arXiv subjects

Tyler Gorda

Publications and source records attributed to Tyler Gorda.

At least 19 recordsLinked to original sources

arXiv2026

As above, so below: assessing extremeness of the neutron-star equation of state based on the unstable branch

Microscopic models of neutron-star matter have been widely used in astrophysical applications. The focus of attention has been on densities up to the maximal densities reached in stable neutron stars. The possibility that the underlying model assumptions may have important implications at higher densities has not been addressed. Here, we show that the behaviour at higher densities is strongly constrained by requiring a causal, stable, and thermodynamically consistent extension to the perturbative-QCD regime. We explicitly reveal what that behaviour must be and provide a tool for constructing and visualizing such extensions. We find that purely hadronic models trusted up to the maximal central density often require radically different behaviour at higher densities from that assumed in the original model, while models with additional degrees of freedom fare better. Our analysis disfavors purely nucleonic models for describing all stable neutron stars and supports the appearance of some type of additional degrees of freedom in stable massive neutron stars.

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arXiv2026

Static linear response of hot and dense QCD matter to electromagnetic fields: Leading hard and soft QCD corrections

We compute the static electromagnetic susceptibilities of a hot and dense quark-gluon plasma using perturbative Quantum Chromodynamics (QCD). Our evaluation includes the leading $\mathcal{O}(\alpha_s)$ correction as well as the leading soft, resummed contribution of $\mathcal{O}(\alpha_s^{3/2})$ within electrostatic QCD. By matching to Lattice QCD at vanishing baryon chemical potential through Lattice perturbation theory, we establish a connection between perturbative results and Lattice simulations and assess the size of higher-order corrections. This extends the electromagnetic susceptibilities to finite baryon chemical potential, where Lattice methods are not applicable and establishes first-principle constraints on the quark-gluon plasma's electromagnetic response at temperatures and densities relevant for intermediate-energy heavy-ion collisions.

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arXiv2025

Strong model-agnostic constraints for twin-star solutions

We perform a model-agnostic Bayesian analysis of the neutron-star-matter equation of state (EoS), using known ab-initio constraints and astrophysical observations to limit its behavior at intermediate densities. Permitting explicit first-order phase transitions allows us to systematically search for twin-star solutions, i.e. the existence of stars degenerate in mass but differing in radius. We find that current observational constraints exclude all but two classes of twin stars. The first is characterized by a first-order transition occurring at a very low density, where the material properties of the system either stay largely intact or move away from the conformal limit. In the second, more interesting class, the discontinuity in the mass-radius curve emerges after a rapid crossover transition at a significantly higher density, with the speed of sound exhibiting two sharp peaks at distinct densities. Since neither class shows clear conformalization upon entering the second branch, the standard twin-star scenario linking the mass-radius discontinuity to deconfinement can be firmly ruled out, while even the remaining solutions -- disfavored by per-mille Bayes factors and in tension with theoretical bounds -- are likely to be excluded in the future.

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arXiv2025

Constrained Gaussian-process bridge prior for neutron-star equation-of-state inference

We set forth a new method for generating model-agnostic, nonparametric priors for neutron star equation-of-state inference that are stable, causal and thermodynamically consistent by construction. This generalizes Gaussian processes to include global thermodynamic constraints, specifically allowing the inclusion of any number of training points in the form $(\mu, n, p)$ while retaining thermodynamic consistency between them. The method is based on constructing constrained Gaussian-process bridges, whose correlation properties can be tuned at will allowing flexibility between a conservative prior and a theory-informed prior. The method does not require any shooting to obey multiple constraints and provides an efficient and informed way to include both chiral effective field theory and perturbative quantum chromodynamics constraints within the same framework.

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arXiv2025

A new approach to determine the thermodynamics of deconfined matter to high accuracy

We demonstrate that at finite density and sufficiently high temperatures, phase-quenched (PQ) lattice simulations combined with perturbation theory provide a new precision approach to determining the thermodynamics of QCD across a wide arc of the phase diagram where the strong coupling constant $\alpha_s$ remains small. In this regime, nonperturbative pairing effects in the PQ theory are parametrically suppressed, so that the difference between the PQ and full QCD pressures becomes a small perturbative correction. We compute this correction up to and including $O(\alpha_s^{7/2})$ using electrostatic QCD together with a novel numerical method to compute four-loop sum-integrals. This enables the determination of the perturbative QCD pressure with precision beyond the current state of the art while including nonperturbative pure-gluonic contributions from the lattice.

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arXiv2025

Listening to the long ringdown: A novel way to pinpoint the EOS in neutron-star cores

Gravitational waves (GWs) from binary neutron star (BNS) merger remnants complement constraints from the inspiral phase, mass-radius measurements, and microscopic theory by providing information about the neutron-star equation of state (EOS) at extreme densities. We perform general-relativistic simulations of BNS mergers using EOS models that span the uncertain high-density regime. We find a robust correlation between the ratio of energy and angular momentum lost during the late-time post-merger GW signal - the long ringdown - and the EOS at the highest densities in neutron star cores. Applying this correlation to post-merger GW signals reduces EOS uncertainty at several times saturation density, where no direct constraints currently exist.

astro-ph.HE↗

arXiv2025

Color superconductivity under neutron-star conditions at next-to-leading order

The equation of state of deconfined strongly interacting matter at high densities remains an open question, with effects from quark pairing in the preferred color-flavor-locked (CFL) ground state possibly playing an important role. Recent studies suggest that at least large pairing gaps in the CFL phase are incompatible with current astrophysical observations of neutron stars. At the same time, it has recently been shown that in two-flavor quark matter, subleading corrections from pairing effects can be much larger than would be na\"ively expected, even for comparatively small gaps. In the present Letter, we compute next-to-leading-order corrections to the pressure of quark matter in the CFL phase arising from the gap and the strong coupling constant, incorporating neutron-star equilibrium conditions and current state-of-the-art perturbative QCD results. We find that the corrections are again quite sizable, and they allow us to constrain the CFL gap in the quark energy spectrum to $\Delta_{\rm CFL} \lesssim 140~{\rm MeV}$ at a baryon chemical potential $\mu_{\rm B} = 2.6~{\rm GeV}$, even when allowing for a wide range of possible behaviors for the dependence of the gap on the chemical potential.

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arXiv2024

NNNLO pressure of cold quark matter: leading logarithm

At high baryon chemical potential $μ_B$, the equation of state of QCD allows a weak-coupling expansion in the QCD coupling $α_s$. The result is currently known up to and including the full next-to-next-to-leading order (NNLO) $α_s^2$. Starting at this order, the computations are complicated by the modification of particle propagation in a dense medium, which necessitates non-perturbative treatment of the scale $α_s^{1/2} μ_B$. In this work, we apply a Hard-Thermal-Loop scheme for capturing the contributions of this scale to the weak-coupling expansion, and use it to determine the leading-logarithm contribution to NNNLO: $α_s^3 \ln^2 α_s$. This result is the first improvement to the equation of state of massless cold quark matter in 40 years. The new term is negligibly small, and thus significantly increases our confidence in the applicability of the weak-coupling expansion.

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arXiv2024

Pressure and speed of sound in two-flavor color-superconducting quark matter at next-to-leading order

Deconfined quark matter at asymptotically high densities is weakly coupled, due to the asymptotic freedom of Quantum Chromodynamics. In this weak-coupling regime, bulk thermodynamic properties of quark matter, assuming a trivial ground state, are currently known to partial next-to-next-to-next-to-leading order. However, the ground state at high densities is expected to be a color superconductor, in which the excitation spectrum of (at least some) quarks exhibit a gap with a non-perturbative dependence on the strong coupling. In this work, we calculate the thermodynamic properties of color-superconducting quark matter at high densities and zero temperature at next-to-leading order (NLO) in the coupling in the presence of a finite gap. We work in the limit of two massless quark flavors, which corresponds to deconfined symmetric nuclear matter, and further assume that the gap is small compared to the quark chemical potential. In these limits, we find that the NLO corrections to the pressure and speed of sound are comparable in size to the leading-order effects of the gap, and further increase both quantities above their values for non-superconducting quark matter. We also provide a parameterization of the NLO speed of sound to guide phenomenology in the high-density region, and we furthermore comment on whether our findings should be expected to extend to the case of three-flavor quark matter of relevance to neutron stars.

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arXiv2024

Constraining the equation of state in neutron-star cores via the long-ringdown signal

Multimessenger signals from binary neutron star (BNS) mergers are promising tools to infer the largely unknown properties of nuclear matter at densities that are presently inaccessible to laboratory experiments. The gravitational waves (GWs) emitted by BNS merger remnants, in particular, have the potential of setting tight constraints on the neutron-star equation of state (EOS) that would complement those coming from the late inspiral, direct mass-radius measurements, or ab-initio dense-matter calculations. To explore this possibility, we perform a representative series of general-relativistic simulations of BNS systems with EOSs carefully constructed so as to cover comprehensively the high-density regime of the EOS space. From these simulations, we identify 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. When applying this correlation to post-merger GW signals, we find a significant reduction of the EOS uncertainty at densities several times the nuclear saturation density, where no direct constraints are currently available. Hence, the long ringdown has the potential of providing new and stringent constraints on the state of matter in neutron stars in general and, in particular, in their cores.

astro-ph.HE↗

arXiv2024

Equation of state at neutron-star densities and beyond from perturbative QCD

We explore the consequences of imposing robust thermodynamic constraints arising from perturbative Quantum Chromodynamics (QCD) when inferring the dense-matter equation-of-state (EOS). We find that the termination density, up to which the EOS modeling is performed in an inference setup, strongly affects the constraining power of the QCD input. This sensitivity in the constraining power arises from EOSs that have a specific form, with drastic softening immediately above the termination density followed by a strong stiffening. We also perform explicit modeling of the EOS down from perturbative-QCD densities to construct a new QCD likelihood function that incorporates additional perturbative-QCD calculations of the sound speed and is insensitive to the termination density, which we make publicly available.

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arXiv2024

Estimate for the bulk viscosity of strongly coupled quark matter using perturbative QCD and holography

Modern hydrodynamic simulations of core-collapse supernovae and neutron-star mergers require knowledge not only of the equilibrium properties of strongly interacting matter, but also of the system's response to perturbations, encoded in various transport coefficients. Using perturbative and holographic tools, we derive here an improved weak-coupling and a new strong-coupling result for the most important transport coefficient of unpaired quark matter, its bulk viscosity. These results are combined in a simple analytic pocket formula for the quantity that is rooted in perturbative Quantum Chromodynamics at high densities but takes into account nonperturbative holographic input at neutron-star densities, where the system is strongly coupled. This expression can be used in the modeling of unpaired quark matter at astrophysically relevant temperatures and densities.

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arXiv2024

Strongly interacting matter exhibits deconfined behavior in massive neutron stars

Neutron-star cores contain matter at the highest densities in our Universe. This highly compressed matter may undergo a phase transition where nuclear matter melts into deconfined quark matter, liberating its constituent quarks and gluons. Quark matter exhibits an approximate conformal symmetry, predicting a specific form for its equation of state (EoS), but it is currently unknown whether the transition takes place inside at least some physical neutron stars. Here, we quantify this likelihood by combining information from astrophysical observations and theoretical calculations. Using Bayesian inference, we demonstrate that in the cores of maximally massive stars, the EoS is consistent with quark matter. We do this by establishing approximate conformal symmetry restoration with high credence at the highest densities probed and demonstrating that the number of active degrees of freedom is consistent with deconfined matter. The remaining likelihood is observed to correspond to EoSs exhibiting phase-transition-like behavior, treated as arbitrarily rapid crossovers in our framework.

astro-ph.HE↗

arXiv2023

Quark matter and nuclear astrophysics: recent developments

Does deconfined cold quark matter occur in nature? This is currently one of the fundamental open questions in nuclear astrophysics. In these proceedings, I review the current state-of-the-art techniques to address this question in a model-agnostic manner, by synthesizing inputs from astrophysical observations of neutron stars and their binary mergers, and first-principles calculations within nuclear and particle theory. I highlight recent improvements in perturbative calculations in asymptotically dense cold quark matter, as well as compelling evidence for a conformalizing transition within the cores of massive neutron stars.

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arXiv2023

Bounding the QCD Equation of State with the Lattice

The equation of state of QCD matter at high densities is relevant for neutron star structure and for neutron star mergers and has been a focus of recent work. We show how lattice QCD simulations, free of sign problems, can provide an upper bound on the pressure as a function of quark chemical potentials. We show that at large chemical potentials this bound should become quite sharp; the difference between the upper bound on the pressure P-phase-quenched and the true pressure P is of order alpha^3 P. The corrections arise from a single Feynman diagram; its calculation would render remaining corrections of order alpha^4 P.

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arXiv2023

Equation of State of Cold Quark Matter to $O(α_s^3 \ln α_s)$

Accurately understanding the equation of state (EOS) of high-density, zero-temperature quark matter plays an essential role in constraining the behavior of dense strongly interacting matter inside the cores of neutron stars. In this Letter, we study the weak-coupling expansion of the EOS of cold quark matter and derive the complete, gauge-invariant contributions from the long-wavelength, dynamically screened gluonic sector at next-to-next-to-next-to-leading order (N3LO) in the strong coupling constant $α_s$. This elevates the EOS result to the $O(α_s^3 \ln α_s)$ level, leaving only one unknown constant from the unscreened sector at N3LO, and places it on par with its high-temperature counterpart from 2003.

hep-ph↗

arXiv2023

Constraints on strong phase transitions in neutron stars

We study current bounds on strong first-order phase transitions (PTs) along the equation of state (EOS) of dense strongly interacting matter in neutron stars, under the simplifying assumption that on either side of the PT the EOS can be approximated by a simple polytropic form. We construct a large ensemble of possible EOSs of this form, anchor them to chiral effective field theory calculations at nuclear density and perturbative QCD at high densities, and subject them to astrophysical constraints from high-mass pulsars and gravitational-wave observations. Within this setup, we find that a PT permits neutron-star solutions with larger radii, but only if the transition begins below twice nuclear saturation density. We also identify a large parameter space of allowed PTs currently unexplored by numerical-relativity studies. Additionally, we locate a small region of parameter space allowing twin-star solutions, though we find them to only marginally pass the current astrophysical constraints. Finally, we find that sizeable cores of high-density matter beyond the PT may be located in the centers of some stable neutron stars, primarily those with larger masses.

astro-ph.HE↗

arXiv2023

Bayesian uncertainty quantification of perturbative QCD input to the neutron-star equation of state

The equation of state of neutron-star cores can be constrained by requiring a consistent connection to the perturbative Quantum Chromodynamics (QCD) calculations at high densities. The constraining power of the QCD input depends on uncertainties from missing higher-order terms, the choice of the unphysical renormalization scale, and the reference density where QCD calculations are performed. Within a Bayesian approach, we discuss the convergence of the perturbative QCD series, quantify its uncertainties at high densities, and present a framework to systematically propagate the uncertainties down to neutron-star densities. We find that the effect of the QCD input on the neutron-star inference is insensitive to the various unphysical choices made in the uncertainty estimation.

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