arXiv subjects

Jan Steinheimer

Publications and source records attributed to Jan Steinheimer.

At least 19 recordsLinked to original sources

arXiv2026

Hadron polarization and equation of state at FAIR/RHIC-BES energies

The $\Lambda$ global polarization indicates that hot and dense matter created in non-central heavy-ion collisions carries large orbital angular momentum. However, the relation between hadronic polarization and the medium's collective rotation remains to be validated. Using the UrQMD transport model, we calculate the thermal vorticity-induced polarization of $\Lambda$s in Ag+Ag and Au+Au collisions from $\sqrt{s_{\rm NN}}=2.24$-$7.7$ GeV and a range of centralities. Two different equations of state used in the UrQMD simulation are compared: one resembles a hadron resonance gas, while the other is based on the chiral mean field (CMF) model, providing a more realistic description of dense nuclear matter including a chiral transition that is consistent with lattice QCD expectations. The polarization is sensitive to the equation of state and a softer EoS leads to smaller values. In addition, we show that the $\Lambda$ polarization in the experimental acceptance and centrality selection does not decrease for even lower beam energies. Our results indicate that the process leading to the large vorticity is a result of the large shear in the baryon current created by its stopping.

nucl-th↗

arXiv2026

Transport simulations with a constrained momentum-dependent Chiral Mean Field EoS at different iso-spin fractions

We present a comparison of the UrQMD model using a chiral mean field EoS (CMF) with flow and pion production data in heavy ion collisions at low beam energies $E_{\mathrm{lab}}=0.2-2.0 A$ GeV and varying iso-spin fraction. The CMF model parameters are constrained by known properties of the high density equation of state of QCD at varying iso-spin fractions. This allows us to calculate the equation of state as well as nuclear interactions for different physical systems like neutron stars and heavy ion collisions in a consistent way. It is found that heavy ion reactions at the upcoming FAIR facility will only have marginal sensitivity on the iso-spin dependence of the high-density equation of state. At lower beam energies, comparing to FOPI, HADES and S$\pi$rit data, the sensitivity is higher but the observed deviations and systematic uncertainties of the existing data are larger than the sensitivity to the symmetry energy.

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arXiv2026

Non-monotonicity of $p_T$ correlations from meson-baryon mixing

The STAR experiment has recently reported data on the $\sqrt{\langle\Delta p_{T,i}\Delta p_{T,j}\rangle}/\langle\langle p_T\rangle\rangle$ charged hadron correlation in Au+Au reactions from $\sqrt{s_{NN}}=3-200$ GeV. The beam energy dependence of this quantity is non-monotonic, showing a pronounced minimum at $\sqrt{s_{NN}} \approx 7.7$ GeV, while being essentially flat at lower and higher energies. It has been proposed that such a non-monotonicity would be consistent with increased momentum correlations due to a critical point of QCD. In the present work it is shown, using a simplified model, that the observed structure can be consistently explained by the transition from a baryon dominated system to a meson dominated system and is therefore not a good observable for the critical point of QCD.

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arXiv2026

Hypernucleus production in p+Au reactions at the FAIR facility

We explore the production of hypernuclei in p+Au reactions using the UrQMD model accompanied by a standard phase space coalescence model. We focus on the proton beam energy range of $E_{\rm lab}= 5 - 30$ GeV as this energy range will be investigated by the CBM-experiment at the upcoming FAIR facility. Starting from proton, $\Lambda$, $\Sigma$, $\Xi$ and $\Omega$ production, we predict the yields, rapidity and transverse momentum distributions of $^{3}_{\Lambda}H$, $^{4}_{\Lambda}H$, $\Xi$N and $\Xi$NN hypernuclei. We conclude that the production rates of novel multi-strange hypernuclei are well within the reach of the CBM-experiment.

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arXiv2026

Enhanced Neutrino Cooling from Parity-Doubled Nucleons in Neutron Star Cooling Simulations

Although restoration of chiral symmetry is predicted by quantum chromodynamics to take place at high baryon density, most modeling of neutron star interiors disregards a chiral phase transition. We model neutron star cores with a parity doublet model, which allows for dynamical chiral symmetry restoration and predicts the appearance of the parity partners of nucleons and hyperons at large densities, as well as deconfined quark matter. We study the thermal evolution of neutron stars, focusing for the first time on the impact of Urca processes involving the parity partners in neutron star cooling simulations. We find that Urca processes for the parity partners of the nucleons significantly affect the thermal evolution of massive stars and allow for improved agreement with observed surface temperature and ages.

astro-ph.HE↗

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 $\chi_{c0}(1P)$ and $\chi_{c1}(1P)$, 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↗

arXiv2025

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.

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arXiv2025

Multi-strange and charmed hadrons: A novel probe for the QCD equation of state at high baryon densities

Nuclear experiments near and below the threshold of hyperon production have shown that the production of Kaons is a sensitive probe for the dense QCD equation of state. At beam energies up to 1.5AGeV, strangeness production can probe the equation of state for densities up to approximately twice nuclear saturation. In this paper we will discuss the possibilities of extending this range in density by the study of multi-strange baryons as well as charmed hadrons in the SIS100 beam energy range up to $10A$GeV. Here, densities up to five times nuclear saturation can be reached and the production of multi-strange and charmed hadrons shows a strong sensitivity to the equation of state. On the other hand a precise prediction of the effect of the equation of state will require knowledge of the fundamental production cross section near the elementary production threshold in p+p collisions which is yet not measured for the hadrons discussed.

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arXiv2025

Electromagnetic probes as signatures for a first-order QCD phase transition

We investigate dimuon production in the context of a first-order phase transition in QCD matter using a chiral fluid dynamics model. This approach incorporates non-equilibrium effects such as entropy production and reheating, which emerge during the dynamical evolution through a first-order phase transition. By comparing equilibrium and non-equilibrium scenarios across a range of beam energies ($\sqrt{s_{NN}}=2.2-6.2$~GeV), we analyze the resulting invariant mass spectra. Our results reveal a substantial enhancement of dilepton yields in the non-equilibrium scenario, particularly pronounced at lower beam energies, where reheating leads to a prolonged lifetime of the fireball and increased emission. The enhancement persists even after normalizing to pion multiplicities, indicating sensitivity beyond effects of entropy production.

hep-ph↗

arXiv2025

$K^*(892)$ Resonance Suppression in Ar+Sc Collisions at SPS Energies

We investigate the production and suppression of short-lived $K^*(892)$ resonances in p+p and Ar+Sc collisions at CERN-SPS energies ($\sqrt{s_{\mathrm{NN}}} = $ 8.8, 11.9, and 16.8~GeV) using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. We present multiplicities, rapidity and transverse momentum distributions, and analyze the $K^*/K$ yield ratios as a function of energy and centrality. We further estimate the time interval between chemical and kinetic freeze-out using the experimental method. A detailed comparison with recent NA61/SHINE data demonstrates that the UrQMD model captures the essential features of resonance dynamics, although the very strong resonance suppression in central collisions observed in the data cannot be quantitatively reproduced.

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arXiv2025

Determination of the $\phi$-meson production process and its absorption cross section via directed flow

We show that the directed flow of $\phi$-mesons in Au+Au collisions at $\sqrt{s_{NN}}=3$ GeV, is sensitive on production and the absorption cross section of the $\phi$ in a nuclear medium. This provides a new observable to constrain the in-medium properties of the $\phi$ which is independent of its absolute production rate. It is shown that the STAR data disfavor any significant $\phi$-N absorption in dense nuclear matter and are consistent with a very small cross section of the $\phi$ comparable to the vacuum cross section. The similarity of the $\phi$-meson and proton directed flow also indicates that the $\phi$ is produced in conjunction with a baryon.

hep-ph↗

arXiv2025

Directed and elliptic flow of light nuclei and hypernuclei in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV: Coalescence vs. Statistical Fragmentation

The harmonic flow coefficients of light nuclei and hypernuclei in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV are investigated using the Ultra-relativistic Quantum Molecular Dynamics transport model. For the Equation-of-State we employ a density and momentum dependent potential from the Chiral-Mean-Field model. Light nuclei and hypernuclei production is described at kinetic freeze-out via a coalescence mechanism or with a statistical multi-fragmentation calculation. The directed flow $v_1$ of p, d, t, $^3$He, $^4$He as well as the $\Lambda$, $^3_\Lambda$H and $^4_\Lambda$H is shown to approximately scale with mass number $A$ of the light cluster in both calculations. This is in agreement with the experimental results for the directed flow measured by STAR. Predictions for the directed and elliptic flow of (hyper)nuclei at further RHIC-FXT and FAIR energies show that the scaling properties should improve as the beam energy is increased.

nucl-th↗

arXiv2025

Explanation of the observed violation of isospin symmetry in relativistic nucleus-nucleus reactions

The violation of isospin symmetry in nucleus-nucleus reactions, as shown in the ratio ${R_K=(K^++K^-)/(K^0+\bar{K}^0)}$ presented by NA61/SHINE, can be understood by introducing results from color-string fragmentation in $e^+e^-$ to nuclear reactions. This novel input allows for a consistent description of the $e^+e^-$ data, proton+proton data and finally nucleus-nucleus data at all investigated energies. We conclude that the observed isospin violation in nucleus-nucleus reactions is explained by asymmetric production of up- and down-quarks in the elementary color field fragmentation process.

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arXiv2025

Charmed hadron production from secondary anti-proton + proton annihilations in p+A reactions at FAIR

We present estimates for the production cross sections of exotic states ($\Lambda_c, \Sigma_c, \Xi_c, D\, \mathrm{and}\, D_s$) from secondary $\overline B + B$ annihilations in p+A reactions from $E_\mathrm{lab}=10-30A$~GeV. We focus specifically on the newly planned hadron physics program of CBM at FAIR. These estimates for the production of exotic states are based on the achievable number of $\overline B + B$ annihilations and their invariant mass distributions calculated in the UrQMD transport model.

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arXiv2025

Ultra fast, event-by-event heavy-ion simulations for next generation experiments

We present a novel deep generative framework that uses probabilistic diffusion models for ultra fast, event-by-event simulations of heavy-ion collision output. This new framework is trained on UrQMD cascade data to generate a full collision event output containing 26 distinct hadron species. The output is represented as a point cloud, where each point is defined by a particle's momentum vector and its corresponding species information (ID). Our architecture integrates a normalizing flow-based condition generator that encodes global event features into a latent vector, and a diffusion model that synthesizes a point cloud of particles based on this condition. A detailed description of the model and an in-depth analysis of its performance is provided. The conditional point cloud diffusion model learns to generate realistic output particles of collision events which successfully reproduce the UrQMD distributions for multiplicity, momentum and rapidity of each hadron type. The flexible point cloud representation of the event output preserves full event-level granularity, enabling direct application to inverse problems and parameter estimation tasks while also making it easily adaptable for accelerating any event-by-event model calculation or detector simulation.

hep-ph↗

arXiv2025

Simultaneous description of high density QCD matter in heavy ion collisions and neutron star observations

A combined constraint on the QCD equation of state, at high densities, from connecting neutron star observations to data from heavy ion reactions is presented. We use the Chiral Mean Field Model which can describe neutron star and iso-spin symmetric matter and allows the consistent calculation of the density and momentum dependent potentials of baryons which are then implemented in the UrQMD transport model. In contrast to previous studies, the same equation of state constrained from neutron star properties is also able to describe experimental observables in heavy ion reactions at the HADES experiment. Unlike many other approaches our results are not constraint to densities up to nuclear saturation or perturbative results which allows a continuous description of the equation of state over a large range in baryon density.

hep-ph↗

arXiv2024

Toward a foundation model for heavy-ion collision experiments based on point-cloud diffusion

A novel point cloud diffusion model for relativistic heavy-ion collisions, capable of ultra-fast generation of complete, event-by-event collision output, is introduced. When trained on UrQMD cascade simulations, the model generates realistic collision event output containing 26 distinct hadron species, as a list of particle momentum vectors along with their particle ID. From solving inverse problems to accelerating model calculations or detector simulations, the model can be a promising general purpose tool for heavy-ion collisions beneficial to both theoretical studies and experimental applications.

hep-ph↗

arXiv2024

The time evolution of light nuclei cumulants and ratios with a first-order phase transition in the UrQMD transport model

The UrQMD model with a density dependent equation of state, including a first-order phase transition, is used to study the time dependence of baryon number and proton number susceptibilities up to third order in heavy ion reactions of $E_{\mathrm{lab}}=2-3 A$ GeV. A significant deviation from the Gaussian fluctuations of the baryon number fluctuation in coordinate space is observed. The proton number fluctuations are always suppressed as they constitute only a small fraction of the total baryon number during the dense phase of the collision. It is found that the only measurable, but small, signal would be an enhancement of the third order (or higher) proton cumulant in a finite rapidity window $\Delta y$ that is larger than one unit of rapidity. In addition, it is found that the coordinate fluctuations will lead to an enhancement of cluster production due to the correlations in coordinate space. However, this enhancement is small and mainly occurs during the dense part of the collision before the system actually freezes out.

hep-ph↗