Strange particle production at RHIC…

1. Strange particle production at RHIC… Anthony Timmins for the STAR Collaboration Hot Quarks 26th August 2008 STAR

2. Contents • Strangeness as a QGP signature… • Strangeness production in light systems… • Model comparisons at RHIC energies… • Empirical scalings… • Discussion… • Summary…

3. Strangeness as a QGP signature Phys. Rev. Lett. 48 (1982) 1066 • Original Predictions… • Strange quark’s dynamical mass drops in quark gluon plasma (QGP) • s+s production cross section increases • s+s in QGP would reach thermal expectations on a small time scale… • QGP saturation time s ~3 fm/c • Hadron gas saturation times (Phys. Rep. 142 (1986) 167) • ~30 fm/c for Kaons (K) • ~200 fm/c Lambdas () and Xis () • Heavy-ion collision lifetime at RHIC < 10 fm/c (HBT)… Strangeness saturation times in QGP for various S…

4. Strangeness as a QGP signature QGP and expansion hadronic phase Prior to collision Initial state hadronization …one naively expects final state strange hadron yields to also reach thermal expectations… If strangeness reaches thermal expectations in a QGP…

5. Strangeness as a QGP signature J. Phys. G32 (2006) S105 • In heavy-ion collisions, we address this in two ways experimentally… • Measure enhancement factor E=strangeness yields per participant relative to pp • Compare to Canonical (thermal) model predictions… • These assume volume  <Npart> • Perform a thermal fit on measured particle ratios and extract the strangeness saturation factor s • Deviation of strangeness production from thermal model expectations… • s=1 strangeness fully equilibrated • s=0 strangeness fully suppressed Phys. Rev. Lett. 98 (2007) 062301

6. Strangeness production in light systems… Phys. Rev. C 60 (1999) 044904 • AGS and SPS energies showed something interesting at lower energies for lighter systems… • Kaons per <Npart> higher in Si+Al, Si+Au compared to Au+Au with similar <Npart> at ~5 GeV • K+/ higher in lighter systems compared to expected Pb+Pb values at 17.3 GeV • What happens at RHIC energies? • Cu+Cu collisions can help us investigate… • What will strangeness measurements tell us about the current theory? Nucl. Phys. A 715 (2003) 474

7. Strangeness production in light systems… STAR Preliminary • Mid-rapidity Cu+Cu 200 GeV yields higher at given <Npart>, contrary to Canonical predictions…

8. STAR Preliminary Strangeness production in light systems… • Evidence for extra multi-strange particle production in central Cu+Cu compared to mid-central Au+Au… •  particle show above unity enhancement at 200 GeV • Also contrary to naive interpretation of the Canonical model • E=1 for particles with closed strangeness… • Differences between Cu+Cu Au+Au not observed within errors…

9. Further model comparisons at RHIC… STAR Preliminary • EPOS (Phys. Rev. Lett. 98 (2007) 152301) • Core hadron production • High density region follows statistical emission… • Expands from initial state and freezes out at critical energy density C=0.22 GeV/fm3 to produce hadrons • Upon hadronization, strangeness oversaturated with s=1.3 • Corona hadron production • Superposition of p+p collisions • Uses successful p+p/d+Au EPOS generator which treat hadron production dynamically • Strangeness therefore undersaturated with s<1 Participant density in central Cu+Cu 200 GeV collisions from MC Glauber

10. Further model comparisons at RHIC… • AMPT (Phys.Rev. C72 (2005) 064901): • HIJING based generator • Soft hadron production is governed by nucleon excitation and string breaking.. • Hard hadron production from (mini) jets whose cross sections are governed by pQCD.. • Mini-jet partons and newly produced hadrons allowed to rescatter… • AMPT results reduce to HIJING without such mechanisms… • Describes various aspects of Au+Au 200 GeV data well… Central Au+Au 200 GeV collisions..

11. Further model comparisons at RHIC… STAR Preliminary Both models reproduce key qualitative features of the data…

12. Further model comparisons at RHIC… STAR Preliminary • Predictions for multi-strange particles follow similar trends.. • EPOS does a better job for’s • Predicts large enhancements for …

13. Empirical scalings… STAR Preliminary • , , K0S yields show similar systematics for Cu+Cu and Au+Au at 200 GeV • Bulk strangeness production must follow suit… • Approximate total strangeness production at y~0 • <Npart> scaling clearly fails for data and theory… • No surprise there given the above definition..

14. Empirical scalings… STAR Preliminary • Test to see if strangeness scales with number of binary collisions <Nbin>… • Hard processes not the dominant contribution to strangeness production ? • Kharzeev-Nardi hypothesis: • Soft production scales with <Npart> • Hard production scales with <Nbin> • x is energy dependent relative weight of each process • Fails for strangeness at RHIC…

15. Empirical scalings… STAR Preliminary • Test to see if strangeness scales with participants that undergo more thanone collision = <Npart>1>… • Data moves most towards a common trend compared to other scalings… • EPOS lies on common trend • AMPT nearly lies on common trend..

16. Discussion… STAR Preliminary • Why does <Npart>1> work best in the models? • AMPT: String Phenomenology • Valence partons in these participants get more pT kicks • When intra-nucleon strings decay, strangeness production is higher compared to participants with just one collision • EPOS: Core Size • Core production major source of strangeness • Core size scales with <Npart>1> as these participants sit in high density region… • In both models, these features lead to.. • Rising per participant yields for a given system • Higher per participant yields in Cu+Cu at given <Npart> • Both are reproduced by data… STAR Preliminary

17. Discussion… • Key question: What do the current comparisons tell us about strangeness production as a QGP signature? • EPOS • Core aims to represent deconfined matter with differing hadron production scheme compared p+p • If strangeness equilibrates/oversaturates in QGP, not inconceivable that s will be >=1 upon hadronisation.. • Are there are other measurements that support core/corona approach? • AMPT • Offers different and more dynamical understanding of particle production in heavy-ion collisions… • Strangeness yield systematics explained via varying levels nucleon excitation E.g. number of participants with more than one collision • In conflict with original hypothesis? • Soft hadron production from independently decaying strings rather than deconfined coupled medium.. • Can these mechanisms also explain s~1 in heavy-ion collisions? • Both models require further comparisons with data to improve distinguishing power…

18. Summary… • Strangeness production in light and heavy systems continues to challenge our understanding of QCD in heavy-ion collisions… • Collision geometry key factor in mid-rapidity strangeness production at RHIC… • Strangeness per <Npart> increases with <Npart> for given system • Strangeness production higher in Cu+Cu collisions at given <Npart> • Both EPOS and AMPT models reproduce these aspects albeit with differing hadron production mechanisms… • Fraction of nucleons that undergo more one collision (<Npart>1>) appears key underlying parameter in each case…

19. Backup.. • Kaon to Pion ratios at SPS

20. Backup..