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Strangeness Production in Collision Systems at RHIC: PANIC 2005

This presentation discusses the understanding of strangeness production in elementary collisions and heavy-ion collisions at RHIC. It explores the models that represent the data and the influence of deconfinement on strangeness production. The talk also covers particle reconstruction techniques and pT spectra at mid-rapidity.

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Strangeness Production in Collision Systems at RHIC: PANIC 2005

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  1. Strangeness production in small and large collision systems at RHIC PANIC 2005, Santa Fe, Oct 24-28 Mark Heinz for the STAR Collaboration Yale University

  2. Outline – Physics topics • Elementary collisions (p+p) at √s=200 GeV • How well do we understand strangeness production in p+p collisions at RHIC energies ? • Which models can represent our data ? • Can we see a difference in our data between quark and gluon jet events ? • Strangeness in Heavy ion collisions (Au+Au) • How does deconfinement influence strangeness production ? • Strangeness Enhancement (SE) • How well do Statistical Hadronization Models predict this ? • Chemical equilibrium conditions PANIC 2005, Santa Fe

  3. Strange Particle Reconstruction • Particle identification: • Secondary decay vertices: • Λ p +  - b.r. 64% • K0short  - + + b.r. 68% • K-(kink)  - + 0 b.r. 21%  - +  b.r. 63% • Ξ-  - + Λ b.r. 99% • Ω- K-+ Λ b.r. 68% • Cuts on dE/dx of daughters • Topological cuts PANIC 2005, Santa Fe

  4. pT-spectra at mid-rapidity • pT-spectra are parameterized with combined function: |y|<0.5 |y|<0.5 |y|<0.5 |y|<0.5 Stat. errors only PANIC 2005, Santa Fe

  5. Eskola et al Nucl. Phys A 713 (2003) STAR STAR Preliminary K0s  - STAR Preliminary STAR Preliminary PYTHIA comparison • Leading Order (LO) “Lund” string fragmentation phenomenology • Version 6.2 (2004) • Tune K-Factor: accounts for NLO processes in hard cross-section • Version 6.3 (January 2005) • New multiple scattering algorithm What are the appropriate K-factor for each particle species? PANIC 2005, Santa Fe

  6. PYTHIA <pT> vs Nch • K-factor does account for increase of <pT> with charged multiplicity • More sensitive observable to compare models to (mini-jet implementation in models) STAR Preliminary STAR Preliminary New PYTHIA version shows good agreement for strange particle species PANIC 2005, Santa Fe

  7. <pT> dNch/d Mini-jet production in p+p • Minijet phenomenology: pQCD model (PYTHIA,HIJING) • High multiplicity p+p events  “Harder” parton interaction - Mini-jets  Higher pT final states  Higher <pT> Njet=2 Nch XN.Wang et al (Phys Rev D45, 1992) PANIC 2005, Santa Fe

  8. Strange baryon ratios • pT dependence of ratios is influenced by differences in gluon vs quark jet fragmentation • Particle ratios increase with strangeness content STAR Preliminary STAR Preliminary Confirms our expectation that p+p interactions are weighted by gluon jet events. PANIC 2005, Santa Fe

  9. Albino et al. ,hep-ph/0502188 UA1 @ 630 GeV K0s STAR @ 200 GeV Albino et al. ,hep-ph/0510173  NLO pT-spectra • NLO Parametrization of Parton Distribution functions (PDF) and Fragmentation function (FF) • KKP FF have been “industry standard” for the last years. Describe pions and kaons well. • 2005: AKK fragmentation functions: • Light-flavour separated parameterization from recent OPAL measurement NLO calculations by W.Vogelsang STAR Preliminary STAR Preliminary FF from Kniehl et al (Nucl.Phys. B582) FF from Vogelsang et al (Phys. Rev. D57)  Major Uncertainties remain in the determination of the Gluon Fragmentation function Dgh(z,μ)  STAR data is used to constrain Gluon→Lambda FF PANIC 2005, Santa Fe

  10. Statistical Equilibrium Model (Redlich et al.) Statistical non-equilibrium Model (Rafelski et al.) RHIC √s=130 GeV STAR Preliminary Europ. Phys. Jour C24 (2002) nucl-th/0506044 Strangeness Enhancement STAR data • Enhancement is very sensitive to Tfo assumed to be 175 MeV here • Strangeness chemical non-equilibrium accounted for by factor s  Rafelski allows for additional non-equilibrium of light quark flavors by factor q PANIC 2005, Santa Fe

  11. Conclusions • Recent improvements in LO-pQCD models and NLO calculations are now more consistent with STAR p+p strangeness data • Further statistics needed to see drop of anti-baryon/baryon ratio vs pT as predicted from quark vs gluon jet phenomenology • Increase in <pT> with Nch due to mini-jets is now better modeled in PYTHIA 6.3 • AKK (Albino,Kniehl,Kramer) NLO calculations using constrained fragmentation functions reproduce STAR and UA1 data nicely. • Centrality dependence of strangeness enhancement does not grow proportionally to Npart. Rafelski’s non-equilibrium model is capable of reproducing this behavior PANIC 2005, Santa Fe

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