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We present an update of the event generator based on the three-fluid dynamics (3FD), complemented by Ultra-relativistic Quantum Molecular Dynamics (UrQMD) for the late stage of the nuclear collision~-- the three-fluid Hydrodynamics-based Event Simulator Extended by UrQMD final State interactions (THESEUS). Two modifications are introduced. The THESEUS table of hadronic resonances is made consistent with that of the underlying 3FD model. The main modification is that the generator is extended to simulate the light-nuclei production in relativistic heavy-ion collisions, on the equal basis with hadrons. These modifications are illustrated by applications to the description of available experimental data. The first run of the updated generator revealed a good reproduction of the NA49 data on the light nuclei. The reproduction is achieved without any extra parameters, while the coalescence approach in 3FD requires special tuning of the coalescence coefficients for each light nucleus separately., Comment: 8 pages, 3 figures, version published in PRC

This report summarizes the presentations and discussions during the Rapid Reaction Task Force "Dynamics of critical fluctuations: Theory -- phenomenology -- heavy-ion collisions", which was organized by the ExtreMe Matter Institute EMMI and held at GSI, Darmstadt, Germany in April 2019. We address the current understanding of the dynamics of critical fluctuations in QCD and their measurement in heavy-ion collision experiments. In addition, we outline what might be learned from studying correlations in other physical systems, such as cold atomic gases., Comment: 89 pages, 11 figures, report on an ExtreMe Matter Institute EMMI Rapid Reaction Task Force

The aim of this paper is to understand resonance production (and more generally particle production) for different collision systems, namely proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) scattering at the LHC. We will investigate in particular particle yields and ratios versus multiplicity, using the same multiplicity definition for the three different systems, in order to analyse in a compact way the evolution of particle production with the system size and the origin of a very different system size dependence of the different particles., Comment: Proceedings of the 17th International Conference on Strangeness in Quark Matter (SQM 2017), https://www.epj-conferences.org/articles/epjconf/abs/2018/06/epjconf_sqm2018_09002/epjconf_sqm2018_09002.html

We estimate the chemical freeze-out of light nuclear clusters for NICA energies of above 2 A GeV. On the one hand we use results from the low energy domain of about 35 A MeV, where medium effects have been shown to be important to explain experimental results. On the high energy side of LHC energies the statistical model without medium effects has provided results for the chemical freeze-out. The two approaches extrapolated to NICA energies show a discrepancy that can be attributed to medium effects and that for the deuteron/proton ratio amounts to a factor of about three. These findings underline the importance of a detailed investigation of light cluster production at NICA energies., Comment: 10 pages, 4 figures, accepted for publication in Phys. Part. Nucl. Lett

We present a new event generator based on the three-fluid hydrodynamics (3FH) approach, followed by a particlization at the hydrodynamic decoupling surface and a subsequent UrQMD afterburner stage based on the microscopic UrQMD transport model that accounts for hadronic final state interactions. First results for Au+Au collisions are presented. The following topics are addressed: the directed flow, transverse-mass spectra, as well as rapidity distributions of protons, pions and kaons for two model equations of state, one with a first-order phase transition, the other with a crossover transition. Preliminary results on the femtoscopy are also discussed. We analyze the accuracy of reproduction of the 3FH results by the new event generator and the effect of the subsequent UrQMD afterburner stage., Comment: 12 pp., 9 figs. Proceedings of 6th Int. Conf. on New Frontiers in Physics (ICNFP 2017), August 17-26, 2017, Kolymbari, Crete, Greece

We study the polarization of particles in relativistic heavy-ion collisions at very high energy along the beam direction within a relativistic hydrodynamic framework. We show that this component of the polarization decreases much slower with center-of-mass energy compared to the transverse component, even in the ideal longitudinal boost-invariant scenario with non-fluctuating initial state, and that it can be measured by taking advantage of its quadrupole structure in the transverse momentum plane. In the ideal longitudinal boost-invariant scenario, the polarization is proportional to the gradient of temperature at the hadronization and its measurement can provide important information about the cooling rate of the Quark Gluon Plasma around the critical temperature., Comment: 5 pages, 3 figures, final version published in Phys. Rev. Lett

Polarization of $\Lambda$ hyperons and their antiparticles is calculated in a 3+1 dimensional viscous hydrodynamic model with initial state from UrQMD hadron/string cascade. We find that, along with recent results from STAR, the mean polarization at fixed centrality decreases as a function of collision energy from 1.5% at $\sqrt{s_{\rm NN}}=7.7$ GeV to 0.2% at $\sqrt{s_{\rm NN}}=200$ GeV. We explore the effects which lead to such collision energy dependence, feed-down corrections and a difference between $\Lambda$ and $\bar\Lambda$., Comment: contribution to proceedings of Quark Matter 2017, 5-11 February 2017, Chicago (IL)

Correlation femtoscopy allows one to measure the space-time characteristics of particle production in relativistic heavy-ion collisions due to the effects of quantum statistics (QS) and final state interactions (FSI). The main features of the femtoscopy measurements at top RHIC and LHC energies are considered as a manifestation of strong collective flow and are well interpreted within hydrodynamic models employing equation of state (EoS) with a crossover type transition between Quark-Gluon Plasma (QGP) and hadron gas phases. The femtoscopy at lower energies was intensively studied at AGS and SPS accelerators and is being studied now in the Beam Energy Scan program (BES) at the BNL Relativistic Heavy Ion Collider in the context of exploration of the QCD phase diagram. In this article we present femtoscopic observables calculated for Au-Au collisions at $\sqrt{s_{NN}} = 7.7 - 62.4$ GeV in a viscous hydro + cascade model vHLLE+UrQMD and their dependence on the EoS of thermalized matter., Comment: 11 pages, 8 figures

We calculate the polarization of $\Lambda$ hyperons in Au-Au collisions at RHIC Beam Energy Scan range $\sqrt{s_{\rm NN}}=7.7, \dots, 200$ GeV in a state-of-the-art 3+1 dimensional cascade + viscous hydro model vHLLE+UrQMD. We find that the polarization of $\Lambda$ in the out-of-plane direction decreases substantially with collision energy. We explore the connection between the polarization signal and thermal vorticity and discuss the feed-down and hadronic rescattering effects on the mean polarization of all produced $\Lambda$ hyperons., Comment: proceedings of Strangeness in Quark Matter 2016 conference, 27 June - 1 July 2016, Berkeley

We present a calculation of the global polarization of Lambda hyperons in relativistic Au-Au collisions at RHIC Beam Energy Scan range sqrt{s}_NN = 7.7 - 200 GeV with a 3+1 dimensional cascade + viscous hydro + cascade model, UrQMD+vHLLE. Within this model, the mean polarization of Lambda in the out-of-plane direction is predicted to decrease rapidly with collision energy from a top value of about 2% at the lowest energy examined. We explore the connection between the polarization signal and thermal vorticity and estimate the feed-down contribution to Lambda polarization due to the decay of higher mass hyperons., Comment: 10 pages, 9 figures. Final version published in EPJC

Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. The comparable hydrodynamic simulations of heavy-ion collisions for (2+1)-flavor and Yang-Mills equations of state performed by using three different hydrodynamic codes are presented. Assuming slow chemical equilibration of quarks, the spectra and elliptic flows of thermal dileptons and photons are calculated for central Pb+Pb collisions at the LHC energy of $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. It is shown that a suppression of quarks at early times leads to a significant reduction of the yield of the thermal dileptons, but only to a rather modest suppression of the $p_T$-distribution of direct photons. It is demonstrated that an enhancement of photon and dilepton elliptic flows might serve as a promising signature of the pure-glue initial state. Calculations based on Bjorken hydrodynamics suggest that collisions of small systems at intermediate energies available at RHIC or future FAIR facilities may show stronger effects associated with initial pure gluodynamic evolution., Comment: 14 pages, 12 figures. Proceedings for 54th International Winter Meeting on Nuclear Physics, 25-29 January 2016, Bormio, Italy

Light cluster production at the NICA accelerator complex offers unique possibilities to use these states as "rare probes" of in-medium characteristics such as phase space occupation and early flow. In order to explain this statement, in this contribution theoretical considerations from the nuclear statistical equilibrium model and from a quantum statistical model of cluster production are supplemented with a discussion of a transport model for light cluster formation and with results from hydrodynamic simulations combined with the coalescence model., Comment: 7 pages, 7 figures, accepted for publication in Eur. Phys. J. A

We present a new event generator based on the three-fluid hydrodynamics approach for the early stage of the collision, followed by a particlization at the hydrodynamic decoupling surface to join to a microscopic transport model, UrQMD, to account for hadronic final state interactions. We present first results for nuclear collisions of the FAIR/NICA energy scan program (Au+Au collisions, $\sqrt{s_{NN}}=4-11$ GeV). We address the directed flow of protons and pions as well as the proton rapidity distribution for two model EoS, one with a first order phase transition the other with a crossover type softening at high densities. The new simulation program has the unique feature that it can describe a hadron-to-quark matter transition which proceeds in the baryon stopping regime that is not accessible to previous simulation programs designed for higher energies., Comment: 18 pages, 16 figures, version accepted for publication in Phys. Rev. C, before proof corrections

Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. To study the implications of such a scenario, the dynamical evolution of a chemically nonequilibrated system is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent (anti)quark fugacity. The equation of state interpolates linearly between the lattice data for the pure gluonic matter and the lattice data for the chemically equilibrated quark-gluon plasma. The spectra and elliptic flows of thermal dileptons and photons are calculated for central Pb+Pb collisions at the CERN Large Hadron Collider energy of $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. We test the sensitivity of the results to the choice of equilibration times, including also the case where the complete chemical equilibrium of partons is reached already at the initial stage. It is shown that a suppression of quarks at early times leads to a significant reduction of the yield of the thermal dileptons, but only to a rather modest suppression of the $p_T$-distribution of direct photons. It is demonstrated that an enhancement of photon and dilepton elliptic flows might serve as a promising signature of the pure-glue initial state., Comment: 27 pages, 13 figures, v3: minor corrections, to match published version

Recent experimental studies of the multiplicity dependence of heavy quark (HQ) production in proton-proton collisions at 7 TeV showed a strong non-linear increase of the HQ multiplicity as a function of the charged particle multiplicity. We try to understand this behavior using the EPOS3 approach. Two issues play an important role: multiple scattering, in particular its impact on multiplicity fluctuations, and the collective hydrodynamic expansion. The results are very robust with respect to many details of the modeling, which means that these data contain valuable information about very basic features of the reaction mechanism in proton-proton collisions., Comment: 11 pages, 20 figures

A state-of-the-art 3+1 dimensional cascade + viscous hydro + cascade model vHLLE+UrQMD has been applied to heavy ion collisions in RHIC Beam Energy Scan range $\sqrt{s_{\rm NN}}=7.7\dots 200$ GeV. Based on comparison to available experimental data it was estimated that an effective value of shear viscosity over entropy density ratio $\eta/s$ in hydrodynamic stage has to decrease from $\eta/s=0.2$ to $0.08$ as collision energy increases from $\sqrt{s_{\rm NN}} = 7.7$ to $39$ GeV, and to stay at $\eta/s=0.08$ for $39\le\sqrt{s_{\rm NN}}\le200$ GeV. In this work we show how an equation of state with first order phase transition affects the hydrodynamic evolution at those collision energies and changes the results of the model as compared to "default scenario" with a crossover type EoS from chiral model., Comment: 4 pages and 3 figures. Proceedings of Quark Matter 2015, Kobe, Japan

We report on the application of a cascade + viscous hydro + cascade model for heavy ion collisions in the RHIC Beam Energy Scan range, $\sqrt{s_{\rm NN}}=6.3\dots200$ GeV. By constraining model parameters to reproduce the data we find that the effective(average) value of the shear viscosity over entropy density ratio $\eta/s$ decreases from 0.2 to 0.08 when collision energy grows from $\sqrt{s_{\rm NN}}\approx7$ to 39 GeV., Comment: 4 pages, proceedings of the Strangeness in Quark Matter 2015, Dubna, July 6-11, 2015

A further development of the evolutionary picture of A+A collisions, which we call the integrated HydroKinetic Model (iHKM), is proposed. The model comprises a generator of the initial state GLISSANDO, pre-thermal dynamics of A+A collisions leading to thermalization, subsequent relativistic viscous hydrodynamic expansion of quark-gluon and hadron medium (vHLLE), its particlization, and finally hadronic cascade ultrarelativistic QMD. We calculate mid-rapidity charged-particle multiplicities, pion, kaon, and antiproton spectra, charged-particle elliptic flows, and pion interferometry radii for Pb+Pb collisions at the energies available at the CERN Large Hadron Collider, $\sqrt{s} = 2.76$ TeV, at different centralities. We find that the best description of the experimental data is reached when the initial states are attributed to the very small initial time 0.1 fm/c, the pre-thermal stage (thermalization process) lasts at least until 1 fm/c, and the shear viscosity at the hydrodynamic stage of the matter evolution has its minimal value, $\eta/s = \frac{1}{4\pi}$. At the same time it is observed that the various momentum anisotropies of the initial states, different initial and relaxation times, as well as even a treatment of the pre-thermal stage within just viscous or ideal hydrodynamic approach, leads sometimes to worse but nevertheless similar results, if the normalization of maximal initial energy density in most central events is adjusted to reproduce the final hadron multiplicity in each scenario. This can explain a good enough data description in numerous variants of hybrid models without a prethermal stage when the initial energy densities are defined up to a common factor., Comment: 22 pages, 11 figures

Hybrid approaches based on relativistic hydrodynamics and transport theory have been successfully applied for many years for the dynamical description of heavy ion collisions at ultrarelativistic energies. In this work a new viscous hybrid model employing the hadron transport approach UrQMD for the early and late non-equilibrium stages of the reaction, and 3+1 dimensional viscous hydrodynamics for the hot and dense quark-gluon plasma stage is introduced. This approach includes the equation of motion for finite baryon number, and employs an equation of state with finite net-baryon density to allow for calculations in a large range of beam energies. The parameter space of the model is explored, and constrained by comparison with the experimental data for bulk observables from SPS and the phase I beam energy scan at RHIC. The favored parameter values depend on energy, but allow to extract the effective value of the shear viscosity coefficient over entropy density ratio $\eta/s$ in the fluid phase for the whole energy region under investigation. The estimated value of $\eta/s$ increases with decreasing collision energy, which may indicate that $\eta/s$ of the quark-gluon plasma depends on baryochemical potential $\mu_B$., Comment: minor changes in the text, results for constant eta*T/w added. Version accepted for publication in Phys. Rev. C

A key ingredient of hydrodynamical modeling of relativistic heavy ion collisions is thermal initial conditions, an input that is the consequence of a pre-thermal dynamics which is not completely understood yet. In the paper we employ a recently developed energy-momentum transport model of the pre-thermal stage to study influence of the alternative initial states in nucleus-nucleus collisions on flow and energy density distributions of the matter at the starting time of hydrodynamics. In particular, the dependence of the results on isotropic and anisotropic initial states is analyzed. It is found that at the thermalization time the transverse flow is larger and the maximal energy density is higher for the longitudinally squeezed initial momentum distributions. The results are also sensitive to the relaxation time parameter, equation of state at the thermalization time, and transverse profile of initial energy density distribution: Gaussian approximation, Glauber Monte Carlo profiles, etc. Also, test results ensure that the numerical code based on the energy-momentum transport model is capable of providing both averaged and fluctuating initial conditions for the hydrodynamic simulations of relativistic nuclear collisions., Comment: 29 pages, 15 figures, minor changes, to be published in Phys. Rev. C

There is little doubt that hydrodynamic flow has been observed in heavy ion collisions at the LHC and RHIC, mainly based on results on azimuthal anisotropies, but also on particle spectra of identified particles, perfectly compatible with hydrodynamic expansions. Surprisingly, in p-Pb collisions one observes a very similar behavior. So do we see flow even in p-Pb? We will try to answer this question., Comment: Talk given at the "Rencontres de Moriond" 2014

We provide, within the hydrokinetic model, a detailed investigation of kaon interferometry in $Pb+Pb$ collisions at LHC energy ($\sqrt{s_{NN}} = 2.76$ TeV). Predictions are presented for 1D interferometry radii of $K^0_SK^0_S$ and $K^{\pm}K^{\pm}$ pairs as well as for 3D femtoscopy scales in out, side and long directions. The results are compared with existing pion interferometry radii. We also make predictions for full LHC energy., Comment: 12 pages, 6 figures

We describe the details of 3+1 dimensional relativistic hydrodynamic code for the simulations of quark-gluon/hadron matter expansion in ultra-relativistic heavy ion collisions. The code solves the equations of relativistic viscous hydrodynamics in the Israel-Stewart framework. With the help of ideal-viscous splitting, we keep the ability to solve the equations of ideal hydrodynamics in the limit of zero viscosities using a Godunov-type algorithm. Milne coordinates are used to treat the predominant expansion in longitudinal (beam) direction effectively. The results are successfully tested against known analytical relativistic inviscid and viscous solutions, as well as against existing 2+1D relativistic viscous code., Comment: 19 pages, 12 figures

Experimental transverse momentum spectra of identified particles in p-Pb collisions at 5.02 TeV show many similarities to the corresponding Pb-Pb results, the latter ones usually being interpreted in term of hydrodynamic flow. We analyse these data using EPOS3, an event generator based on a 3D+1 viscous hydrodynamical evolution starting from flux tube initial conditions, which are generated in the Gribov-Regge multiple scattering framework. An individual scattering is referred to as Pomeron, identified with a parton ladder, eventually showing up as flux tubes (or strings). Each parton ladder is composed of a pQCD hard process, plus initial and final state linear parton emission. Nonlinear effects are considered by using saturation scales $Q_{s}$, depending on the energy and the number of participants connected to the Pomeron in question. We compute transverse momentum ($p_{t}$) spectra of pions, kaons, protons, lambdas, and $\Xi$ baryons in p-Pb and p-p scattering, compared to experimental data and many other models. In this way we show in a quantitative fashion that p-Pb data (and even p-p ones) show the typical ``flow effect'' of enhanced particle production at intermediate $p_{t}$ values, more and more visible with increasing hadron mass.

We apply a 3+1D viscous hydrodynamic + cascade model to the heavy ion collision reactions with $\sqrt{s_{NN}}=6.3\dots39$ GeV. To accommodate the model for a given collision energy range, the initial conditions for hydrodynamic phase are taken from UrQMD, and the equation of state at finite baryon density is based on Chiral model coupled to the Polyakov loop. We study the collision energy dependence of pion and kaon rapidity distributions and $m_T$-spectra, as well as charged hadron elliptic flow and how shear viscosity affects them. The model calculations are compared to the data for Pb-Pb collisions at CERN SPS, as well as for Au-Au collisions in the Beam Energy Scan (BES) program energies at BNL RHIC. The data favours the value of shear viscosity $\eta/s\gtrsim0.2$ for this collision energy range., Comment: 5 pages, 4 figures. Talk given at FAIRNESS 2013 workshop, September 15-21 2013, Berlin, Germany. arXiv admin note: substantial text overlap with arXiv:1310.0702

Hadrons inclusively produced with large pT in high-energy collisions originate from the jets, whose initial virtuality and energy are of the same order, what leads to an extremely intensive gluon radiation and dissipation of energy at the early stage of hadronization. Besides, these jets have a peculiar structure: the main fraction of the jet energy is carried by a single leading hadron, so such jets are very rare. The constraints imposed by energy conservation enforce an early color neutralization and a cease of gluon radiation. The produced colorless dipole does not dissipate energy anymore and is evolving to form the hadron wave function. The small and medium pT region is dominated by the hydrodynamic mechanisms of hadron production from the created hot medium. The abrupt transition between the hydrodynamic and perturbative QCD mechanisms causes distinct minima in the pT dependence of the suppression factor R_{AA} and of the azimuthal asymmetry v2. Combination of these mechanisms allows to describe the data through the full range of pT at different collision energies and centralities., Comment: Presented at EDS Blois 2013 (arXiv:1309.5705)

Following the experimental program at BNL RHIC, we perform a similar "energy scan" using 3+1D viscous hydrodynamics coupled to the UrQMD hadron cascade, and study the collision energy dependence of pion and kaon rapidity distributions and $m_T$-spectra, as well as charged hadron elliptic flow. To this aim the equation of state for finite baryon density from a Chiral model coupled to the Polyakov loop is employed for hydrodynamic stage. 3D initial conditions from UrQMD are used to study gradual deviation from boost-invariant scaling flow. We find that the inclusion of shear viscosity in the hydrodynamic stage of evolution consistently improves the description of the data for Pb-Pb collisions at CERN SPS, as well as of the elliptic flow measurements for Au-Au collisions in the Beam Energy Scan (BES) program at BNL RHIC. The suggested value of shear viscosity is $\eta/s\ge0.2$ for $\sqrt{s_{NN}}=6.3\dots39$ GeV., Comment: 4 pages, to appear in the proceedings of Strangeness in Quark Matter 2013 conference

The 2D azimuth & rapidity structure of the two-particle correlations in relativistic A+A collisions is altered significantly by the presence of sharp inhomogeneities in superdense matter formed in such processes. The causality constraints enforce one to associate the long-range longitudinal correlations observed in a narrow angular interval, the so-called (soft) ridge, with peculiarities of the initial conditions of collision process. This study's objective is to analyze whether multiform initial tubular structures, undergoing the subsequent hydrodynamic evolution and gradual decoupling, can form the soft ridges. Motivated by the flux-tube scenarios, the initial energy density distribution contains the different numbers of high density tube-like boost-invariant inclusions that form a bumpy structure in the transverse plane. The influence of various structures of such initial conditions in the most central A+A events on the collective evolution of matter, resulting spectra, angular particle correlations and v_n-coefficients is studied in the framework of the HydroKinetic Model (HKM)., Comment: 18 pages, 6 figures, the paper to be published in Advances of High Energy Physics (2013, in press)

The three-dimensional pion and kaon emission source functions are extracted from the HKM model simulations of the central Au+Au collisions at the top RHIC energy $\sqrt{s_{NN}}=200$ GeV. The model describes well the experimental data, previously obtained by the PHENIX and STAR collaborations using the imaging technique. In particular, the HKM reproduces the non-Gaussian heavy tails of the source function in the pair transverse momentum (out) and beam (long) directions, observed in the pion case and practically absent for kaons. The role of the rescatterings and long-lived resonances decays in forming of the mentioned long range tails is investigated. The particle rescatterings contribution to the out tail seems to be dominating. The model calculations also show the substantial relative emission times between pions (with mean value 14.5 fm/c in LCMS), including those coming from resonance decays and rescatterings. The prediction is made for the source functions in the LHC Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV, which are still not extracted from the measured correlation functions., Comment: 23 pages, 4 figures

We show that a fluid dynamical scenario describes quantitatively the observed mass splitting of the elliptical flow coefficients v2 for pions, kaons, and protons. This provides a strong argument in favor of the existence of a fluid dynamical expansion in pPb collisions at 5TeV.

With short resumes and highlights the discussions in the different working groups of the workshop MPI@LHC 2012 is documented.

EPOS is a Monte-Carlo event generator for minimum bias hadronic interactions, used for both heavy ion interactions and cosmic ray air shower simulations. Since the last public release in 2009, the LHC experiments have provided a number of very interesting data sets comprising minimum bias p-p, p-Pb and Pb-Pb interactions. We describe the changes required to the model to reproduce in detail the new data available from LHC and the consequences in the interpretation of these data. In particular we discuss the effect of the collective hadronization in p-p scattering. A different parametrization of flow has been introduced in the case of a small volume with high density of thermalized matter (core) reached in p-p compared to large volume produced in heavy ion collisions. Both parametrizations depend only on the geometry and the amount of secondary particles entering in the core and not on the beam mass or energy. The transition between the two flow regimes can be tested with p-Pb data. EPOS LHC is able to reproduce all minimum bias, Comment: 27 pages, 34 figures, change to 1 column style, add ALICE data on PbPb dN/deta and link to source code

A method for quantum corrections of Hanbury-Brown/Twiss (HBT) interferometric radii produced by semi-classical event generators is proposed. These corrections account for the basic indistinguishability and mutual coherence of closely located emitters caused by the uncertainty principle. A detailed analysis is presented for pion interferometry in $p+p$ collisions at LHC energy ($\sqrt{s}=7$ TeV). A prediction is also presented of pion interferometric radii for $p+$Pb collisions at $\sqrt{s}=5.02$ TeV. The hydrodynamic/hydrokinetic model with UrQMD cascade as 'afterburner' is utilized for this aim. It is found that quantum corrections to the interferometry radii improve significantly the event generator results which typically overestimate the experimental radii of small systems. A successful description of the interferometry structure of $p+p$ collisions within the corrected hydrodynamic model requires the study of the problem of thermalization mechanism, still a fundamental issue for ultrarelativistic $A+A$ collisions, also for high multiplicity $p+p$ and $p+$Pb events., Comment: 22 pages, 7 figures

A simultaneous description of hadronic yields; pion, kaon, and proton spectra; elliptic flows; and femtoscopy scales in the hydrokinetic model of A+A collisions is presented at different centralities for the top BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) 2.76-TeV energies. The initial conditions are based on the Glauber Monte-Carlo simulations. When going from RHIC to LHC energy in the model, the only parameters changed are the normalization of the initial entropy defined by the number of all charged particles in most central collisions, contribution to entropy from binary collisions and baryonic chemical potential. The hydrokinetic model is used in its hybrid version (hHKM), which provides the correct match (at the isochronic hypersurface) of the decaying hadron matter evolution with hadronic ultrarelativistic quantum molecular dynamics cascade. The results are compared with the standard hybrid models where hydrodynamics and hadronic cascade are matching just at the non-space-like hypersurface of chemical freeze-out or on the isochronic hypersurface. The modification of the particle number ratios at LHC caused, in particular, by the particle annihilations at the afterburn stage is also analyzed., Comment: 14 pages, 7 figures. Version accepted for publication in Phys.Rev. C

We discuss a theoretical scheme that accounts for bulk matter, jets, and the interaction between the two. The aim is a complete description of particle production at all transverse momentum ($p_{t}$) scales. In this picture, the hard initial scatterings result in mainly longitudinal flux tubes, with transversely moving pieces carrying the $p_{t}$ of the partons from hard scatterings. These flux tubes constitute eventually both bulk matter (which thermalizes and flows) and jets. We introduce a criterion based on parton energy loss to decide whether a given string segment contributes to the bulk or leaves the matter to end up as a jet of hadrons. Essentially low $p_{t}$ segments from inside the volume will constitute the bulk, high $p_{t}$ segments (or segments very close to the surface) contribute to the jets. The latter ones appear after the usual flux tube breaking via q-qbar production (Schwinger mechanism). Interesting is the transition region: Intermediate $p_{t}$ segments produced inside the matter close to the surface but having enough energy to escape, are supposed to pick up q-qbar pairs from the thermal matter rather than creating them via the Schwinger mechanism. This represents a communication between jets and the flowing bulk matter (fluid-jet interaction). Also very important is the interaction between jet hadrons and the soft hadrons from the fluid freeze-out. We employ the new picture to investigate Pb-Pb collisions at 2.76 TeV. We discuss the centrality and $p_{t}$ dependence of particle production and long range dihadron correlations at small and large $p_{t}$.

We study untriggered di-hadron correlations in Pb+Pb at 2.76 TeV, based on an event-by-event simulation of a hydrodynamic expansion starting from flux tube initial conditions. The correlation function shows interesting structures as a function of the pseudorapidity difference $\Delta\eta$ and the azimuthal angle difference $\Delta\phi$, in particular comparing different centralities. We can clearly identify a peak-like nearside structure associated with very low momentum components of jets for peripheral collisions, which disappears towards central collisions. On the other hand, a very broad ridge structure from asymmetric flow seen at central collisions, gets smaller and finally disappears towards peripheral collisions.

Using a fluid dynamical scenario for $pp$ scattering at 7 TeV, we compute correlation functions for $\pi^+\pi^+$ pairs. Femtoscopic radii are extracted based on three-dimensional parametrizations of the correlation functions. We study the radii as a function of the transverse momenta of the pairs, for different multiplicity classes, corresponding to recent experimental results from ALICE. We find the same decrease of the radii with $k_T$, more and more pronounced with increasing multiplicity, but absent for the lowest multiplicities. In the model we understand this as transition from string expansion (low multiplicity) towards a three-dimensional hydrodynamical expansion (high multiplicity).

A study of energy behavior of the pion spectra and interferometry scales is carried out for the top SPS, RHIC and for LHC energies within the hydrokinetic approach. The main mechanisms that lead to the paradoxical, at first sight, dependence of the interferometry scales with an energy growth, in particular, a decrease $R_{out}/R_{side}$ ratio, are exposed. The hydrokinetic predictions for the HBT radii at LHC energies are compared with the recent results of the ALICE experiment., Comment: Based on the talks given at the Sixth Workshop on Particle Correlations and Femtoscopy, BITP, Kiev, September 14 - 18, 2010 and GSI/EMMI Seminar, January 14, 2011

The hydrokinetic model is applied to restore the initial conditions and space-time picture of the matter evolution in central Au+Au collisions at the top RHIC energy. The analysis is based on the detailed reproduction of the pion and kaon momentum spectra and femtoscopic data in whole interval of the transverse momenta studied by both STAR and PHENIX collaborations. A good description of the pion and kaon transverse momentum spectra and interferometry radii is reached with both initial energy density profiles motivated by the Glauber and Color Glass Condensate (CGC) models, however, at different energy densities., Comment: Talk given at the the Sixth Workshop on Particle Correlations and Femtoscopy, BITP, Kiev, September 14-18, 2010

Two-particle angular correlation for charged particles emitted in Au+Au collisions at the center-of-mass of 200 MeV measured at RHIC energies revealed novel structures commonly referred to as a near-side ridge. The ridge phenomenon in relativistic A+A collisions is rooted probably in the initial conditions of the thermal evolution of the system. In this study we analyze the evolution of the bumping transverse structure of the energy density distribution caused by fluctuations of the initial density distributions that could lead to the ridge structures. We suppose that at very initial stage of collisions the typical one-event structure of the initial energy density profile can be presented as the set of longitudinal tubes, which are boost-invariant in some space-rapidity region and are rather thin. These tubes have very high energy density comparing to smooth background density distribution. The transverse velocity and energy density profiles at different times of the evolution till the chemical freeze-out (at the temperature T=165 MeV) willbe reached by the system are calculated for sundry initial scenarios., Comment: Talk given at the The Sixth Workshop on Particle Correlations and Femtoscopy (WPCF2010), Kiev, September 14 - 18, 2010

The hydrokinetic model and the scheme of dynamical freeze out proposed earlier for the RHIC energy are extended to lower colliding energy for non-vanishing baryon chemical potential. In this case a new two-phase equation of state is applied where baryon-rich hadronic matter is described within modified relativistic mean-field theory. As an example, the approach is employed for analyzing pion and kaon spectra and interferometry radii of Pb+Pb collisions at laboratory energies 40 and 158 AGeV. A good agreement is observed for the transverse mass dependence of the longitudinal radius for both energies studied. The transverse radii $R_{out}, R_{side}$ reproduce experiment at $E_{lab}=$158\ AGeV but at $E_{lab}=$40\ AGeV there is a marked (20 \%) difference from experimental points. This discrepancy is more noticeable in analysis of the ratio $R_{out}/R_{side}$. Nevertheless, considered examples of application of the hydrokinetic model with equation of state including the first order phase transition are quite successful and further development of the model is required., Comment: 12 pages, 3 figures

Since 2006, EPOS hadronic interaction model is being used for very high energy cosmic ray analysis. Designed for minimum bias particle physics and used for having a precise description of SPS and RHIC heavy ion collisions, EPOS brought more detailed description of hadronic interactions in air shower development. Thanks to this model it was possible to understand why there were less muons in air shower simulations than observed in real data. With the start of the LHC era, a better description of hard processes and collective effects is needed to deeply understand the incoming data. We will describe the basic physics in EPOS and the new developments and constraints which are taken into account in EPOS 2., Comment: Contributed presentation to the XVI International Symposium on Very High Energy Cosmic Ray Interactions (ISVHECRI 2010), Batavia, IL, USA (28 June 2 July 2010). 4 pages, 6 figures

One of the most important experimental results for proton-proton scattering at the LHC is the observation of a so-called "ridge" structure in the two particle correlation function versus the pseudorapidity difference $\Delta\eta$ and the azimuthal angle difference $\Delta\phi$. One finds a strong correlation around $\Delta\phi=0$, extended over many units in $\Delta\eta$. We show that a hydrodynamical expansion based on flux tube initial conditions leads in a natural way to the observed structure. To get this result, we have to perform an event-by-event calculation, because the effect is due to statistical fluctuations of the initial conditions, together with a subsequent collective expansion. This is a strong point in favour of a fluid-like behavior even in $pp$ scattering, where we have to deal with length scales of the order of 0.1 fm., Comment: 5 pages, 4 figures

In $pp$ scattering at LHC energies, large numbers of elementary scatterings will contribute significantly, and the corresponding high multiplicity events will be of particular interest. Elementary scatterings are parton ladders, identified with color flux-tubes. In high multiplicity events, many of these flux tubes are produced in the same space region, creating high energy densities. We argue that there are good reasons to employ the successful procedure used for heavy ion collisions: matter is assumed to thermalizes quickly, such that the energy from the flux-tubes can be taken as initial condition for a hydrodynamic expansion. This scenario gets spectacular support from very recent results on Bose-Einstein correlations in $pp$ scattering at 900 GeV at LHC., Comment: 11 pages, 20 figures

The hydrokinetic approach, that incorporates hydrodynamic expansion of the systems formed in A+A collisions and their dynamical decoupling, is applied to restore the initial conditions and space-time picture of the matter evolution in central Au+Au collisions at the top RHIC energy. The analysis is based on the detailed reproduction of the pion and kaon momentum spectra and femtoscopic data in whole interval of the transverse momenta studied by both STAR and PHENIX collaborations. The fitting procedure utilizes the two parameters: the maximal energy density at supposed thermalization time 1 fm/c and the strength of the pre-thermal flows developed to this time. The quark-gluon plasma and hadronic gas is supposed to be in complete local equilibrium above the chemical freeze-out temperature $T_{ch}$ = 165 MeV with the equation of states (EoS) at high temperatures as in the lattice QCD. Below $T_{ch}$ the EoS in the expanding and gradually decoupling fluid depends on the composition of the hadron-resonance gas at each space-time point and accounts for decays of resonances into the non-equilibrated medium. A good description of the pion and kaon transverse momentum spectra and interferometry radii is reached at both used initial energy density profiles motivated by the Glauber and Color Glass Condensate (CGC) models, however, at different initial energy densities. The discussion as for the approximate pion and kaon $m_T$-scaling for the interferometry radii is based on a comparison of the emission functions for these particles., Comment: 31 pages, 3 figures

We present a realistic treatment of the hydrodynamic evolution of ultrarelativistic heavy ion collisions, based on the following features: initial conditions obtained from a flux tube approach, compatible with the string model and the color glass condensate picture; event-by-event procedure, taking into the account the highly irregular space structure of single events, being experimentally visible via so-called ridge structures in two-particle correlations; use of an efficient code for solving the hydrodynamic equations in 3+1 dimensions, including the conservation of baryon number, strangeness, and electric charge; employment of a realistic equation-of-state, compatible with lattice gauge results; use of a complete hadron resonance table, making our calculations compatible with the results from statistical models; hadronic cascade procedure after an hadronization from the thermal matter at an early time., Comment: 29 pages, 49 figures

A study of energy behavior of the pion spectra and interferometry scales is carried out for the top SPS, RHIC and LHC energies within the hydrokinetic approach. The latter allows one to describe evolution of quark-gluon and hadron matter as well as continuous particle emission from the fluid in agreement with the underlying kinetic equations. The main mechanisms that lead to the paradoxical, at first sight, behavior of the interferometry scales, are exposed. In particular, a slow decrease and apparent saturation of $R_{out}/R_{side}$ ratio with an energy growth happens due to a strengthening of positive correlations between space and time positions of pions emitted at the radial periphery of the system. Such en effect is a consequence of a developing of the pre-thermal collective transverse flows and an increase of the initial energy density in the fireball., Comment: The results are re-calculated for more realistic Glauber-like initial conditions instead of the Gaussian ones. The analytic estimates of the ratio of out- to side- HBT radii are presented in more transparent form. The discussions concerning the viscosity effects are extended. The main results and conclusions are the same as in the previous version of this Letter. Accepted for Phys.Lett.B

The results related to non-equilibrium phenomena at the very early and late stages of the processes of A+A collisions are presented. A good description of the hadron momentum spectra as well as pion and kaon interferometry data at RHIC is reached within the realistic dynamical picture of A+A collisions: HydroKinetic Model (HKM). The model accumulates the following features: not too early thermalization time; $\tau\geq 1$ fm/c; a developing of the pre-thermal transverse flows; the effectively more hard, than in the case of chemical equilibrium, equation of state of expanding chemically non-equilibrated multi-hadronic gas; a continuous non-equilibrated emission of hadrons. All these factors lead to a good description of the mentioned RHIC data, in particular, the observed $R_{out}/R_{side}$ ratios, solving, therefore, the HBT puzzle in detailed realistic model., Comment: 10 pages, 2 figures; talks given at XXXIX International Symposium on Multiparticle Dynamics, Gomel, Belarus September 4-9, 2009 and ECT Workshop on flows and dissipation in ultrarelativistic A+A collisions September 14-18, 2009

We investigate the rapidity dependence of the elliptical flow in heavy ion collisions at 200 GeV (cms), by employing a three-dimensional hydrodynamic evolution, based on different initial conditions, and different freeze-out scenarios. It will be shown that the form of pseudo-rapidity ($\eta$) dependence of the elliptical flow is almost identical to space-time-rapidity ($\eta_{s}$) dependence of the initial energy distribution, independent of the freeze-out prescriptions.

In this note we review our ideas, first published in year 2006, and corresponding results, including the new ones, which show that whereas the assumption of (partial) thermalization in relativistic A+A collisions is really crucial to explain soft physics observables, the hypotheses of {\it early} thermalization at times less than 1 fm/c is not necessary. The reason for the later conclusion is that the initial transverse flow in thermal matter as well as its anisotropy, leading to asymmetry of the transverse momentum spectra, could be developed at pre-thermal, either partonic or classical field - Glasma, stage with even more efficiency than in the case of very early perfect hydrodynamics. Such radial and elliptic flows develop no matter whether a pressure already established. The general reason for them is an essential finiteness of the system in transverse direction., Comment: Presented at Workshop on Particle Correlations and Femtoscopy: WPCF-2008