Strangeness Production in Microscopic Transport Models

1. Strangeness Production in Microscopic Transport Models Steffen A. Bass Duke University & RIKEN BNL Research Center • Microscopic Transport Models from SIS to RHIC • Mechanisms for Strangeness Production • Strangeness close to Threshold • Strangeness at SPS and RHIC • Review of Model Predictions (and Postdictions) • Energy-, Mass- and Centrality-Dependence • What can we learn? • Summary and Outlook Steffen A. Bass

2. Probing the QCD Equation of State Finding evidence for deconfinement (e.g. a QGP) is only the first step in exploring a novel domain of elementary matter! hadronic phase and freeze-out QGP and hydrodynamic expansion initial state pre-equilibrium hadronization Steffen A. Bass

3. Why use Microscopic Transport Models? hadronic phase and freeze-out QGP and hydrodynamic expansion initial state pre-equilibrium hadronization Steffen A. Bass

4. Microscopic Transport Models from SIS to RHIC Steffen A. Bass

5. A Typical Microscopic Model: UrQMD • elementary degrees of freedom: hadrons, const. (di)quarks • classical trajectories in phase-space (relativistic kinematics) • initial high energy phase of the reaction is modeled via the excitation and fragmentation of strings • 55 baryon- and 32 meson species, among those 25 N*, Δ* resonances and 29 hyperon/hyperon resonance species • full baryon-antibaryon and isospin symmetry • ideal for the description of excited hadronic matter • main physics input and parameters: • cross sections: total and partial cross sections, angular distributions • resonance parameters: total and partial decay widths • string fragmentation scheme: fragmentation functions, formation time • An interaction takes place if at the time of closes approach dminof two hadrons the following condition is fulfilled: Steffen A. Bass

6. Strangeness Production at Threshold • explicit parameterization of 3 and 4-body exit-channels: BB  NYK or BB  NNK+K- (B=N,N*,Δ, Δ* and Y=Λ,Σ) • many different channels to parameterize • loss of detailed balance • ease of implementation • resonance model of strangeness production BB  BN* and N*  YK • difficult to fit to experimental data • preservation of detailed balance • seamless transition to higher energies Steffen A. Bass

7. Parameterization vs. Resonance Model UrQMD 1.2 H. Weber, FFM Sibirtsev & Cassing, nucl-th/9820019 both approaches yield good fits to experimental data beware of old parameterizations: incompatible with COSY data! Steffen A. Bass

8. K- Rescattering in UrQMD • model degrees of freedom determine the interaction to be used • calculate cross section according to: Steffen A. Bass

9. Particle Production at SPS/RHIC Steffen A. Bass

10. Model Fits to Hadron-Hadron Data RQMD 2.3 S. Soff et al, J.Phys.G27 (2001) 449 F. Wang et al., PRC 61 (2000) models provide in general a good fit to elementary hadron-hadron data beware: tolerable deviations in individual yields may adversely affect ratios! Steffen A. Bass

11. Exotic Scenarios for Strangeness Production • Droplets: • particles in high ε regions form a mini-QGP fireball • Ropes: • overlapping strings form flux-tube with increased κ • Baryon Junctions Loops: • multi-string fragmentation into (anti-)hyperons • Diquarks from the nucleon sea: • enhanced strange diquark production • increased fragmentation into Y, Ξ, and Ω Changing κ from 1 Gev/fm to 3 GeV/fm increases the proability for a string to fragement into an Ω by a factor of 10! M. Bleicher, PRC62 (2000) 061901 Steffen A. Bass

12. Hadrons in Medium: K- Flow and Yield S. Pal et al., Phys.Rev.C62:061903, 2000 C. Hartnack et al., nucl-th/0109016 • K- flow very sensitive to K- N potential • K- yield depends strongly on K+ N potential: • linked via associate production to Λ yield - relevant process: ΛπK- N Steffen A. Bass

13. Strangeness at SIS: Medium vs. Cross Section C. Hartnack et al. J. Phys. G27 2001 C. Hartnack, Nantes • model calculations describe K+ yield well, but with conflicting statements concerning the need of potentials • ambiguity due to different parameterization of unknown cross sections Steffen A. Bass

14. Strangeness at SIS: EOS Dependence C. Fuchs et al. PRL 86 (2001) 1974 C. Hartnack, Nantes • taking the ratio of yields for different system sizes removes dependency on cross section parameterization • strong unambiguous sensitivity to EoS is recovered Steffen A. Bass

15. Excitation Function of the K+/K- Ratio data, compiled by J. Aichelin, Nantes RQMD, F. Wang et al, PRC 61 (2000) 064904 • SIS to AGS/SPS: K-/K+ ratio scales as π/A • K- yield is dominated by σ(ΛπK- N) and M(K+)  M(Λ) • pions serve as catalytic converters : ΛK- • SPS/RHIC: K-pair production dominates Steffen A. Bass

16. Energy-Dependence of the K/π Ratio RQMD F. Wang et al, PRC 61 (2000) 064904 HSD Cassing et al, NPA674 (2000) 249 UrQMD, FFM (plots/comparisons to data courtesy of R. Bramm & T. Kollegger, IKF, FFM) UrQMD • K/π ratio is very sensitive to the internal microscopic dynamics of the models! • HSD: lacks strangeness prod. via high-mass res. • UrQMD: overpredicts pion yield at AGS, SPS Steffen A. Bass

17. Energy Density in String/Hadron Models sub-hadronic degrees of freedom: • hadrons created in string fragmentations within their formation time high energy density dominated by sub-hadronic degrees of freedom up to 2000 valence quarks in medium with ε>2 GeV/fm3 Lattice: εcrit calculated for infinite time / periodic boundaries RHIC: dynamic system with short lifetime and finite size Steffen A. Bass

18. Strangeness at SPS & RHIC: Baseline for Deconfinement Physics S. Vance & M. Gyulassy PRL 83 (1999) 1735 • conventional strangeness production cannot explain measured Ω yield • introduction of ropes/modified string tension fits data • strong indication for non-hadronic physics!! Steffen A. Bass

19. Flavor Dynamics: Radial Flow • Hydro: linear mass-dependence of slope parameter, strong radial flow • Hydro+Micro: softening of slopes for multistrange baryons • early decoupling due to low collision rates • nearly direct emission from the phase boundary Steffen A. Bass

20. Radial Flow: Model Comparison H. van Hecke et al. Phys.Rev.Lett. 81 (1998) 5764. S. Pal et al. nucl-th/0106073 • good agreement among models concerning flavor dependent slopes • sequential decoupling of hadrons from the reaction Steffen A. Bass

21. Sources of Final State Kaons and Hyperons S. Soff, Ph.D. thesis the dominant final source for K and Λ,Σ are resonances for K: baryon- and meson-resonances contribute to similar extent less than 10% are directly emitted by primordial or 2nd generation strings Steffen A. Bass

22. Summary • Transport Models • study dynamics of strangeness production • insight is gained as much by disagreement with data as with perfect fits! • Strangeness at SIS/AGS • Kaon-flow: probes K-N potential • Kaon-yield/spectra: robust probe for EoS • Strangeness Excitation Functions • K/π ratio: convolution of K and π production dynamics • mass dependence: strong indications of collectivity • Strangeness at SPS/RHIC • Mechanisms beyond known hadron-hadron physics necessary • Radial flow of multi-strange baryons: probe collective expansion at phase boundary Steffen A. Bass