Simulating Many Accelerated Strongly-interacting Hadrons

Collisional Broadening within a Hadronic Transport Approach

We explore the emergence of the collisional broadening of hadrons under the influence of different media using the hadronic transport approach SMASH (Simulating Many Accelerated Strongly interacting Hadrons), which employs vacuum properties and contains no a priori information about in-medium effects. In this context, we define collisional broadening as a decrease in the lifetime of hadrons, and it arises from an interplay between the cross-sections for inelastic processes and the available phase space. We quantify this effect for various hadron species, in both a thermal gas in equilibrium and in nuclear collisions. Furthermore, we distinguish the individual contribution of each process and verify the medium response to different vacuum assumptions; we see that the decay width that depends on the resonance mass leads to a larger broadening than a mass-independent scenario.

Effective spectral function of vector mesons via lifetime analysis

We reconstruct effective spectral functions of the ρ meson by considering the effective lifetimes inside different media. Due to inelastic scatterings, resonance lifetimes are dynamically shortened (collisional broadening), even though SMASH assumes vacuum resonance properties. Analyzing the ρ meson lifetimes lets us quantify an effective broadening of the decay width and spectral function, which is important in order to distinguish dynamical effects from additional genuine medium-modified spectral functions, indicating e.g. an onset chiral symmetry restoration. We show that the broadening in a thermalized system is consistent with other theoretical calculations, and also present it for the dynamical evolution of heavy-ion collisions, finding a clear correlation of the broadening to system size. Furthermore, we explore the difference in the results between the thermal system and full collision dynamics, which may point to non-equilibrium effects.

Particle Production in AgAg Collisions at E_Kin = 1.58A GeV within a Hadronic Transport Approach

In March 2019, the HADES collaboration has taken data for AgAg collisions and in this work, we provide predictions for particle production within SMASH. The multiplicities of strange and non-strange particles follow the expected trends as a function of system size. In particular, in ArKCl (and pNb) collisions, much higher yields of double-strange baryons were observed experimentally than expected from a thermal model. Therefore, we incorporate a previously suggested mechanism to produce Ξ baryons via rare decays of high mass N* resonances and predict the multiplicities. In addition, we predict the dilepton emission and explore the most important sources of dileptons above 1 GeV, that are expected to indicate the temperature of the medium. Interestingly, the overall dilepton emission is very similar to the one in AuAu collisions, a hint that the smaller system at a higher energy behaves very similar to the larger system at lower beam energy.

Constraining resonance properties through kaon production in pion-nucleus collisions at low energies

The properties and decay channels of resonances are an essential ingredient of hadronic transport approaches. The HADES collaboration measured particle production in collisions of pions with carbon and tungsten nuclei at 1.7 GeV kinetic energy. Such reactions are of high interest, because they allow to probe the properties of baryonic resonances produced in a much cleaner environment than the usual nucleus-nucleus collisions. We study these reactions with two transport approaches: SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) and UrQMD (Ultra relativistic Quantum Molecular Dynamics) which follow the same underlying concept but with different implementations. It is found that the data favor the production of more particles with lower momentum over the production of few particles with higher momentum in these decays. In addition, the shape of the rapidity distribution of the kaons strongly depends on the angular distribution of the elastic kaon-nucleon cross section.

Particle Production via Strings and Baryon Stopping within a Hadronic Transport Approach

The stopping of baryons in heavy ion collisions at beam momenta of p_lab= 20−160A GeV is lacking a quantitative description within theoretical calculations. Since the net baryon density is determined by the amount of stopping, this is the pre-requisiste for any investigation of other observables related to structures in the QCD phase diagram such as a first-order phase transition or a critical endpoint. In this work we employ a string model for treating hadron-hadron interactions within a hadronic transport approach. The model is applied to heavy ion collisions, where the experimentally observed change of the shape of the proton rapidity spectrum from a single peak structure to a double peak structure with increasing beam energy is reproduced. In the future, the presented approach can be used to create event-by-event initial conditions for hybrid calculations.

Strangeness production via resonances in heavy-ion collisions at SIS energies

Production of strange hadrons in elementary and heavy-ion reactions is studied within SMASH. The poorly known branching ratios of the relevant hadronic resonances are constrained from the known elementary hadronic cross sections and from invariant mass spectra of dileptons. The constrained model is employed as a baseline to compare to heavy-ion-collision experiments at low energies (E_kin = 1−2AGeV) and to predict some of the upcoming pion-beam results by HADES, which are expected to be sensitive to the resonance properties. The employed vacuum-resonance approach proves to be viable for small systems at these energies, but for large systems additional in-medium effects might be required.

Constraints on the Equation of State of Nuclear Matter from Systematically Comparing SMASH Calculations to HADES Data

We aim to constrain the equation of state of nuclear matter by comparing calculations to directed and elliptic flow measurements for protons and deuterons performed by the HADES collaboration in a systematic way. A momentum-dependent term is included in the potential for which we show that it is needed to describe flow data. We further incorporate a simple symmetry potential in the transport model and present constraints on the stiffness of the equation of state of nuclear matter at saturation density and on the symmetry potential. The constraints are obtained by performing a Bayesian analysis such that we can also provide an uncertainty for the estimated parameters.

Flow and Equation of State of nuclear matter at E_kin/A=0.25-1.5 GeV with the SMASH transport approach

We present a comparison of directed and elliptic flow data by the FOPI collaboration in Au-Au, Xe-CsI, and Ni-Ni collisions at beam kinetic energies from 0.25 to 1.5 GeV per nucleon to simulations using the SMASH hadronic transport model. The Equation of State is parameterized as a function of nuclear density and momentum dependent potentials are newly introduced in SMASH. With a~statistical analysis, we show that the collective flow data at lower energies is in best agreement with a soft momentum dependent potential, while the elliptic flow at higher energies requires a harder momentum dependent Equation of State.

Collective flow at SIS energies within a hadronic transport approach: Influence of light nuclei formation and equation of state

In this work SMASH is applied to study the first four anisotropic flow coefficients in Au+Au collisions at 1.23A GeV in the context of the recently measured data by the HADES collaboration. In particular, the formation of light nuclei is important in this energy regime. Two different approaches are contrasted to each other: A clustering algorithm inspired by coalescence as well as microscopic formation of deuterons via explicit cross-sections. The sensitivity of directed and elliptic flow observables to the strength of the Skyrme mean field is explored.

Symmetry energy investigation with pion production from Sn+Sn systems

In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central 132Sn+124Sn, 112Sn+124Sn, and 108Sn+112Sn collisions at E/A = 270 MeV with the SπRIT Time Projection Chamber. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron and proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data.

Phase transitions and critical behavior in hadronic transport with a relativistic density functional equation of state

We develop a flexible, relativistically covariant parameterization of dense nuclear matter equation of state suited for inclusion in computationally demanding hadronic transport simulations. Within an implementation in the hadronic transport code SMASH, we show that effects due to bulk thermodynamic behavior are reproduced in dynamic hadronic systems, demonstrating that hadronic transport can be used to study critical behavior in dense nuclear matter, both at and away from equilibrium. We also show that two-particle correlations calculated from hadronic transport simulation data follow theoretical expectations based on the second order cumulant ratio, and constitute a clear signature of crossing the phase diagram above the critical point.

Deuteron, triton, helium-3 and hypertriton production in relativistic heavy ion collisions via stochastic multi-particle reactions

The production of light nuclei in heavy ion collisions is an excellent probe for studying the phase diagram of quantum chromodynamics and for the search of a critical end point. In this work we apply a hybrid approach in which we study the light nuclei production in the afterburner stage of central Au+Au collisions at $\sqrt{\mathrm{s_NN}}$=7.7, 14.5 and 19.6 GeV. In this stage, light nuclei are produced dynamically in 4↔2 catalysis reactions. A comparison of the dynamic production and a coalescence approach is presented for transverse momentum spectra of deuterons, tritons, ³He nuclei and hypertritons and ratios of light nuclei yields. A good agreement with the experimentally measured yield of nuclei is found and we find that the afterburner stage is essential for the description of light nuclei formation in heavy-ion collisions.

"Smashing more than two": Deuteron production in relativistic heavy ion collisions via stochastic multi-particle reactions

We study the deuteron production via the deuteron pion and nucleon catalysis reactions, πpn↔πd and Npn↔Nd, by employing stochastic multi-particle reactions for the first time. This is an improvement compared to previous studies, which introduced an artificial fake resonance d' to simulate these 3↔2 reactions as a chain of 2↔2 reactions. The derivation of the stochastic criterion for multi-particle reactions is presented in a comprehensive fashion and its implementation is tested against an analytic expression for the scattering rate and the equilibrating particle yields in box calculations. We then study Au + Au collisions at $\sqrt{\mathrm{s_NN}}$ = 7.7 GeV, where we find that multi-particle collisions substantially reduce the time required for deuterons to reach partial chemical equilibrium with nucleons.

Deuteron production in AuAu collisions at $\sqrt{\mathrm{s_NN}}$= 7 - 200 GeV via pion catalysis

We study deuteron production using no-coalescence hydrodynamic + transport simulations of central AuAu collisions at $\sqrt{\mathrm{s_NN}}$ = 7 - 200 GeV. Deuterons are sampled thermally at the transition from hydrodynamics to transport, and interact in transport dominantly via π p n ↔ π d reactions. The measured proton, Lambda, and deuteron transverse momentum spectra and yields are reproduced well for all collision energies considered. We further provide a possible explanation for the measured minimum in the energy dependence of the coalescence parameter, B₂ ($\sqrt{\mathrm{s_NN}}$) as well as for the difference between B₂(d) for deuterons and that for anti-deuterons, B₂ (d).

Microscopic study of deuteron production in PbPb collisions at √s=2.76 TeV via hydrodynamics and a hadronic afterburner

The deuteron yield in Pb+Pb collisions at center-of-mass energies of 2.76 TeV is consistent with thermal production at a freeze-out temperature of T=155 MeV. The existence of deuterons with binding energy of 2.2 MeV at this temperature was described as "snowballs in hell". We provide a microscopic explanation of this phenomenon, utilizing relativistic hydrodynamics and switching to a hadronic afterburner at the above mentioned temperature of T=155 MeV.

Distinguishing the sources of dielectron anisotropic flow at low beam energies

We present calculations of dielectron anisotropic flow in heavy-ion collisions at HADES beam energies from a hadronic transport approach. The ongoing experimental analysis employs the traditional reaction plane method to evaluate the flow coefficients v_n$ and claims to see isotropic radiation from the thermal quark-gluon plasma. We show in this work, that in the region above the pion mass, the dilepton flow measurement might suffer from cancellation effects that mask the complicated underlying dynamics. Contributions from different baryonic and mesonic resonances show collective behaviour with different signs and lead to an overall vanishing elliptic flow. To differentiate the different contributions, we propose to employ the scalar product method, which exploits the previously measured hadronic flow to create different reference planes.

Out-of-Equilibrium Photon Production in the Late Stages of Relativistic Heavy-Ion Collisions

In this work, we assess the importance of non-equilibrium dynamics in the production of photons from the late stages of relativistic heavy-ion collisions. The p_T-differential spectra and v₂ of photons from the late hadronic stage are computed within a non-equilibrium hadron transport approach, and compared to the results of a local equilibrium evolution using ideal relativistic hydrodynamics. It is found that non-equilibrium dynamics enhance the late-stage photon production at low p_T and decreases it at higher p_T compared to the estimate from hydrodynamics. This same comparison points to a significant increase in the momentum anisotropies of these photons due to non-equilibrium dynamics. Once combined with photons produced above the particlization temperature in the hydrodynamics evolution, the differences between the two approaches appear modest in what concerns the p_T-differential spectra, but are clearly noticeable at low p_T for the elliptic flow: non-equilibrium dynamics enhance the photon v₂ below p_T≈1.4 GeV.

Benchmarking a Non-Equilibrium Approach to Photon Emission in Relativistic Heavy-Ion Collisions

The production of direct photons from hadronic scatterings is implemented and validated within SMASH. Cross sections for photon production in binary, mesonic scattering processes are derived from chiral field theory and applied to describe photon production in equilibrated hadronic systems. The sensitivity of the thermal photon rate to incorporating form factors and describing non-stable ρ mesons is further investigated. It is found that photon processes involving ω mesons provide a major contribution to the total photon production and that considering non-stable ρ mesons results in a significant enhancement of photon production at low photon energies. This benchmark is the first step towards a consistent treatment of photon emission in hybrid hydrodynamics+transport approaches and a genuine dynamical description.

Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data

The dilepton emission in heavy-ion reactions at low beam energies is examined within SMASH. The calculations are systematically confronted with HADES data in the kinetic energy range of 1−3.5A GeV for elementary, proton-nucleus and nucleus-nucleus reactions. The present approach employing a resonance treatment based on vacuum properties is validated by an excellent agreement with experimental data up to system sizes of carbon-carbon collisions. After establishing this well-understood baseline in elementary and small systems, the significance of in-medium effects is investigated with a coarse-graining approach based on the same hadronic evolution.

Fate of critical fluctuations in an interacting hadronic medium using maximum entropy distributions

We study the evolution of critical fluctuations in an expanding system within a hadronic transport approach. The initialization of the system with critical fluctuations is achieved by coupling the ideal hadron resonance gas cumulants to the ones from the 3d Ising model and generating the net and total particle number distribution from the principle of maximum entropy. We find that resonance regeneration and isospin randomization processes have the strongest influence on the evolution of the fluctuations. We find that in the scaling region of the critical point correlations are propagated through the whole collisional history and are still present after the kinetic freeze-out of the matter if the coupling strength is large enough.

Thermodynamics of an updated hadronic resonance list and influence on hadronic transport

Hadron lists based on experimental studies summarized by the Particle Data Group (PDG) are a crucial input for the equation of state and thermal models used in the study of strongly-interacting matter produced in heavy-ion collisions. It has been shown that even the most uncertain states listed in the PDG from 2016 are required to reproduce partial pressures and susceptibilities from Lattice Quantum Chromodynamics with the hadronic list known as the PDG2016+. Here, we update the hadronic list for use in heavy-ion collision modeling by including the latest experimental information for all states listed in the Particle Data Booklet in 2021. Furthermore, we develop a novel scheme based on intermediate decay channels that allows for only binary decays, such that PDG2021+ will be compatible with the hadronic transport framework SMASH.

Collision term dependence of the hadronic shear viscosity and diffusion coefficients

The value of the shear viscosity η and the diffusion coefficients of conserved charges κ_ij with i,j in {B,Q,S} strongly depend on the microscopical interactions of the constituents. We find that multi-particle reactions decrease the shear viscosity in a simplified hadron gas whereas the electric charge diffusion coefficient is not modified. Furthermore, additional elastic cross sections have a strong impact on both η and κ_ij whereas anisotropic scatterings enhance the shear viscosity in the full hadron gas. When increasing the number of degrees of freedom the shear viscosity is only slightly modified in comparison to the diffusion coefficients. Finally, the calculation within a finite baryon chemical potential reveals that the shear viscosity itself does not depend on μ_B but on the ratio η/s.

Impact of hadronic interactions and conservation laws on cumulants of conserved charges in a dynamical model

We calculate the role of hadronic interactions and momentum cuts on cumulants of conserved charges up to fourth order in a system in equilibrium. In our model the net baryon, net charge and net strangeness is perfectly conserved on an event-by-event basis and the cumulants are calculated as a function of subvolume sizes and compared to analytic expectations. We find a modification of the kurtosis due to charge annihilation processes in systems with simplified degrees of freedom. Furthermore the result of the full SMASH hadron gas for the net baryon and net proton number fluctuations is presented for systems with zero and finite values of baryon chemical potential. Additionally we find that due to dynamical correlations the cumulants of the net baryon number cannot easily be recovered from the net protons. Finally the influence of deuteron cluster formation on the net proton and net baryon fluctuations in a simplified system is shown.

Inclusive and effective bulk viscosities in the hadron gas

The temperature dependence of the bulk viscosity in a relativistic hadron gas. Employing the Green-Kubo formalism in the SMASH transport approach, we study different hadronic systems in increasing order of complexity. We analyze the (in)validity of the single exponential relaxation ansatz for the bulk-channel correlation function and the strong influence of the resonances and their lifetimes. We discuss the difference between the inclusive bulk viscosity of an equilibrated, long-lived system, and the effective bulk viscosity of a short-lived mixture like the hadronic phase of relativistic heavy-ion collisions, where the processes whose inverse relaxation rate are larger than the fireball duration are excluded from the analysis. We compare our final results with previous hadron gas calculations and confirm a decreasing trend of the inclusive bulk viscosity over entropy density as temperature increases, whereas the effective bulk viscosity to entropy ratio, while being lower than the inclusive one, shows no strong dependence to temperature.

Cross-conductivity: novel transport coefficients to constrain the hadronic degrees of freedom of nuclear matter

In general, the constituents of the bulk matter produced in heavy-ion collisions carry, besides electric charge, multiple other conserved quantum numbers like baryon number and strangeness. Therefore, an electric field will not only generate an electric current but, at the same time, also currents in baryon number and strangeness. We propose that the impact of the electric field on these conserved currents should be characterized by additional transport coefficients, which we call cross-conductivities. In this paper, we introduce and present a calculation of these cross-conductivities from the Green-Kubo formalism for different chemical compositions of hadron resonance gases. We find that the coefficients underlie an ordering in the active degrees of freedom and that thus the chemical composition of the system plays a crucial role. Further, we argue that in future comparisons of lattice QCD calculations with these findings, one could constrain which degrees of freedom and their corresponding charge properties are relevant for the QCD dynamics of the system.

Jet quenching in the hadron gas: an exploratory study

The suppression of high momentum particles in heavy-ion collisions in comparison to elementary reactions is one of the main indications for the formation of a quark-gluon plasma. In this work, the effect of the late stage hadronic interactions are explored. High momentum particles are incorporated in a radially expanding hadron gas to analyse the corresponding angular distributions, also refered to as `jet shape' observables. We find that for such an observable the full hadron gas can be approximated with a pion gas with constant elastic cross-sections of 100 mb. In addition, the temperature and probe energy dependence of diffusion coefficients qtilde and etilde quantifying the transverse and parallel momentum transfers are extracted. The species dependence and the importance of different interaction types are investigated. Parametrizations are presented that can be employed in future jet quenching calculations to include the effect of the hadronic phase.

Electrical conductivity and relaxation via colored noise in a hadronic gas

Motivated by the theory of relativistic hydrodynamic fluctuations we make use of the Green-Kubo formula to compute the electrical conductivity and the (second-order) relaxation time of the electric current of an interacting hadron gas. We use SMASH to explore the role of the resonance lifetimes in the determination of the electrical relaxation time. As opposed to a previous calculation of the shear viscosity we observe that the presence of resonances with lifetimes of the order of the mean-free time does not appreciably affect the relaxation of the electric current fluctuations. We compare our results to other approaches describing similar systems, and provide the value of the electrical conductivity and the relaxation time for a hadron gas at temperatures between T=60 MeV and T=150 MeV.

Shear viscosity of a hadron gas and influence of resonance lifetimes on relaxation time

We address a discrepancy between different computations of η/s (shear viscosity over entropy density) of hadronic matter. Substantial deviations of this coefficient are found between transport approaches mainly based on resonance propagation with finite lifetime and other (semianalytical) approaches with energy-dependent cross sections, where interactions do not introduce a timescale. We provide an independent extraction of this coefficient by using SMASH, which is an example of a mainly resonance-based approach. Our conclusion is that the hadron interaction via resonance formation/decay strongly affects the transport properties of the system, resulting in significant differences in η/s with respect to other approaches where binary collisions dominate.

Role of initial transverse momentum in a hybrid approach

This work studies the event-by-event correlations of elliptic and triangular flow when exchanging the initial condition models in a hybrid approach. This study is performed in the hybrid approach SMASH-vHLLE. The initial condition models investigated are SMASH IC, Trento and IP-Glasma. Correlations are calculated on an event-by-event basis between the eccentricities and momentum anisotropies of the initial state as well as the momentum anisotropies in the final state at 200 GeV. This work demonstrates that, although averaged values for the eccentricities of these models are very similar, substantial differences exist both in the distributions of eccentricities, the correlations amongst the initial state properties as well as in the correlations between initial state and final state properties. Notably, whereas initial state momentum anisotropy is shown to not affect the final state flow, the presence of radial flow affects the emergence of final state momentum anisotropies. Inclusion of radial flow in the linear fit improves the prediction of final state flow from initial state properties. The presence of momentum in the initial state has an effect on the emergence of flow and is therefore a relevant part of initial state models, challenging the common understanding of final state momentum anisotropies being a linear response to initial state eccentricity only.

Temperature and net baryochemical potential dependence of η/s in a hybrid approach

The qualitative impact of the net baryochemical potential dependence of the shear viscosity to entropy density ratio η/s in hydrodynamical simulations is studied. The effect of a predicted non-constant η/s(μB​) is largely unexplored in hydrodynamic simulations. This work addresses this issue by studying qualitatively the effect of a generalized η/s(T,μB​) in the hybrid approach SMASH-vHLLE. In order to reduce the bias of the result on the equation of state used in the hydrodynamic part of the model, η/s is parameterized directly in the energy density and net baryon number density. The parameterization takes into account the constraints of matching to the transport coefficients in the hadronic phase, as well as pQCD results. It is shown that the effect of an explicit net baryon number dependence on the elliptic flow is negligible and only relevant in the early stages of the collision. Additionally, we find that the proposed parameterization could be a good proxy for the shear viscosity in the non-equilibrium hadronic transport stage.

The applicability of hydrodynamics in heavy ion collisions at $\sqrt{\mathrm{s_NN}}$ = 2.4-7.7 GeV

By using a coarse-graining method, we compute the energy momentum tensor of the system at fixed time steps and evaluate the degree of isotropy of the diagonal terms and the relative magnitude of the off-diagonal terms. This study focuses mostly on Au+Au collisions in the energy range \snn = 2.4-7.7 GeV, but central collisions of lighter ions like C+C, Ar+KCl and Ag+Ag are considered as well. We find that the conditions concerning local equilibration for a hydrodynamic description are reasonably satisfied in a large portion of the system for a significant amount of time (several fm/c) when considering the average evolution of many events, yet they are rarely fulfilled on an event by event basis. This is relevant for the application of hybrid approaches at low beam energies as they are or will be reached by the HADES experiment at GSI, the future CBM experiment at FAIR as well as the beam energy scan program at RHIC.

The role of proton-antiproton regeneration in the late stages of heavy-ion collisions

We investigate the long-standing question of the effect of proton-antiproton annihilation on the (anti-)proton yield, while respecting detailed balance for the 5-body back-reaction for the first time in a full microscopic description of the late stages of heavy-ion collisions. This is achieved by employing a stochastic collision criterion, which allows to treat arbitrary multi-particle reactions. It is used to account for the regeneration of (anti-)protons via 5π → ppbar. Our results show that a back-reaction happens for a fraction of 15-20% of all annihilations. Within a viscous hybrid approach Au+Au/Pb+Pb collisions from $\sqrt{\mathrm{s_NN}}$=17.3 GeV-5.02 TeV are investigated and the quoted fraction is independent of the beam energy or centrality of the collision. Taking the back-reaction into account results in regeneration of half of the (anti-)proton yield that is lost due to annihilations at midrapidity. We also find that, concerning the multiplicities, treating the back-reaction as a chain of 2-body reactions is equivalent to a single 5-to-2 reaction.

Resonance production in PbPb collisions at 5.02 TeV via hydrodynamics and hadronic afterburner

Using a relativistic hydrodynamics + hadronic afterburner simulation we explore resonance production in PbPb collisions at 5.02 TeV, and demonstrate that many resonance yields, mean transverse momenta, and flows are very sensitive to the late stage hadronic rescattering. Out of all measured resonances Λ(1520) is affected strongest by the hadronic rescattering stage, which allows to estimate its duration, and even constrain branching ratios of Σ^* -> Λ(1520)π decays. We find that some resonances like Δ(1232), f₀(980), a₀(980), Λ(1405) are enhanced rather than suppressed by the afterburner.

Multi-system Bayesian constraints on the transport coefficients of QCD matter

Note the appendix containing a detailed comparison of running a hybrid approach with UrQMD or SMASH as a hadronic afterburner.

We study the properties of the strongly-coupled quark-gluon plasma with a multistage model of heavy ion collisions that combines the Trento initial condition ansatz, free-streaming, viscous relativistic hydrodynamics, and a relativistic hadronic transport. A model-to-data comparison with Bayesian inference is performed, revisiting assumptions made in previous studies. Our study combines measurements from the Large Hadron Collider and the Relativistic Heavy Ion Collider, achieving a good simultaneous description of a wide range of hadronic observables from both colliders. The selected experimental data provide reasonable constraints on the shear and the bulk viscosities of the quark-gluon plasma at T ∼ 150--250 MeV.

Forced canonical thermalization in a hadronic transport approach at high density

At high densities, the assumption of binary interactions often used in hadronic transport approaches may not be applicable anymore. Therefore, we effectively simulate the high-density regime using the local forced canonical thermalization. This framework provides the opportunity to interpolate in a dynamical way between two different limits of kinetic theory: the dilute gas approximation and the ideal fluid case. This approach will be important for studies of the dynamical evolution of heavy ion collisions at low and intermediate energies as experimentally investigated at the beam energy scan program at RHIC, and in the future at FAIR and NICA.

Transport Model Comparison Studies of Intermediate-Energy Heavy-Ion Collisions

Transport models are the main method to obtain physics information from low to relativistic-energy heavy-ion collisions. The Transport Model Evaluation Project (TMEP) has been pursued to test the robustness of transport model predictions in reaching consistent conclusions from the same type of physical model. Calculations under controlled conditions of physical input and set-up were performed with various participating codes. These included both calculations of nuclear matter in a box with periodic boundary conditions, and more realistic calculations of heavy-ion collisions. In this intermediate review, we summarize and discuss the present status of the project. We also provide condensed descriptions of the 26 participating codes, which contributed to some part of the project.

Comparison of Heavy-Ion Transport Simulations: Mean-field Dynamics in a Box

Within the transport model evaluation project (TMEP) of simulations for heavy-ion collisions, the mean-field response is examined here. Specifically, zero-sound propagation is considered for neutron-proton symmetric matter enclosed in a periodic box, at zero temperature and around normal density. The results of several transport codes belonging to two families (BUU-like and QMD-like) are compared among each other and to exact calculations. The significance of these results for the description of real heavy-ion collisions is discussed.

Comparison of heavy-ion transport simulations: Collision integral in a box

Simulations by transport codes are indispensable to extract valuable physical information from heavy-ion collisions. In order to understand the origins of discrepancies among different widely used transport codes, we compare 15 such codes under controlled conditions of a system confined to a box with periodic boundary, initialized with Fermi-Dirac distributions at saturation density and temperatures of either 0 or 5 MeV. In such calculations, one is able to check separately the different ingredients of a transport code. In this second publication of the code evaluation project, we only consider the two-body collision term; i.e., we perform cascade calculations.

Equilibration and freeze-out of an expanding gas in a transport approach in a Friedmann–Robertson–Walker metric

Motivated by a recent finding of an exact solution of the relativistic Boltzmann equation in a Friedmann–Robertson–Walker spacetime, we implement this metric into SMASH. We study the numerical solution of the transport equation and compare it to this exact solution for massless particles. Having passed these checks for the SMASH code, we study a gas of massive particles within the same spacetime, where the particle decoupling is forced by the Hubble expansion. The results might be of interest for their potential application to relativistic heavy-ion collisions, for the characterization of the freeze-out process in terms of hadron properties.

Global angular momentum generation in heavy-ion reactions within a hadronic transport approach

In 2017, the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC) has measured finite global angular momentum in heavy-ion collisions through a spin polarization measurement of Lambda hyperons. This measurement revealed a high angular momentum of the heavy ions and provided experimental evidence for vorticity in the quark-gluon plasma (QGP) for the first time. In order to investigate the underlying mechanisms, a dynamic description of the transfer of angular momentum is required. As SMASH provides access to the whole phase-space evolution of every particle at any given time, it allows to assess the fraction of angular momentum generated in the fireball by all participants.

Influence of the neutron-skin effect on nuclear isobar collisions at RHIC

The unambiguous observation of a Chiral Magnetic Effect (CME)-driven charge separation is the core aim of the isobar program at RHIC consisting of Zr+Zr and Ru+Ru collisions at E_CM= 200A GeV. We quantify the role of the spatial distributions of the nucleons in the isobars on both eccentricity and magnetic field strength. In particular, we introduce isospin-dependent nucleon-nucleon spatial correlations in the geometric description of both nuclei, deformation for Ru and the so-called neutron skin effect for the neutron-rich isobar i.e. Zr. The main result of this study is a reduction of the magnetic field strength difference between Ru+Ru and Zr+Zr by a factor of 2, from 10\% to 5\% in peripheral collisions when the neutron-skin effect is included.