On chemical equilibrium in nuclear collisions

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off-equilibrium version allowing for partial strangeness saturation is significantly better. The freeze-out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in e+e-, pp and p{\bar p} collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor gamma_S increases from about 0.45 for pp interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions. The quark strangeness suppression factor lambda_S is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding syste

Perfect-fluid hydrodynamics with constant acceleration along the stream lines and spin polarization

A global equilibrium state of a spin polarized fluid that undergoes constant acceleration along the stream lines is described as a solution of recently introduced perfect-fluid hydrodynamic equations with spin 1/2

Hydrodynamics of fluids with spin

We discuss the possibility of a non-vanishing spin tensor in relativistic hydrodynamics and its relevance to the description of Quark-Gluon-Plasma evolution in relativistic heavy ion collisions. After a short historical introduction, we report on some recent theoretical results for fully equilibrated fluids

Thermodynamic equilibrium in relativity: Four-temperature, Killing vectors and Lie derivatives

The main concepts of general relativistic thermodynamics and general relativistic statistical mechanics are reviewed. The main building block of the proper relativistic extension of the classical thermodynamics laws is the four-temperature vector \beta, which plays a major role in the quantum framework and defines a very convenient hydrodynamic frame. The general relativistic thermodynamic equilibrium condition demands \beta to be a Killing vector field. We show that a remarkable consequence is that all Lie derivatives of all physical observables along the four-temperature flow must then vanish.Comment: 13 page

The QCD Phase Diagram from Statistical Model Analysis

Ideally, the Statistical Hadronization Model (SHM) freeze-out curve should reveal the QCD parton-hadron phase transformation line in the ($T$,$\mu_B$) plane. We discuss the effects of various final state interaction phenomena, like baryon-antibaryon annihilation, core-corona effects or QCD critical point formation, which shift or deform the SHM freezeout curve. In particular, we present a method to remove the annihilation effects by quantifying them with the microscopic hadron transport model UrQMD. We further discuss the new aspects of hadronization that could be associated with the relatively broad cross-over phase transformation as predicted by lattice-QCD theory at low $\mu_B$. That opens up the possibility that various observables of hadronization, e.g. hadron formation or susceptibilities of higher order (related to grand canonical fluctuations of conserved hadronic charges) may freeze out at different characteristic temperatures. This puts into question the concept of a universal \textit{(pseudo-)critical} temperature, as does the very nature of a cross-over phase transformation.Comment: 24 pages, 11 figures. Submitted as part of the Walter Greiner memorial boo

Strangeness production from SPS to LHC

Global strangeness production in relativistic heavy ion collisions at SPS and RHIC is reviewed. Special emphasis is put on the comparison with the statistical model and the canonical suppression mechanism. It is shown that recent RHIC data on strange particle production as a function of centrality can be explained by a superposition of a fully equilibrated hadron gas and particle emission from single independent nucleon-nucleon collisions in the outer corona.Comment: Plenary talk given by F. Becattini at Quark Matter 2008, Jaipur (India). Additions to the tex

Lambda polarization in heavy ion collisions: from RHIC BES to LHC energies

STAR collaboration at RHIC has recently measured the polarization of $\Lambda$ hyperons in non-central heavy ion collisions in the RHIC Beam Energy Scan (BES) program. The magnitude of the polarization was found to decrease from few percents at the lowest BES energies to $\approx0.2$% at the top RHIC energy. The polarization signal has been reproduced in different hydrodynamic calculations assuming a thermodynamic spin-vorticity coupling mechanism at the Cooper-Frye hypersurface. In this work an extension of our existing calculations of the $\Lambda$ polarization in the RHIC BES program to the top RHIC and 2.76 TeV LHC energies is presented. The longitudinal component of the $\Lambda$ polarization, which is the dominant component of the polarization at the LHC energies, is discussed. Finally we show that the global polarization of $\Lambda$ originates dominantly from the relativistic analogue of the classical vorticity, whereas the quadrupole longitudinal component originates from the gradients of temperature and acceleration of the medium when the $\Lambda$s are produced out of the fluid.Comment: talk given at Quark Matter 2018 conference (Venice, Italy, 13-19 May 2018). 4 pages, 2 figure

Exact spin polarization of massive and massless particles in relativistic fluids at global equilibrium

We present the exact form of the spin polarization vector and the spin density matrix of massive and massless free particles of any spin and helicity at general global equilibrium in a relativistic fluid with non-vanishing thermal vorticity, thus extending the known expression at the linear order. The exact form is obtained by means of the analytic continuation of the relativistic density operator to imaginary thermal vorticity and the resummation of the obtained series. The phenomenological implications for the polarization of the $\Lambda$ hyperon in relativistic heavy-ion collisions are addressed.Comment: 14 pages + 3 pages appendices. 2 figure