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 Large Hadron Collider (LHC) experiments have provided a number of very interesting data sets comprising minimum bias $p\text{−}p,p$-Pb, and Pb-Pb interactions. We describe the changes required to the model to reproduce in detail the new data available from the LHC and the consequences in the interpretation of these data. In particular we discuss the effect of the collective hadronization in $p\text{−}p$ 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 $p\text{−}p$ 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 results for all particles with transverse momentum from $p_{\mathrm{t}}=0$ to a few GeV/$c$.

The system created in ultrarelativistic nuclear collisions is known to behave as an almost ideal liquid. In noncentral collisions, because of the large orbital momentum, such a system might be the fluid with the highest vorticity ever created under laboratory conditions. Particles emerging from such a highly vorticous fluid are expected to be globally polarized with their spins on average pointing along the system angular momentum. Vorticity-induced polarization is the same for particles and antiparticles, but the intense magnetic field generated in these collisions may lead to the splitting in polarization. In this paper we outline the thermal approach to the calculation of the global polarization phenomenon for particles with spin and we discuss the details of the experimental study of this phenomenon, estimating the effect of feed-down. A general formula is derived for the polarization transfer in two-body decays and, particularly, for strong and electromagnetic decays. We find that accounting for such effects is crucial when extracting vorticity and magnetic field from the experimental data.

We present a sophisticated 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; an 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; the use of an efficient code for solving the hydrodynamic equations in $3+1$ dimensions, including the conservation of baryon number, strangeness, and electric charge; the employment of a realistic equation of state, compatible with lattice gauge results; the use of a complete hadron resonance table, making our calculations compatible with the results from statistical models; and a hadronic cascade procedure after hadronization from the thermal matter at an early time.

We present a calculation of the global polarization of \(\Lambda \) hyperons in relativistic Au–Au collisions at RHIC Beam Energy Scan range \(\sqrt{s_\mathrm {NN}}=7.7{-}200\) GeV with a \(3+1\)-dimensional cascade\(\,+\,\)viscous hydro 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.

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 nonfluctuating 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.

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 terms of hydrodynamic flow. We analyze these data using epos3, an event generator based on a $3\mathrm{D}+1$ 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 perturbative QCD 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.