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The first principle lattice QCD methods allow to calculate the thermodynamic observables at finite temperature and imaginary chemical potential. These can be compared to the predictions of various phenomenological models. We argue that Fourier coefficients with respect to imaginary baryochemical potential are sensitive to modeling of baryonic interactions. As a first application of this sensitivity, we consider the hadron resonance gas (HRG) model with repulsive baryonic interactions, which are modeled by means of the excluded volume correction. The Fourier coefficients of the imaginary part of the netbaryon density at imaginary baryochemical potential – corresponding to the fugacity or virial expansion at real chemical potential – are calculated within this model, and compared with the Nt = 12 lattice data. The lattice QCD behavior of the first four Fourier coefficients up to T 185 MeV is described fairly well by an interacting HRG with a single baryon–baryon eigenvolume interaction parameter b 1 fm3, while the available lattice data on the difference χB 2 − χB 4 of baryon number susceptibilities is reproduced up to T 175 MeV.

The statistical model with exact conservation of baryon number, electric charge, and strangeness – the Canonical Statistical Model (CSM) – is used to analyze the dependence of yields of light nuclei at midrapidity on charged pion multiplicity at the LHC. The CSM calculations are performed assuming baryon-symmetric matter, using the recently developed Thermal-FIST package. The light nuclei-to-proton yield ratios show a monotonic increase with charged pion multiplicity, with a saturation at the corresponding grand-canonical values in the high-multiplicity limit, in good qualitative agreement with the experimental data measured by the ALICE collaboration in pp and Pb–Pb collisions at different centralities and energies. Comparison with experimental data at low multiplicities shows that exact conservation of charges across more than one unit of rapidity and/or a chemical freeze-out temperature which decreases with the charged pion multiplicity improves agreement with the data.

Study of hard core repulsive interactions in an hadronic gas from a comparison with lattice QCD
(2016)

We study the influence of hard-core repulsive interactions within the Hadron-Resonace Gas model in comparison to first principle calculation performed on a lattice. We check the effect of a bag-like parametrization for particle eigenvolume on flavor correlators, looking for an extension of the agreement with lattice simulations up to higher temperatures, as was yet pointed out in an analysis of hadron yields measured by the ALICE experiment. Hints for a flavor depending eigenvolume are present.

The thermal fit to preliminary HADES data of Au+Au collisions at sNN=2.4 GeV shows two degenerate solutions at T≈50 MeV and T≈70 MeV. The analysis of the same particle yields in a transport simulation of the UrQMD model yields the same features, i.e. two distinct temperatures for the chemical freeze-out. While both solutions yield the same number of hadrons after resonance decays, the feeddown contribution is very different for both cases. This highlights that two systems with different chemical composition can yield the same multiplicities after resonance decays. The nature of these two minima is further investigated by studying the time-dependent particle yields and extracted thermodynamic properties of the UrQMD model. It is confirmed, that the evolution of the high temperature solution resembles cooling and expansion of a hot and dense fireball. The low temperature solution displays an unphysical evolution: heating and compression of matter with a decrease of entropy. These results imply that the thermal model analysis of systems produced in low energy nuclear collisions is ambiguous but can be interpreted by taking also the time evolution and resonance contributions into account.

We study the correlation between the distributions of the net-charge, net-kaon, net-baryon and net-proton number at hadronization and after the final hadronic decoupling by simulating ultra relativistic heavy ion collisions with the hybrid version of the ultrarelativistic quantum molecular dynamics (UrQMD) model. We find that due to the hadronic rescattering these distributions are not strongly correlated. The calculated change of the correlation, during the hadronic expansion stage, does not support the recent paradigm, namely that the measured final moments of the experimentally observed distributions do give directly the values of those distributions at earlier times, when the system had been closer to the QCD crossover.

The theory of strong interactions — Quantum Chromodynamics (QCD) — is well-defined mathematically. However, direct applications of this theory to experiment are rather limited due to significant technical obstacles. Even some general features of QCD remain unclear to date.
Hence, phenomenological input is important and needed for practical applications, e.g. for theoretical analysis of the heavy-ion collision experiments. In this thesis the role of hadronic interactions is studied in the hadron resonance gas (HRG) model — a popular model for the confined phase of QCD. The description of hadronic interactions is based on the famous van der Waals (VDW) equation and its quantum statistical generalization. While this is not the conventional choice for nuclear/hadronic physicspplications, the simplicity of the VDW approach makes it extremely useful.
In particular, this framework allows to include the two most basic ingredients of hadron-hadron interaction: the short-range repulsion, modeled by excluded-volume (EV) corrections, and the intermediate range attraction. The first part of the thesis considers just the repulsive EV interactions between hadrons. A hitherto unknown, but surprisingly strong sensitivity of the long known thermal fits to heavy-ion hadron yield data to the choice of hadron eigenvolumes is uncovered. It challenges the robustness of the chemical freeze-out temperature and baryochemical potential determination from the thermal fits. However, at the same time, the extracted value of the entropy per baryon is found to be a robust observable which depends weakly on this systematic uncertainty of the HRG model.
A Monte Carlo procedure to treat EV interactions in HRG is also introduced in this thesis. It allows to study simultaneous effects of EV and of exact charge conservation in HRG for the first time. Generalizations of the classical VDW equation are required for its applications in hadronic physics. he grand canonical ensemble (GCE) formulation of the classical VDW equation is presented. Remarkably, this important aspect of the VDW equation was not discovered before. The GCE formulation yields the analytic structure of the critical fluctuations, both in the vicinity of and far off the critical point. These critical fluctuations are presently actively being used as probes for the QCD critical point. Another extension is the hitherto undiscovered generalization of the VDW equation to include quantum Bose-Einstein and Fermi-Dirac statistics. It is performed for both single-component and multi-component fluids. The Fermi-Dirac VDW equation is applied for the first time. It is used to describe nucleons and basic properties of nuclear matter. The quantum statistical generalization of the VDW equation developed in this work is quite general, and can be applied for any fluid. Thus, its applications are not restricted to QCD physics, but may also find themselves in chemistry and/or industry. The quantum statistical VDW equation is used to describe baryonic interactions in full HRG. The VDW parameters $a$ and $b$ are fixed to the nuclear ground state and the predictions of the model are confronted with lattice QCD calculations. The inclusion of baryonic interactions leads to a qualitatively different behavior of the fluctuations of conserved charges in the crossover region. In many cases it resembles the lattice data. These results suggest that hadrons do not melt quickly with increasing temperature, as one could conclude on the basis of the common simple ideal HRG model. Calculations at finite chemical potentials show that the nuclear liquid-gas transition manifests itself by non-trivial fluctuations of the net baryon number in heavy ion collisions. In the final part of the thesis the pure glue initial scenario for high-energy hadron and heavy-ion collisions is explored. This scenario is shown not to spoil the existing agreement of the hadronic and electromagnetic observables description in Pb+Pb collisions at energies available at the CERN Large Hadron Collider. Hydrodynamic calculations suggest that collisions of small-sized nuclei at lower collision energies available at the BNL Relativistic Heavy Ion Collider are promising in the search for the traces of the chemically non-equilibrium gluon-dominated phase transition.

The creation of loosely bound objects in heavy ion collisions, e.g. light clusters, near the phase transition temperature () has been a puzzling observation that seems to be at odds with Big Bang nucleosynthesis suggesting that deuterons and other clusters are formed only below a temperature . We solve this puzzle by showing that the light cluster abundancies in heavy ion reactions stay approximately constant from chemical freeze-out to kinetic freeze-out. To this aim we develop an extensive network of coupled reaction rate equations including stable hadrons and hadronic resonances to describe the temporal evolution of the abundancies of light (anti-)(hyper-)nuclei in the late hadronic environment of an ultrarelativistic heavy ion collision. It is demonstrated that the chemical equilibration of the light nuclei occurs on a very short timescale as a consequence of the strong production and dissociation processes. However, because of the partial chemical equilibrium of the stable hadrons, including the nucleon feeding from Δ resonances, the abundancies of the light nuclei stay nearly constant during the evolution and cooling of the hadronic phase. This solves the longstanding contradiction between the thermal fits and the late stage coalescence (and the Big Bang nucleosynthesis) and explains why the observed light cluster yields are compatible with both a high chemical production temperature and a late state emission as modeled by coalescence. We also note in passing that the abundancies of the light clusters in the present approach are in excellent agreement with those measured by ALICE at LHC.

In this talk we discuss the effects of the hadronic rescattering on final state observables in high energy nuclear collisions. We do so by employing the UrQMD transport model for a realistic description of the hadronic decoupling process. The rescattering of hadrons modifies every hadronic bulk observable. For example apparent multiplicity of resonances is suppressed as compared to a chemical equilibrium freeze-out model. Stable and unstable particles change their momentum distribution by more than 30% through rescattering. The hadronic rescattering also leads to a substantial decorrelation of the conserved charge distributions. These findings show that it is all but trivial to conclude from the final state observables on the properties of the system at an earlier time where it may have been in or close to local equilibrium.

The production of light (anti-)(hyper-)nuclei in heavy-ion collisions at the LHC is considered in the framework of the Saha equation, making use of the analogy between the evolution of the early universe after the Big Bang and that of “Little Bangs” created in the lab. Assuming that disintegration and regeneration reactions involving light nuclei proceed in relative chemical equilibrium after the chemical freeze-out of hadrons, their abundances are determined through the famous cosmological Saha equation of primordial nucleosynthesis and show no exponential dependence on the temperature typical for the thermal model. A quantitative analysis, performed using the hadron resonance gas model in partial chemical equilibrium, shows agreement with experimental data of the ALICE collaboration on d, 3He, HΛ3, and 4He yields for a very broad range of temperatures at T≲155 MeV. The presented picture is supported by the observed suppression of resonance yields in central Pb–Pb collisions at the LHC. Keywords: Light (anti-)(hyper-)nuclei production, Saha equation, Partial chemical equilibrium.

We develop a framework to relate proton number cumulants measured in heavy-ion collisions within a momentum space acceptance to the susceptibilities of baryon number, assuming that particles are emitted from a fireball with uniform distribution of temperature and baryochemical potential, superimposed on a hydrodynamic flow velocity profile. The rapidity acceptance dependence of proton cumulants measured by the HADES Collaboration in √sNN = 2.4 GeV Au-Au appears to be consistent with thermal emission of nucleons from a grand-canonical heat bath, with the extracted baryon number susceptibilities exhibiting an hierarchy χB 4 >> −χB 3 >> χB 2 >> χB 1 . Naively, this could indicate large nonGaussian fluctuations that might point to the presence of the QCD critical point close to the chemical freeze-out at T ∼ 70 MeV and μB ∼ 850 − 900 MeV. However, the description of the experimental data at large rapidity acceptances becomes challenging once the effect of exact baryon number conservation is incorporated, suggesting that more theoretical and experimental studies are needed to reach a firm conclusion.