, neutron stars are the most compact observed stars. Their average density exceeds that found inside atomic nuclei. Neutron stars are also endowed with the highest magnetic fields known, which can reach millions of billion times that of the Earth. According to our current understanding, a neutron star is stratified into distinct layers. The surface is probably covered by a metallic ocean. The solid layers beneath consist of a crystal lattice of pressure-ionized atoms embedded in a highly degenerate relativistic electron gas. With increasing density, nuclei become progressively more neutron rich until neutrons start to drip out of nuclei thus delimiting the boundary between the outer and inner regions of the crust, where neutron-proton clusters are immersed in a neutron liquid. At about half the nuclear saturation density, the crust dissolves into a homogeneous liquid mixture of nucleons and leptons. A mantle of nuclear pasta might also be present at the interface between the crust and the core.
Over the past years, we have developed a series of unified equations of state of dense matter in neutron stars. Based on the nuclear energy-density functional theory, these equations of state provide a thermodynamically consistent treatment of all regions of the star and were calculated using functionals that were precision fitted to experimental and theoretical nuclear data. These equations of state were specifically constructed to assess the role of nuclear uncertainties on neutron-star properties. Our latest developments concern the inclusion of neutron pairing in the inner crust, refined calculations of the pressure and chemical potentials, and a more realistic description of nuclear pasta. These equations of state will be compared to constraints inferred from the detection of the gravitational-wave signal GW170817 from a binary neutron-star merger and from observations of the electromagnetic counterparts. Constraints inferred from other observations including NICER will be also discussed.
Speaker: Prof. Nicholas Chamel (Univ. Libre de Bruxelles) -
12:00
New ab initio constrained extended Skyrme equations of state for simulations of neutron stars, supernovae and binary mergers 30m
A large sample of equation of state (EOS) models built within a Bayesian inference is used to investigate the thermal behavior of homogeneous matter with Brussels extended Skyrme interactions over wide ranges of density and temperature, in both symmetric nuclear matter and pure neutron matter. Our key finding is that, via a lower limit on the effective mass, the condition imposed on neutron Fermi velocity not to exceed the speed of light leads to negative values of thermal pressure, previously predicted by $\chi$-ETF calculations.
Tables of general purpose EOS models, suitable to input in astrophysical simulations, are being produced are made available for the community. We expect that, once implemented in numerical simulations of binary neutron star mergers or core-collapse simulations, these EOS tables will lead to evolutions qualitatively different than those obtained based on traditional Skyrme interactions.
Speaker: Adriana R. Raduta (IFIN-HH, Bucharest) -
12:30
Inferring neutron star crust properties from unified EoS models with ab-initio constraints 25m
Bayesian inference offers a powerful framework for constraining the nuclear equation of state (EoS) across a wide range of densities by combining information from astrophysical observations, ab-initio nuclear theory, and heavy-ion collisions. In this work, we refine a unified meta-modeling framework for the EoS by incorporating low-density corrections based on energy density functionals constrained by ab-initio neutron matter calculations. This refinement improves consistency with nuclear physics in the dilute regime, which is critical for accurately modeling neutron star crustal properties.
We explore how these improvements impact predictions of the crust-core transition density and pressure, crustal composition, and the moment of inertia fraction. Our results emphasize the importance of combining theoretical and experimental constraints across densities to robustly model the EoS, paving the way for future high-precision multimessenger studies.Speaker: Stefano Burrello (LNS (INFN))
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Lunch
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15+5 minute talks: 2.3
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A Novel View on the Inner Crusts of Neutron Stars: thermodynamic stability, diffusional stability, and exotic light nuclei 20m
We investigate the properties of non-accreted crusts of neo-neutron stars, i.e., of inhomogeneous subsaturation warm dense matter in beta equilibrium. We present two novel results and one known, but frequently ignored property of such matter. The new features include: the presence of an exotic light nucleus, $^{14}$He, starting from the baryon density of $\approx~0.01~$fm$^{-3}$ and up to the density of the transition to homogeneous matter; an instability with respect to diffusion (buoyancy) in the inner crusts of isolated neo-neutron stars in some models. We also demonstrate and emphasize that nuclear matter with electrons is thermodynamically stable and does not have a first-order phase transition between inhomogeneous and homogeneous phases.
Speaker: Dr Mikhail Beznogov (National Institute for Physics and Nuclear Engineering (IFIN-HH)) -
14:50
Interpolating EoS between different regimes 20m
Matching the equation of state (EoS) of pure neutron matter (PNM) with the EoS of the crust (outer and inner) of a neutron star (NS) is fundamental to understanding the properties of neutron stars, but remains a major challenge.
Our starting point here is the EoS band obtained for PNM at zero temperature and very low densities, expressed in terms of the nucleon scattering data [1], and subsequently interpolated to high-density physics by two steps (2). The first one is performed between the uncertainty band provided by (1) and the saturation density, constrained by the nuclear experimental data (symmetry energy and its slope). Then, a second interpolation is performed between this first band and the pQCD regime, where we also obtain the bands corresponding to the cases of including or not the constraints of the astrophysical observables.
In this work we incorporate to the band obtained with the previous procedure several crust models, which are connected by the coincidence of all thermodynamic quantities: pressure, energy density and baryon density. We study different matching options along the inner crust, analyzing how the matching of the crust EoS with the EoS of this band affects the NS radius, since the models describing the inner crust and the PNM EoS are not the same.
(1). J. M. Alarcón and J. A. Oller, Phys. Rev. C 107 (2023) no.4, 044319.
(2). J. M. Alarcón, E. Lope-Oter and J. A. Oller, [arXiv:2410.14776 [nucl-th]].
Speaker: Eva Lope Oter (Universidad Complutense de Madrid) -
15:10
Bayesian inference on nuclear data and neutron star observations for the nuclear equation of state 20m
The Equation of State (EoS) of nuclear matter is related to many topics in nuclear physics. In particular, it is crucial for understanding the structure of compact objects such as neutron stars. In the conservative hypothesis of a purely nucleonic composition of neutron star matter, the EoS is fully determined in terms of the so-called nuclear matter parameters (NMPs), which, in principle, can be determined from nuclear theory and experiments, though with error bars. However, analyses that try to infer the NMPs from nuclear experiments often present one of the following limitations: (i) the control over the quality of the simultaneous reproduction of different observables is limited; (ii) independent inferences of single NMPs give poor knowledge of the correlations among parameters.
The main objective of our work is to address both limitations. Within the standard Skyrme functional ansatz, we build a reliable probability distribution for a combination of nuclear matter parameters and Skyrme parameters (which are needed to constrain all the terms of the functional) using a combined Bayesian inference of a large set of EoS-sensitive nuclear structure data. Beyond the usual ground state properties like binding energies and charge radii, we also included the much-discussed polarizabilities and parity-violating asymmetries of $^{208}$Pb and $^{48}$Ca, which put stringent constraints on the NMPs $J$ and $L$, both crucial for the symmetry energy.
The Bayesian analysis final result is a 10-dimensional multivariate probability distribution for the NMPs and Skyrme parameters. Marginalizing the distribution over all parameters but one allows for comparison with previous simpler analyses in the literature, which will be presented during the talk.
Furthermore, the posterior distribution can be used as a prior distribution in a successive Bayesian analysis, this time using astrophysical observations as constraints. This way, this second posterior distribution of NMPs will be informed by both nuclear physics and astrophysics. We will show that the constraints from nuclear experiments are well compatible with the theoretical predictions for infinite pure neutron matter from ab initio modelling, and those constraints additionally indicate the existence of interesting structures in the EoS of neutron stars. We will discuss the final predictions on some selected static properties of neutron stars, which can be computed from the distribution of NMPs. We will devote further attention to the composition of the star crust, which is computed consistently with the star EoS within the extended Thomas-Fermi formalism.
Speaker: Pietro Klausner (Università degli studi di Milano / LPC Caen - Université de Normandie Caen) -
15:30
Bayesian Constraints on Neutron Star Matter with Δ-Resonances 20m
We perform a Bayesian inference of the neutron star equation of state (EoS) including Δ-resonances, hyperons, and nucleons within a density-dependent relativistic hadron (DDRH) model. Using constraints from nuclear saturation properties, chiral effective field theory (χEFT), and astrophysical observations, we systematically investigate the impact of Δ-baryons on neutron star structure. Our EoS models are compatible with tidal deformability measurements from GW170817. Additionally, the presence of Δ-resonances enables a natural description of the low-mass compact object observed in HESS J1731-347. We further analyze the quasi-normal f-mode oscillations within a fully relativistic framework, highlighting strong correlations between the f-mode frequency, stellar compactness, and tidal deformability, offering potential observational signatures of the dense matter composition.
Speaker: Vishal Parmar (INFN Pisa) -
15:50
Frozen and β-equilibrated f and p modes of cold neutron stars: nuclear metamodel predictions 20m
This talk investigates the impact of assuming a barotropic equation of state (EoS) for neutron stars, which neglects potential out-of-β-equilibrium effects, on the calculation of oscillation modes. We focus on how the assumption of β-equilibrated EoS influences the frequencies of the non-radial fundamental (f) and first pressure (p1) modes. These calculations are performed using a wide set of neutron star models, each governed by different nucleonic EoS, generated through the metamodel technique. This framework integrates astrophysical observations, experimental nuclear physics, and chiral effective field theory. Additionally, we exploit quasi-universal relations to propose a method for estimating general relativity corrections to the commonly used Cowling approximation. Finally, we present a distribution of the expected f and p1 mode frequencies, which could be detectable by next-generation gravitational wave interferometers, offering new insights into neutron star structure.
Speaker: Gabriele Montefusco (CNRS - LPC Caen) -
16:10
Moat hyperons in cold neutron stars 20m
We investigate the hyperonic equation of state (EoS) within the non-linear derivative model that incorporates a momentum dependence on the interactions, with a special emphasis on properly establishing the conditions for hyperon appearance in neutron star matter. We demonstrate that hyperons can appear at finite momentum, forming a so-called “moat” region, even when they are
absent at zero momentum. Our study shows that this phenomenon significantly alters the composition and EoS of hyperonic matter as compared to the cases when it is not taken into account, highlighting the importance of treating the momentum dependence of
the baryon fields in dense matter appropriately.Speaker: Hristijan Kochankovski (Univ. Barcelona)
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Coffee Break
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25+5 minute talks: 1.9
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17:00
Locating the QCD critical point using neutron star observations 30m
We present a holographic framework designed to model strongly interacting matter relevant for the interiors of neutron stars. This approach captures key features of QCD at finite density and provides a controlled approximation for the nonperturbative regime inaccessible to standard methods. By calibrating the model to lattice data at low density, we obtain predictions for the equation of state across a wide density range. Our results are consistent with known nuclear matter properties at low densities and smoothly interpolate to the deconfined phase at higher densities, consistent with pQCD. We discuss implications for neutron star phenomenology and constraints from astrophysical observations.
Speaker: Niko Jokela (University of Helsinki) -
17:30
Strong coupling estimates of bulk viscosity in compact stars 30m
Estimating the value of bulk viscosity in neutron stars is crucial to determine the dampening of oscillations and gravitational wave emission in pulsars and binary mergers. The main contribution originates from weak processes through beta equilibration rates and depends both on the equation of state and weak equilibration rates. We use symmetry arguments in the form of Ward identities to derive general formulas for weak equilibration rates in terms of spectral functions of flavor currents. We then apply the general formulas to obtain the temperature dependence of weak rates and the bulk viscosity in strongly coupled quark matter using a holographic model. We compare the values obtained from the holographic model with perturbative estimates and with the bulk viscosity of pure QCD contribution at strong coupling.
Speaker: Carlos Hoyos (Universidad de Oviedo) -
18:00
Pseudoconformal behavior color-flavor-locked quark matter within a nonlocal chiral quark model 30m
We propose a three-flavor nonlocal Nambu–Jona-Lasinio model of quark matter with attractive scalar and diquark, and repulsive vector interaction channels to study the question whether an approximately conformal behavior of the strongly interacting quark matter in neutron star interiors is possible. The model qualitatively agrees with the perturbative quantum chromodynamics (pQCD), which predicts asymptotically conformal behavior of quark matter. In particular, the color-flavor-locked (CFL) color superconducting state is shown to be the ground state at asymptotically high densities. The conformal limit for the speed of sound and the dimensionless interaction measure is also shown to be reached from below and above, respectively. The developed equation of state is constrained by the results of a physics-informed Bayesian analysis using modern multi-messenger neutron star observations. It is shown that in the phenomenologically relevant range of parameters the model exhibits a narrow interval of densities close to the central densities of the heaviest neutron stars, where the speed of sound and dimensionless interaction measure simultaneously attain almost conformal values. A microscopic quantity, which characterizes single particle excitations of quarks and quantifies deviation from the conformal behavior of quark matter, is constructed to test the hypothesis of approximately conformal behavior of quark matter in neutron stars. Analysis of this quantity does not support the assumption about the nearly conformal behavior of quark matter even in the mentioned density range. Therefore, the apparent behavior of speed of sound and dimensionless interaction measure is denoted as pseudoconformal behavior.
[1] O. Ivanytskyi, Asymptotically conformal color-flavor-locked quark matter within a nonlocal chiral quark model, Phys. Rev. D 111 (2025) 034004.
[2] A. Ayriyan et al., Bayesian analysis of neutron star EOS with asymptoticaly conformal color-flavor-locked quark cores, in preparation (2025).Speaker: Oleksii Ivanytskyi (University of Wroclaw)
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