arXiv subjects

Volodymyr Vovchenko

Publications and source records attributed to Volodymyr Vovchenko.

At least 19 recordsLinked to original sources

arXiv2026

QCD thermodynamics through the crossover as a gas of confining strings with repulsive interactions

We investigate QCD thermodynamics in the intermediate-temperature regime using a gas of confining strings supplemented by excluded volume (EV) corrections. In this description, the discrete hadron resonance spectrum is replaced by an exponentially growing open-string spectrum characterized by a Hagedorn temperature $T_H$, related to the confining string tension. With a common EV parameter shared by mesons and baryons, the model gives a good description of bulk lattice-QCD thermodynamics above the pseudocritical temperature. Among the values considered, $T_H \simeq 300\,\text{MeV}$ provides the best agreement, lower than the value $T_H \simeq 320\!-\!340\,\text{MeV}$ inferred from ideal-string-gas fits to the vacuum hadron spectrum. A simultaneous calibration of the mesonic and baryonic EV parameters gives $b_M \simeq 0.19\,\text{fm}^3$ and $b_B \simeq 0.73\,\text{fm}^3$. Within the present spectral ansatz, the larger baryonic EV parameter indicates that stronger effective suppression is required in the baryon sector, although noticeable tensions remain in several conserved-charge observables.

hep-ph↗

arXiv2026

Subensemble Acceptance Method 3.0: General Corrections to Cumulants from Exact Conservation Constraints

We present the subensemble acceptance method 3.0 (SAM-3.0), which corrects cumulants of an observable measured in a subsystem of a large system for the effect of exact global conservation of multiple charges. The required input is the set of joint grand-canonical cumulants of the acceptance observable with the total event charges, from which the canonical cumulants follow algebraically via a closed recursion based on (multivariate) partial exponential Bell polynomials. The framework accommodates any number of observables, including non-conserved quantities such as net protons, and any number of simultaneously conserved charges, including the total energy, which yields the microcanonical ensemble. The mapping contains SAM-1.0 and SAM-2.0 as special cases and, unlike SAM-2.0, reproduces the exact binomial-acceptance limit. We also derive the leading finite-size corrections from the saddle-point expansion. We apply the method to update the hydrodynamics-based non-critical baseline (Hydro-EV) for net-proton cumulants at RHIC-BES energies, finding a refined baseline that agrees with direct canonical Monte Carlo sampling and stays close to the earlier SAM-2.0 result. We further validate the formalism against direct Monte Carlo sampling with exact simultaneous conservation of baryon number, electric charge, and strangeness, including hadronic-afterburner effects.

hep-ph↗

arXiv2026

Hadron resonance gas with density-dependent interactions for neutron stars and heavy-ion collisions

We present a density-dependent generalization of the van der Waals hadron resonance gas model (DD-HRG) for describing both the hot hadronic matter created in heavy-ion collisions and the cold, dense matter inside neutron stars. Non-resonant interactions are incorporated through a generalized excluded-volume prescription with a density-dependent available-volume fraction, supplemented by an arbitrary density-dependent mean field. With isospin-dependent interaction parameters constrained by empirical properties of nuclear matter, the resulting equation of state extends the causality range to include neutron-star interiors and supports two-solar-mass stars. It also improves the description of lattice QCD thermodynamics and conserved-charge susceptibilities at vanishing baryochemical potential, with lattice data favoring reduced repulsion among strange baryons. This DD-HRG framework is available within the latest version of the open-source Thermal-FIST package.

nucl-th↗

arXiv2026

Extraction of baryon number susceptibilities at finite density from heavy-ion collisions

We present, to our knowledge, the first Bayesian extraction of baryon number susceptibilities of QCD matter at finite baryon density from heavy-ion collision data on proton number cumulants. The framework embeds arbitrary equation-of-state susceptibilities $χ_n^B$ into realistic hydrodynamic particlization hypersurfaces through maximum-entropy freeze-out, maps the resulting (anti)baryon fluctuations onto protons, applies the experimental kinematic acceptance, and accounts for exact baryon number conservation. Applying the framework to measurements of (net-)proton number fluctuations in 0--5\% central Au-Au collisions from the RHIC Beam Energy Scan in the collider mode, we extract, at each collision energy, the second-order susceptibility normalized by the hadron resonance gas value, $χ_{2}^B/\barχ_{2}^B$, and the higher-order susceptibility ratios $χ_{3}^B/χ_{1}^B$ and $χ_{4}^B/χ_{2}^B$. We obtain tight constraints on $χ_{2}^B$, with extracted values in quantitative agreement with lattice QCD based estimates along the chemical freeze-out line for $μ_B \lesssim 300$~MeV. At larger $μ_B$, the extracted values indicate an enhancement of baryon number fluctuations relative to the noncritical lattice-based extrapolation and HRG baseline considered here. The third- and fourth-order susceptibilities are only weakly constrained. In particular, we find that the observed nonmonotonic collision-energy dependence of the proton factorial cumulant ratio $\hat{C}_{3}/\hat{C}_{1}$ can be described without irreducible three- or four-baryon correlations. This behavior emerges from the interplay between the energy dependence of $χ_{2}^B$ and exact baryon number conservation.

hep-ph↗

arXiv2026

Lattice-based equation of state with a critical point from constant entropy contours and its comparison to effective QCD approaches

In this work, we systematically assess the performance of a new method from [H. Shah et al., Phys. Rev. C 113, L012201] for locating the QCD critical point using constant-entropy contours by testing it against various effective QCD approaches. We demonstrate that, while the method yields spurious critical points in purely hadronic models (HRG) due to non-parabolic contour behavior at low temperatures ($T \lesssim 120$ MeV), it accurately reproduces the CP location in frameworks that feature a genuine phase transition and benchmarked against lattice QCD, such as Holographic Einstein-Maxwell-Dilaton, and Functional QCD approaches. Building on our previous determination of constant entropy contours using lattice data, we extend that analysis to construct a complete Lattice-based Equation of State (EoS) at finite density, which features a critical point at $(T, μ_B) \approx (114, 602)$ MeV. By integrating the extrapolated entropy density with respect to temperature, we reconstruct the pressure, baryon density, susceptibility, and speed of sound in the critical region, and analyze the focusing behavior of isentropic trajectories in the vicinity of the critical point.

hep-ph↗

arXiv2026

Hadronic and partonic composition of QCD matter across the crossover

We construct a simple equation of state of strongly interacting matter at zero chemical potentials that provides a unified description of lattice QCD thermodynamics in terms of hadronic and partonic degrees of freedom. The hadronic phase is described by the quantum van der Waals hadron resonance gas, extended by excluded-volume repulsion between mesons, while the quark-gluon plasma is modeled as an ideal gas of quarks and gluons supplemented with a phenomenological interaction term proportional to $T^3$. The two regimes are connected by a smooth crossover switching function. The three model parameters - the meson hard-core radius, the strength of the partonic interaction term, and the switching temperature - are determined from a fit to lattice QCD results for the trace anomaly. The resulting equation of state reproduces the lattice data on the pressure, entropy density, energy density, and speed of sound in the temperature range $T=100$-$500$ MeV. The fit yields a meson hard-core radius $r_M \simeq 0.2$ fm, a partonic interaction scale $A \simeq 600$ MeV, and a switching temperature $T_0 \simeq 216$ MeV, substantially exceeding both the pseudocritical temperature of the QCD chiral crossover and the chemical freeze-out temperature. This finding suggests that the transition from hadronic to partonic degrees of freedom is considerably more gradual than indicated by the chiral pseudocritical temperature alone, with hadronic states remaining an important component of strongly interacting matter up to temperatures of about $250$ MeV, well above the QCD chiral crossover.

nucl-th↗

arXiv2026

QCD critical surface from constant entropy contours

We provide the first mapping of the critical surface in (2+1)-flavor QCD in the full $(T,μ_B,μ_Q,μ_S)$ space, anchored on lattice QCD results at vanishing chemical potentials and obtained within an expansion along contours of constant entropy density. In the pure $μ_B$ direction, this framework yields a critical point at $(T_c,μ_{B,c}) \simeq (114,\, 602)$ MeV. Here we extend the construction to arbitrary directions in the three-dimensional chemical-potential space, parametrized by spherical coordinates $(μ,θ,φ)$, with the radial expansion truncated at $\mathcal{O}(μ^2)$. The resulting two-dimensional surface carries a direction-dependent critical temperature $T_c(θ,φ)$ and baryochemical potential $μ_{B,c}(θ,φ)$, which quantify the shift of the critical point relative to the pure $μ_B$ direction. We find that $μ_{B,c}$ increases by 40-100 MeV along the approximately strangeness neutral direction [$μ_S \approx (0.15$--$0.33)\, μ_B$, $μ_Q \approx 0$] relevant for heavy-ion collisions, while the critical temperature stays essentially unchanged. In the charge-neutral, weak-equilibrium direction~[$μ_Q \approx -(0.05$--$0.1) \,μ_B$, $μ_S = 0$] relevant for neutron star mergers, the critical point, and the associated first-order phase transition, remain present at essentially the same location in the $(T,μ_B)$ plane. We find no evidence for a critical point at large isospin densities, $|μ_Q| / μ_B \gtrsim 1$, relevant for cosmic trajectories in the early Universe, nor along the pure electric-charge or strangeness directions, at least outside the regions where pion or kaon condensation may occur.

nucl-th↗

arXiv2026

Studying the QCD Matter produced in Heavy-Ion Collisions using the MUSES Calculation Engine

The equation of state of hot and dense matter is essential for describing heavy-ion collisions at all collision energies. Here, we explore the capabilities of the latest version of the MUSES Calculation Engine, $\textit{Calliope}$, focusing on software modules and workflows that compute the equation of state and observable properties of the matter produced in heavy-ion collisions. These include several equations of state, ranging from first-principles lattice QCD to phenomenological approaches, with or without a critical point, and with phase-space dimensionality ranging from two dimensions defined by temperature $T$ and baryon chemical potential $μ_B$, to four dimensions after the addition of strangeness and electric-charge chemical potentials $μ_S$ and $μ_Q$. We also discuss modules that provide additional thermodynamic quantities and observables relevant for heavy-ion modeling, including elements of the pressure Hessian matrix and transport coefficients. Workflow examples are constructed that merge two equations of state thermodynamically consistently to extend phase-diagram coverage, and feed the results into an equation of state inverter to produce inputs suitable for hydrodynamic simulations. Finally, we apply this framework to perform a relativistic viscous hydrodynamic simulation with equations of state with an extended $T$ and $μ_B$ coverage and a movable critical point, including effects from transport coefficients that phenomenologically encode critical scaling, at collision energies $\sqrt{s_{NN}}=7.7, 19.6$, and $39$ GeV.

nucl-th↗

arXiv2026

Electric charge fluctuations from lattice QCD in the continuum limit

Electric charge fluctuations $χ_n^Q$ allow comparisons between theory and experiment, but are elusive on the lattice due to severe cutoff effects. We use a 4HEX action to obtain $χ_2^Q$ and, for the first time ever, $χ_4^Q$ in the continuum limit. We find disagreement with the hadron resonance gas (HRG) model, which we cannot explain with finite volume effects. We include light meson interactions in the HRG model via the S-matrix, reducing the tension for $χ_4^Q$, but worsening the agreement for $χ_2^Q$. We propose measuring the ratio $χ_4^Q/χ_2^Q$ at the LHC to investigate this tension.

hep-lat↗

arXiv2026

Exploring the QCD phase diagram through correlations and fluctuations

The exploration of the Quantum Chromodynamics (QCD) phase diagram is a central goal of relativistic heavy-ion collision experiments. This review focuses on the role of fluctuations and correlations as sensitive probes of the phase structure. We discuss theoretical advancements and experimental methodologies employed to map the QCD phase diagram, highlighting constraints derived from both lattice QCD calculations and existing experimental data. Key observables such as cumulants and factorial cumulants of conserved charges (e.g., net-proton, net-charge) are explored as promising signatures of phase transitions and the QCD critical point. We discuss how these quantities are measured experimentally and compared with theoretical predictions, addressing challenges and best practices for meaningful comparisons. Special attention is given to predictions and current experimental results at high baryon density, including recent findings from the STAR collaboration at RHIC. Finally, we identify open issues and future directions for fluctuation and correlation studies at lower collision energies, relevant for future measurements, for example by the CBM experiment.

nucl-th↗

arXiv2026

Canonical statistical hadronization with local baryon conservation for higher-order cumulants

We study higher-order cumulants of the conserved baryon number at the LHC within the canonical ensemble with local baryon conservation. We generalize the density correlations approach of [Phys. Rev. C 110, L061902 (2024)] to incorporate the effect of Gaussian local conservation in spatial rapidity space in cumulants up to 6th order. Gaussian local conservation improves upon the commonly employed $V_c$ approach, yielding comparable predictions at midrapidity, but marked differences for larger rapidity acceptances. Our coordinate-space results are in exact agreement with the diffusion master equation approach for all cumulant ratios up to $κ_6/κ_2$. Using the blast-wave model to apply kinematic cuts, we obtain predictions for net-proton cumulants in O--O and Pb--Pb collisions at the LHC that establish an ideal hadron gas baseline. We find that local baryon conservation alone can drive $κ_6/κ_2$ to small or even negative values in restricted acceptance, a behavior often associated with chiral criticality. The conservation baseline must therefore be carefully accounted for when interpreting upcoming LHC measurements.

hep-ph↗

arXiv2026

Why are the dilepton temperatures at the relativistic heavy-ion colliders are constant, T ~ 0.3 GeV?

The STAR collaboration at RHIC and the ALICE collaboration at the LHC have reported dielectron spectra in the intermediate mass region, M = (1-3) GeV, which reveal a strikingly constant, energy-independent emission temperature $T_{IMR} \simeq 0.3~\textrm{GeV}$ over a broad range of collision energies, $\sqrt{s_{NN}} = 27 - 5020~\textrm{GeV}$. This unexpected ''thermostat'' behavior raises fundamental questions: why does the temperature remain constant despite increasing collision energy,and what mechanism governs this apparent universality?

hep-ph↗

arXiv2026

Disentangling baryon stopping and neutron skin effects in heavy-ion collisions

We analyse the net baryon (B) and net electric charge (Q) stopping in heavy-ion collisions using the statistical model. Focusing first on isobar collisions $\rm{Ru}+\rm{Ru}$ and $\rm{Zr}+\rm{Zr}$ at $\sqrt{s_{\rm NN}}=200$~GeV, we show that the excess baryon-stopping parameter $γ_B \equiv (N_B/N_Q)\,(Z/A)$ can be quantitatively extracted from forthcoming RHIC measurements of charge- and baryon-stopping ratios. We then generalize the approach to other collision systems at RHIC and LHC energies and introduce an oxygen-baseline observable, $r^{OX}$, whose central-to-peripheral ratio exhibits strong and systematic sensitivity to the neutron-skin thickness of the target nucleus $X$.

hep-ph↗

arXiv2026

Charmed nuclei and exotic charmed meson production at CBM@FAIR and ALICE@LHC

We make predictions for the expected multiplicities of exotic charmed hadrons and charmed nuclei in Au+Au collisions at SIS100 and LHC beam energies, using input on light hadron and charm production from the UrQMD transport model, and applying the Thermal-FIST model. We demonstrate that the CBM experiment has the capability to explore these states with production rates of one per 3 seconds for $χ_{c0}(1P)$ and $χ_{c1}(1P)$, and one every 3 minutes for $X(3872)$ at the expected data taking rates. Due to the higher baryon density at CBM compared to the LHC, charmed nuclei, if they exist, will be equally abundant at CBM as at the LHC, even though the total charm production at CBM is much lower.

hep-ph↗

arXiv2026

Locating the QCD critical point through contours of constant entropy density

We propose a new method to investigate the existence and location of the conjectured high-temperature critical point of strongly interacting matter via contours of constant entropy density. By approximating these lines as a power series in the baryon chemical potential $μ_B$, one can extrapolate them from first-principle results at zero net-baryon density, and use them to locate the QCD critical point, including the associated first-order and spinodal lines. As a proof of principle, we employ currently available continuum-extrapolated first-principle results from the Wuppertal--Budapest collaboration to find a critical point at a temperature and a baryon chemical potential of $T_c = 114.3 \pm 6.9$ MeV and $μ_{B,c} = 602.1 \pm 62.1$ MeV, respectively. We advocate for a more precise determination of the required expansion coefficients via lattice QCD simulations as a means of pinpointing the location of the critical endpoint in the phase diagram of strongly interacting matter.

hep-ph↗

arXiv2026

Systematics of the chemical freeze-out line in the high baryon density regime explored at SIS100

The systematic uncertainties of chemical freeze-out fits at SIS100 energies (Au+Au reactions at $\sqrt{s_{NN}}=3-5$ GeV) are studied using UrQMD simulations. Although hadron production in UrQMD does not occur on a sharp chemical freeze-out hyper-surface, the extracted fit quality is shown to be very good. The extracted chemical parameters depend on the selected hadron species as well as the underlying equation of state (EoS) of the matter. Including light nuclei and anti-protons in the fit increases the expected freeze-out temperature, while a stiffer EoS increases the obtained chemical potential. Similarly, the baryon densities extracted by the thermal fits depend on the choice of hadrons as well as the underlying equation of state. These results are important for the upcoming CBM@FAIR physics program and highlight that a degree of caution is advised when one relates the chemical freeze-out curve to features on the QCD phase diagram like the critical endpoint or a possible phase transition.

nucl-th↗

arXiv2026

Exploring the properties of the Hadronic Phase in Heavy-Ion Collisions at RHIC Energies via Partial Chemical Equilibrium

The hadronic phase in heavy-ion collisions plays a crucial role in shaping the final-state hadron abundances. In this work, we study Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7-200 GeV using the Hadron Resonance Gas model in Partial Chemical Equilibrium (HRG-PCE). By fitting the yields of stable hadrons and short-lived resonances such as K$^*(892)^0$, we extract both chemical and kinetic freeze-out temperatures as functions of center-of-mass energy and centrality. The analysis, performed using the Thermal-FIST package, avoids assumptions about radial flow profile or freeze-out hypersurfaces. Furthermore, we estimate the baryon annihilation freeze-out temperature from the experimentally measured $\bar{\rm p}/$p ratio, using the HRG-PCE framework extended to include $B\bar{B} \leftrightarrow nπ$ reactions. The inferred annihilation freeze-out temperature lies between the chemical and kinetic freeze-out temperatures, suggesting that baryon annihilation remains active in the early hadronic phase but ceases prior to kinetic freeze-out. These results provide a consistent picture of the sequential decoupling of hadronic processes and demonstrate that inelastic hadronic interactions significantly influence the chemical composition of the system between chemical and kinetic freeze-outs at RHIC energies.

nucl-th↗

arXiv2025

Indications for freeze-out of charge fluctuations in the quark-gluon plasma at the LHC

The D-measure of net-charge fluctuations quantifies the variance of net charge in strongly interacting matter. It was introduced over 20 years ago as a potential signal of quark-gluon plasma (QGP) in heavy-ion collisions, where it is expected to be suppressed due to the fractional electric charges of quarks. Measurements have been performed at RHIC and LHC, but the conclusion has been elusive in the absence of quantitative calculations for both scenarios. We address this issue by employing a recently developed formalism of density correlations and incorporate resonance decays, local charge conservation, and experimental kinematic cuts. We find that the hadron gas scenario is in fair agreement with the ALICE data for $\sqrt{s_{\rm NN}} = 2.76$ TeV Pb-Pb collisions only when a very short rapidity range of local charge conservation is enforced, while the QGP scenario is in excellent agreement with experimental data and largely insensitive to the range of local charge conservation. A Bayesian analysis of the data utilizing different priors yields moderate evidence for the freeze-out of charge fluctuations in the QGP phase relative to hadron gas. The upcoming high-fidelity measurements from LHC Run 2 will serve as a precision test of the two scenarios.

hep-ph↗