arXiv subjects

Marcus Bleicher

Publications and source records attributed to Marcus Bleicher.

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.

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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

Isospin-symmetry violation -- kaons and beyond (ISO-BREAK 25: summary and outlook)

This report summarizes the presentations and discussions during the ISO-BREAK 25 Workshop ``Isospin symmetry violation: kaons and beyond'', which was held at Jan Kochanowski University in Kielce on October 23-25, 2025. We address the current status of the isospin-symmetry breaking discovered by NA61/SHINE in nucleus-nucleus collisions at the CERN SPS, its confirmation by other experiments and studies in \ee and deep inelastic scattering. In addition, we discuss the theoretical status as well as we outline experimental and theoretical priorities towards understanding this currently unexplained phenomenon.

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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

Road map for the tuning of hadronic interaction models with accelerator-based and astroparticle data

In high-energy and astroparticle physics, event generators play an essential role, even in the simplest data analyses. As analysis techniques become more sophisticated, e.g. based on deep neural networks, their correct description of the observed event characteristics becomes even more important. Physical processes occurring in hadronic collisions are simulated within a Monte Carlo framework. A major challenge is the modeling of hadron dynamics at low momentum transfer, which includes the initial and final phases of every hadronic collision. QCD-inspired phenomenological models used for these phases cannot guarantee completeness or correctness over the full phase space. These models usually include parameters which must be tuned to suitable experimental data. Until now, event generators have been developed and tuned mainly on the basis of data from high-energy physics experiments at accelerators. The wealth of data available from the latest generation of astroparticle experiments has not yet been fully exploited, and in many cases is not satisfactorily described. Both kinds of data sets are complementary as astroparticle experiments provide sensitivity especially to hadrons produced nearly parallel to the collision axis and cover center-of-mass energies up to several hundred TeV, well beyond those reached at colliders so far. In this report, we provide an overview of state-of-the-art event generators and their tuning, including the most relevant inputs from high-energy accelerator and astroparticle experiments. We present a road map that shows, for the first time, how the unified tuning of event generators with accelerator-based and astroparticle data can be performed.

astro-ph.HE↗

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

Kadanoff-Baym approach to bound states in open quantum systems

In this paper, we extend the method of Kadanoff-Baym equations for open quantum systems to arbitrary kinds of systems and heat baths, either fermionic or bosonic. This includes three spacial dimensions and different potentials for the system-bath interaction or external traps. We study the quantum-mechanical formation of bound states in one and also in three dimensions with the full Kadanoff-Baym equations and compare them to more simplified approaches with and without memory effects. An in-depth examination of the thermodynamics of open systems is performed, showing perfect equilibration of the system's degrees of freedom along with a comprehensive investigation of the influence of the heat bath on the system's wave functions. The formation time, decay time and regeneration of bound states and their dependence on the temperature and coupling strength is explored We evaluate the non-equilibrium Kadanoff-Baym equations for the system particles, assuming that interactions are elastic two-particle collisions with the heat-bath particles. Finally, we describe in detail the method used to numerically solve the corresponding spatially heterogeneous integro-differential equations for the set of one-particle Green's functions.

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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.

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arXiv2025

The quest for the quark-gluon plasma from the perspective of dynamical models of relativistic heavy-ion collisions

The physics of heavy-ion collisions is one of the most exciting and challenging directions of science for the last four decades. On the theoretical side one deals with a non-abelian field theory, while on the experimental side today's largest accelerators are needed to enable these studies. The discovery of a new stage of matter - called the quark-gluon plasma (QGP) - and the study of its properties is one of the major achievements of modern physics. In this contribution we briefly review the history of theoretical descriptions of heavy-ion collisions based on dynamical models, focusing on the personal experiences in this inspiring field.

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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

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↗