arXiv subjects

Aleksi Kurkela

Publications and source records attributed to Aleksi Kurkela.

At least 19 recordsLinked to original sources

arXiv2026

Imaging non-hydrodynamic modes with jet wakes

While studies of ultra-relativistic heavy-ion collisions have established that the quark--gluon plasma exhibits hydrodynamic behavior, direct signatures of non-hydrodynamic modes have remained elusive, and no observable is known to be exclusively sensitive to them. Here, we show that the angular structure of the jet wake provides such a probe. In the long-wavelength limit, hydrodynamics contributes only to the lowest angular moments of the detector image of the jet wake, while higher moments directly encode microscopic non-equilibrium dynamics. The jet wake thus serves as a spectroscopic probe of the medium's non-hydrodynamic sector. We develop a general kinetic-theory framework relating the angular moments of the late-time energy flux generated by a jet to the relaxation spectrum of the collision operator. In all models considered, non-hydrodynamic modes leave distinct imprints on the higher angular moments. Our results motivate precision measurements of the higher angular moments of the negative jet wake.

hep-ph↗

arXiv2026

Causality alone bounds the maximum radius difference between different-mass neutron stars

We investigate how the assumption of a common causal equation of state (EoS) correlates the radii of neutron stars at different masses and thereby reduces the uncertainties inferred from independent observations. We show that causality, anchored only to the chiral effective field theory ($χ$EFT) EoS near saturation density, places a closed-form upper bound on the radius difference, $R(2.0\,M_\odot)\le 1.16\,R(1.4\,M_\odot)-1.1\,$km. The bound is saturated exactly by a one-parameter family of EoSs that we construct analytically. Imposing this prior-independent causal ceiling on the independent NICER posteriors of PSR J0437-4715, PSR J0614-3329, and PSR J0740+6620 retains only 7.5% of their joint product distribution and removes the large-radius tail of the PSR J0740+6620 posterior. Unlike full EoS-informed inferences, our construction cleanly isolates the consequences of the generic physical assumptions of a common causal EoS from those associated with a particular choice of EoS prior, providing a transparent benchmark for interpreting neutron-star observations.

astro-ph.HE↗

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.

nucl-th↗

arXiv2026

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 $(μ, n, p)$ 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.

astro-ph.HE↗

arXiv2025

Solving the QCD effective kinetic theory with neural networks

Event-by-event QCD kinetic theory simulations are hindered by the large numerical cost of evaluating the high-dimensional collision integral in the Boltzmann equation. In this work, we show that a neural network can be used to obtain an accurate estimate of the collision integral in a fraction of the time required for the ordinary Monte Carlo evaluation of the integral. We demonstrate that for isotropic and anisotropic distribution functions, the network accurately predicts the time evolution of the distribution function, which we verify by performing traditional evaluations of the collision integral and comparing several moments of the distribution function. This work sets the stage for an event-by-event modeling of the pre-equilibrium initial stages in heavy-ion collisions.

hep-ph↗

arXiv2025

Machine learning approach to QCD kinetic theory

The effective kinetic theory (EKT) of QCD provides a possible picture of various non-equilibrium processes in heavy- and light-ion collisions. While there have been substantial advances in simulating the EKT in simple systems with enhanced symmetry, eventually, event-by-event simulations will be required for a comprehensive phenomenological modeling. As of now, these simulations are prohibitively expensive due to the numerical complexity of the Monte Carlo evaluation of the collision kernels. In this talk, we show how the evaluation of the collision kernels can be performed using neural networks paving the way to full event-by-event simulations.

hep-ph↗

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

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

astro-ph.HE↗

arXiv2025

Updated Astrophysical Equation-of-State Constraints on the Color-Superconducting Gap

We summarize and update using new NICER measurements the results of arXiv:2401.16253, in which we used various astrophysical neutron-star observations to set an upper bound on the CFL color-superconducting gap in a range of baryon chemical potentials $μ_B \in [2.1,3.2]$, above those reached within neutron stars. We also corroborate the ``reasonable" constraint from arXiv:2401.16253 on the maximum value of the color-superconducting gap by performing a new Bayesian analysis using a prior that extends a two-segment Gaussian process connecting the whole density range between CEFT and pQCD.

hep-ph↗

arXiv2025

From existing and new nuclear and astrophysical constraints to stringent limits on the equation of state of neutron-rich dense matter

Through continuous progress in nuclear theory and experiment and an increasing number of neutron-star observations, a multitude of information about the equation of state (EOS) for matter at extreme densities is available. To constrain the EOS across its entire density range, this information needs to be combined consistently. However, the impact and model-dependency of individual observations vary. We present a broad compendium of different constraints and apply them individually to a large set of EOS candidates within a Bayesian framework. Specifically, we explore different ways how chiral effective field theory and perturbative quantum chromodynamics can be used to place a likelihood on EOS candidates. We also investigate the impact of nuclear experimental constraints, as well as different radio and X-ray observations of neutron star (NS) masses and radii. This is augmented by reanalyses of the existing data from BNS coalescences, in particular of GW170817, with improved models for the tidal waveform and kilonova light curves, which we also utilize to construct a tight upper limit of 2.39$\,$M$_\odot$ on the TOV mass based on GW170817's remnant. Our diverse set of constraints is eventually combined to obtain stringent limits on NS properties. We organize the combination in a way to distinguish between constraints where the systematic uncertainties are deemed small and those that rely on less conservative assumptions. For the former, we find the radius of the canonical 1.4$\,$M$_\odot$ neutron star to be $R_{1.4}= 12.26_{-0.91}^{+0.80}\,$km and the TOV mass at $M_{\rm TOV}= 2.25_{-0.22}^{+0.42}\,$M$_\odot$ (95% credibility). Including all the presented constraints yields $R_{1.4}= 12.20_{-0.48}^{+0.50}\,$km and $M_{\rm TOV}= 2.30_{-0.20}^{+0.07}\,$M$_\odot$.

astro-ph.HE↗

arXiv2025

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

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.

hep-ph↗

arXiv2024

Jet quenching parameter in QCD kinetic theory

We study the jet quenching parameter $\hat q$ in a non-equilibrium plasma using the QCD effective kinetic theory. We discuss subleading terms at large jet momentum p, show that our expression for $\hat q$ reproduces thermal results at small and large transverse momentum cutoffs for infinite p and construct an interpolation between these limits to be used in phenomenological applications. Using simple non-equilibrium distributions that model pertinent features of the bottom-up thermalization scenario, we analytically assess how anisotropy, under- or overoccupation affect the jet quenching parameter. Our work provides more details on the $\hat q$ formula used in our preceding work [arXiv:2303.12595] and sets the stage for further numerical studies of jet momentum broadening in the initial stages of heavy-ion collisions from QCD kinetic theory.

hep-ph↗

arXiv2024

Astrophysical Equation-of-State Constraints on the Color-Superconducting Gap

We demonstrate that astrophysical constraints on the dense-matter equation of state place an upper bound on the color-superconducting gap in dense matter above the transition from nuclear matter to quark matter. Pairing effects in the color-flavor locked (CFL) quark matter phase increase the pressure at high density, and if this effect is sufficiently large then the requirements of causality and mechanical stability make it impossible to reach such a pressure in a way that is consistent with what is known at lower densities. The intermediate-density equation of state is inferred by considering extensions of chiral effective field theory (CEFT) to neutron star densities, and conditioning these using current astrophysical observations of neutron star radius, maximum mass, and tidal deformability (PSR J0348+0432, PSR J1624-2230, PSR J0740+6620, GW170817). At baryon number chemical potential $μ= 2.6~\text{GeV}$ we find a 95% upper limit on the CFL pairing gap $Δ$ of $457~\text{MeV}$ using overly conservative assumptions and $216~\text{MeV}$ with more reasonable assumptions. This constraint may be strengthened by future astrophysical measurements as well as by future advances in high density QCD calculations.

astro-ph.HE↗

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.

hep-ph↗

arXiv2024

Jet momentum broadening during initial stages in heavy-ion collisions

We study the jet quenching parameter $\hat q$ in the initial pre-equilibrium stages of heavy-ion collisions using the QCD kinetic theory description of the anisotropic quark-gluon plasma. This allows us to smoothly close the gap in the literature between the early glasma stage of the collision and the onset of hydrodynamics. We find that the pre-hydrodynamic evolution of $\hat q$ during the bottom-up kinetic scenario shows little sensitivity to the initial conditions, jet energies and models of the transverse momentum cutoff. We also observe that, similarly to the glasma case, the jet quenching parameter is enhanced along the beam axis as compared to the transverse direction during most of the kinetic evolution.

hep-ph↗

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.

nucl-th↗

arXiv2024

AMY Lorentz invariant parton cascade -- the thermal equilibrium case

We introduce ALPACA, a Lorentz invariant parton cascade encoding the AMY effective kinetic theory of QCD at high temperatures. It solves the Boltzmann equation by explicitly simulating the evolution of parton ensembles corresponding to single events. We discuss how the effective masses and temperature entering the elastic collision and splitting/merging rates can be estimated from just a single event. We perform an extensive validation of the framework by showing that it reproduces the expected behaviour in thermal equilibrium.

hep-ph↗