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Helmut Oeschler Darmstadt University of Technology

Centrality Dependence of Strange Particle Yields from SIS up to RHIC. Helmut Oeschler Darmstadt University of Technology. Brookhaven Nat. Lab., February 17 th , 2006. Observations from SIS up to RHIC: Do particle ratios change with centrality? Strange/non-strange particles?

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Helmut Oeschler Darmstadt University of Technology

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  1. Centrality Dependence of Strange Particle Yields from SIS up to RHIC Helmut Oeschler Darmstadt University of Technology Brookhaven Nat. Lab., February 17th, 2006

  2. Observations from SIS up to RHIC: Do particle ratios change with centrality? Strange/non-strange particles? Are periph. PbPb collisions and central SiSi the same or very similar? Interpretations: What do we expect based on our present knowledge?

  3. SIS: • Ni+Ni and Au+Au • 1.5 A GeV • Förster, F. Uhlig, . • PRL 91 (2003) 152301 • KaoS Coll. • Rise of K+ and K- similar • At the same Apart, Ni+Ni central and Au+Au periph. similar

  4. AGS E802 PRC 60 (1999) K+ and K- similar rise Periph. AuAu very different from SiAl.

  5. SPS NA49 Coll.You have the choice! C. Höhne, PhD thesis

  6. K+ and K- very similar

  7. Phenix NP A757 (2005) K+ and K- very similar! Lighter systems?

  8. K-/K+ Ratio from SIS up to RHIC Astonishing only at low incident energies! K+ threshold at 1.58 AGeV K- threshold at 2.5 AGeV

  9. K- and K+ are linked Au+Au and Ni+Ni 1.5 AGeV A. Förster, F. Uhlig et al., KaoS PRL 91 (2003) 152301 dashed line: stat. Model Step 1 NN -> Λ K+ N Step 2 Λπ -> K- N K- and K+ are linked via strangeness exchange „Law of mass action“ J. Cleymans, et al. PLB603(2004)

  10. Strangeness Exchange If equilibrium, then K- yield just proportional to the density of K+ and the density of pions! K+ proportional to Λ! (associate production!) Hence: K-/K+~ pion density!

  11. Test of the Law of Mass ActionJ. Cleymans et al., PLB κ = ([π] [Y])/([K-] [N]) [Y] ~ [K+]

  12. Transport model calculations repulsive K+N attractive K-N In-medium effects just cancel in the K- yield!

  13. Strangeness Exchange at AGS? AGS: L. Ahle et al., PLB 490 J. Klay et al., PRC68 PLB603

  14. T(K-) < T(K+) at 1 – 2 A GeV

  15. Canonical Statistical Model Ni+Ni J. Cleymans, HO, K. Redlich, PR C59 (1999) 1663

  16. Expected Centrality Dependence (SM) Pion density n(π) = exp(-Eπ/T) Strangeness is conserved! Kaon density NN N Λ K+ n(K) = exp(-EK/T) [g V∫ … exp[-(EΛ-µB)/T] J. Cleymans, HO, K. Redlich, PRC 60 (1999)

  17. Thanks, Claudia! Qualitative agreement! Except for AGS!

  18. Chemical Freeze Out J. Cleymans and K. Redlich, PRL 81 (1998) 5284

  19. S. Wheaton, canonical statistical model Λ/π Corr. Volume prop to Apart Drastic change expected at AGS energies! Depends on the size of the correlation volume! Grand-canonical limit

  20. AGS Au+Au 6 A GeV P. Chung et al., E895 Coll. PRL 91(2003) updated

  21. SPS NA49 Coll.You have the choice! C. Höhne, PhD thesis

  22. Two serious problems: (i) How to get Apart in a reliable way? From measurements? But what is Apart? (ii) From the fraction of σ(Reac)? Then it depends on the chosen value! Divide by pions? Not Bad! But are we sure that π/Apart is flat? Better: Compare central collisions of different systems!

  23. NA49 Data - 158 AGeV PRL … Ingrid Kraus et al., to be published Corr. vol. NOT prop to Apart

  24. Modification of the Stat. Model • Instead of: strangeness undersaturation factor gS Fit parameter • Alternative: small clusters (RC) in fireball (R): • Chemical equilibrium in subvolumes: canonical suppression • RC free parameter R RC

  25. Model setting with RC • RHIC : clusters still much smaller than fireball

  26. Above RC~ 2 fm, nearly grand-canonical !

  27. Conclusions • Centrality dependence only qualitative! Apart is ill-defined. • Canonical description roughly okay! • Periph. PbPb -- central light system?? • From system-size dependence: Correlation volume NOT prop. to Apart Various ideas! Here: fitting the corr. vol.! Small Rc

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