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The Role of Strangeness in Ultrarelativistic Nuclear

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The Role of Strangeness in Ultrarelativistic Nuclear HUTFT THE ROLE OF STRANGENESS IN a ULTRARELATIVISTIC NUCLEAR COLLISIONS Josef Sollfrank Research Institute for Theoretical Physics PO Box FIN University of Helsinki Finland and Ulrich Heinz Institut f ur Theoretische Physik Universitat Regensburg D Regensburg Germany ABSTRACT We review the progress in understanding the strange particle yields in nuclear colli sions and their role in signalling quarkgluon plasma formation We rep ort on new insights into the formation mechanisms of strange particles during ultrarelativistic heavyion collisions and discuss interesting new details of the strangeness phase di agram In the main part of the review we show how the measured multistrange particle abundances can b e used as a testing ground for chemical equilibration in nuclear collisions and how the results of such an analysis lead to imp ortant con straints on the collision dynamics and spacetime evolution of high energy heavyion reactions a To b e published in QuarkGluon Plasma RC Hwa Eds World Scientic Contents Introduction Strangeness Pro duction Mechanisms QuarkGluon Pro duction Mechanisms Hadronic Pro duction Mechanisms Thermal Mo dels Thermal Parameters Relative and Absolute Chemical Equilibrium The Partition Function The Phase Diagram of Strange Matter The Strange Matter Iglo o Isentropic Expansion Trajectories The T ! Limit of the Phase Diagram Hadronization of Strange Quark Matter at T 6 Ingredients and Extensions of the Thermal Mo del The Strangeness Neutrality Condition Transverse Flow and Transverse Momentum Sp ectra Resonance Decays Particle Ratios The Sp ecic Entropy Chemical Parameters from Strange Particle Ratios Thermal and Chemical Parameters from the WA Exp eriment Thermal and Chemical Parameters from the NA Exp eriment Thermal and Chemical Parameters from the NA Exp eriment Thermal and Chemical Parameters from the AGS Exp eriments Sp ecic Entropy from Heavy Ion Exp eriments Discussion of the Results Consequences for FreezeOut Mo dels Sequential Freezeout Sudden Hadronization Scenarios Conclusions and Outlo ok A App endix Literature Guide to Strange Particle Measurements References THE ROLE OF STRANGENESS IN ULTRARELATIVISTIC NUCLEAR COLLISIONS Josef Sollfrank Research Institute for Theoretical Physics PO Box FIN University of Helsinki Finland and Ulrich Heinz Institut f ur Theoretische Physik Universitat Regensburg D Regensburg Germany ABSTRACT We review the progress in understanding the strange particle yields in nuclear colli sions and their role in signalling quarkgluon plasma formation We rep ort on new insights into the formation mechanisms of strange particles during ultrarelativistic heavyion collisions and discuss interesting new details of the strangeness phase di agram In the main part of the review we show how the measured multistrange particle abundances can b e used as a testing ground for chemical equilibration in nuclear collisions and how the results of such an analysis lead to imp ortant con straints on the collision dynamics and spacetime evolution of high energy heavyion reactions Introduction The high energy heavyion program at the Bro okhaven Alternating Gradient Syn chrotron AGS and at the CERN Sup er Proton Synchrotron SPS is devoted to the investigation of nuclear matter under extreme conditions high temp eratures andor densities The most exciting prosp ect is the p ossible observation of a phase tran 1 sition from normal nuclear matter to a socalled QuarkGluon Plasma QGP A large amount of exp erimental and theoretical eort has gone into working out clear signatures for the QGP Although the ultimate hop e is to eventually see the QGP 2 shine via direct electromagnetic radiation of photons and lepton pairs for prac tical reasons larger signals and b etter understanding of backgrounds exp erimental eorts have up to now fo cussed on the pro duction of hadrons with newly pro duced quark avors strangeness charm etc At present the largest b o dy of data with the so far most promising indications for unconventional physics has b een accumulated 3 in the sector of strange particles following the early suggestions of Rafelski well over years ago There are two ma jor asp ects of the socalled strangeness signal The more sp ec ulative one is connected with the search for exotic strange matter which requires the intermediate pro duction of a QGP state The list of these so far undetected phenom 4 6 ena contains strangelets small droplets of metastable strange quark matter the 7 ultimate pro of of the existence the quarkgluon plasma but also the Hdibaryon 8 and more recently the socalled MEMOs Metastable Exotic Multistrange Ob jects For this asp ect of the strangeness signal we refer the reader to Refs and to the contribution by C Greiner and J Schaner in this volume The other more conventional asp ect of strangeness as a QGP signature was in tro duced by Rafelski who predicted an enhanced pro duction of strange hadrons in heavyion collisions when compared to nucleonnucleon or nucleonnucleus collisions 311 at the same energy The idea is simple and is based on the dierent pro duc tion mechanisms of strangeness in a QGP individual strange quark pair pro duction from a dense system of gluons and light quarks and in a hadron gas pro duction of hadrons with opp osite strangeness in inelastic hadronhadron interactions which have strongly dierent intrinsic time scales But the quantitative interpretation of the observed enhanced nuclear strangeness pro duction as a QGP signal is a subtle issue Clear and unambiguous statements require a quantitative theoretical understanding of strangeness pro duction in a QGP and in a hadronic environment including the initial preequilibrium stages of the evolution of strangeness during the reball life time in particular during the dicult hadronization stage and of the variations of the nonstrange multiplicities as one go es from nucleonnucleon to nuclear collisions Some of these issues are still not satisfactorily resolved but signicant progress has b een made in the last few years which we will review in this article An imp ortant not always appreciated advantage of strangeness is the large variety of strange particles sp ecies Due to the long life time of strange quarks against weak decay one has by now b een able to identify and measure the yields and sp ectra of dierent sp ecies of strange hadrons in heavyion exp eriments K K K s An uptodate list of references is given in Tables and in the App endix an account of the latest results can b e found in Refs These data triggered a lot of studies within various mo dels for nuclear collisions Esp ecially 14 26 thermal mo dels which are characterized by a rather small number of adjustable parameters and thus predict various restrictive relations b etween the various particle sp ecies can thus b e tested very precisely We will devote a large fraction of this article to these investigations not only b ecause large progress has b een made in particular in this sector but also b ecause these studies provide a simple and intuitive picture which gives valuable qualitative and quantitative insight into the physics of hadron pro duction in nuclear collisions The thermal picture allows for a straightforward investigation of the question of chemical equilibration in the strange and nonstrange sectors and provides a simple characterization of the nal stages of the the heavyion collision in terms of freezeout parameters entropy pro duction and collective ow dynamics It provides an intuitive bridge b etween the data on the one hand and complete kinetic simulations of the phasespace evolution of the collision region on the other hand In this review we concentrate on the developments after the publication of the 1 9 27 rst volume of this b o ok for the earlier work we refer to the reviews by Ko ch In addition to the discussion of strange hadron yields and momentum sp ectra we also rep ort on some recent progress on the the microscopic mechanisms of strange particle pro duction in a hot and dense environment and on the general structure of the strangeness phase diagram The article is divided into the following parts In Section we cover some new developments on strangeness pro duction mechanisms Section provides the basic equations for the thermo dynamics of hadronic matter These are used in Section to study the phase diagram of strange matter In Section we discuss additional ingredients like collective ow and deviations from chemical equilibrium which are necessary when testing the thermal mo del against exp erimental data This is then done in Section for various heavy exp eriments on strange particle pro duction In Section we shortly discuss the asp ect of entropy pro duction in these heavyion exp eriments A comprehensive discussion of the outcome of the thermal analysis follows in Section In Section we draw some sp eculative conclusions from this discussion on the nature of the hadronization and freezeout pro cesses and discuss the internal consistency of our interpretation of the strange particle abundances
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