Dependence of lepton pair emission on EoS and initial state

We present results from a hydrodynamic calculation for thermal emission of lepton pairs in central lead-lead collisions at the CERN SPS energy. Dependence of the emission on the initial conditions and Equation of State (EoS) is considered and the spectra are compared with CERES data and calculated distribution of Drell--Yan pairs.Comment: 4 pages, includes 4 ps-figures, talk at Quark Matter'97, Tsukuba, Japa

On local and global equilibrium in heavy ion collisions

The thermal model is commonly used in two different ways for the description of hadron production in ultra-relativistic heavy ion collision. One is the application of the thermal model to 4pi integrated data and the other is the thermal description of central dN/dy ratios. While the first method implicitly assumes global equilibrium the other scenario assumes Bjorken scaling within the investigated rapidity range. Both assumptions are only approximations for real physical collision systems. We study the impact of both approximations for the extraction of thermal parameters on the exemplary case of S+S collisions at SPS energies. The particle distributions are modeled by a hydrodynamical description of the relevant collision system.Comment: 13 pages, 2 figures included, uses REVTE

The Role of Strangeness in Ultrarelativistic Nuclear Collisions

We review the progress in understanding the strange particle yields in nuclear collisions and their role in signalling quark-gluon plasma formation. We report on new insights into the formation mechanisms of strange particles during ultrarelativistic heavy-ion collisions and discuss interesting new details of the strangeness phase diagram. In the main part of the review we show how the measured (multi-)strange particle abundances can be used as a testing ground for chemical equilibration in nuclear collisions, and how the results of such an analysis lead to important constraints on the collision dynamics and space-time evolution of high energy heavy-ion reactions.Comment: 82 pages, uuencoded compressed postscript file. The full postscript file can also be obtained via www at http://www.physics.helsinki.fi/tft/tft_preprints.html or via anonymous ftp at ftp://rock.helsinki.fi/pub/preprints/tft/Year1995/HU-TFT-95-27/paper.ps.Z or directly from authors. To appear in Quark-Gluon Plasma 2, R.C Hwa (Eds

THE THING Hamburg:A Temporary Democratization of the Local Art Field

THE THING Hamburg was an experimental Internet platform whose vocation was to contribute to the democratization of the art field, to negotiate new forms of art in practice, and to be a site for political learning and engagement. We, the authors, were actively involved in the project on various levels. In this paper, we trace the (local) circumstances that led to the emergence of the project and take a look at its historical precursor, we reflect on the organizational form of this collectively-run and participatory platform, and we investigate the role locality can play in the development of political agency. As a non-profit Internet platform built with free software, the project also invites a reflection of the role technology can play for the creation of independent experimental spaces for social innovation and how they make a difference against the backdrop of corporate social media. Relating the project to both the conceptual innovations of the Russian avant-garde as well as media-utopian projections shows that THE THING Hamburg stands in the tradition of an art that expands its own field by invoking a self-issued social assignment. Challenging the norms and in stitutions of the art field does not remain an exercise in self-referentiality; it rather redefines the role of art as an agent for political learning and how the use of technology in society at large can be emancipatory. And just as small projects like the THE THING Hamburg draw on old utopias for their contemporary negotiations of art, they equally produce more questions than they provide answers

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

Hadronic matter compressibility from event-by-event analysis of heavy-ion collisions

We propose a method to measure the hadronic matter compressibility by means of the event-by-event analysis of heavy-ion collisions at high energies. The method, which utilizes the thermodynamical relation between the compressibility and the particle number fluctuations, requires a simultaneous measurement of the particle source size, temperature and particle multiplicity.Comment: 5 pages, no macro

Anisotropic flow from AGS to LHC energies

Within hydrodynamics we study the effects of the initial spatial anisotropy in non-central heavy-ion collisions on the momentum distributions of the emitted hadrons. We show that the elliptic flow measured at midrapidity in 158 A GeV/c Pb+Pb collisions can be quantitatively reproduced by hydrodynamic expansion, indicating early thermalization in the collision. We predict the excitation functions of the 2nd and 4th harmonic flow coefficients from AGS to LHC energies and discuss their sensitivity to the quark-hadron phase transition.Comment: 5 pages ReVTeX, incl. 4 figures, subm. to Phys. Lett. B. Improved discussion of results and a few added reference

Mass number scaling in ultra-relativistic nuclear collisions from a hydrodynamical approach

We study the different nucleus-nucleus collisions, O+Au, S+S, S+Ag, S+Au and Pb+Pb, at the CERN-SPS energy in a one-fluid hydrodynamical approach using a parametrization based on baryon stopping in terms of the thickness of colliding nuclei. Good agreement with measured particle spectra is achieved. We deduce the mass number scaling behaviour of the initial energy density. We find that the equilibration time is nearly independent of the size of the colliding nuclei.Comment: 27 pages, figures included, submitted to European Physical Journa

Strange Messages: Chemical and Thermal Freeze-out in Nuclear Collisions

Thermal models are commonly used to interpret heavy-ion data on particle yields and spectra and to extract the conditions of chemical and thermal freeze-out in heavy-ion collisions. I discuss the usefulness and limitations of such thermal model analyses and review the experimental and theoretical evidence for thermalization in nuclear collisions. The crucial role of correlating strangeness production data with single particle spectra and two-particle correlation measurements is pointed out. A consistent dynamical picture for the heavy-ion data from the CERN SPS involves an initial prehadronic stage with deconfined color and with an appreciable isotropic pressure component. This requires an early onset of thermalization.Comment: 15 pages, 2 figures, talk given at Strange Quark Matter '98, Padova, Italy, 20-24 July 1998, to be published in J. Phys. G 25; final version with updated reference