Collisions at RHIC are very strange

1. Collisions at RHIC are very strange Outline • Bulk matter • Equilibrium • Enhancement • Beyond the bulk • Intermediate pT

2. RHIC - a strange particle factory

3. Are we in thermal/chemical equilibrium? • Statistical Thermal Model • Assume: • Ideal hadron resonance gas • thermally and chemically equilibrated fireball at hadro-chemical freeze-out • Recipe: • GRAND CANONICAL ensemble to describe partition function  density of particles of species i • fixed by constraints: Volume V, , strangeness chemical potentialS,isospin • input: measured particle ratios • output: temperature T and baryo-chemical potential B Particle density of each particle: Qi : 1 for u and d, -1 for u and d si : 1 for s, -1 for s gi:spin-isospin freedom mi : particle mass Tch : Chemical freeze-out temperature mq : light-quark chemical potential ms : strangeness chemical potential gs : strangeness saturation factor Compare particle ratios to experimental data

4. Canonical vs Grand Canonical • Canonical (small system i.e. p-p): Quantum Numbers conserved exactly. Computations take into account energy to create companion to ensure conservation of strangeness. Relative yields given by ratios of phase space volumes Pn/Pn’ = fn(E)/fn’(E) • Grand Canonical limit (large system i.e. central AA): Quantum Numbers conserved on average via chemical potential Just account for creation of particle itself. The rest of the system “picks up the slack”. Not new idea pointed out by Hagedorn in 1960’s (and much discussed since)

5. Comparison to data Au-Au √sNN = 200 GeV STAR Preliminary p-p √s = 200 GeV STAR Preliminary Canonical ensemble

6. Centrality and energy dependence Close to net-baryon free Tch flat with centrality ● p, K,p ● p, K,p ● p, K,p, L, X ● p, K,p, L, X Chem. equilibrium ! and 62 GeV STAR preliminary Au+Au at √sNN=200GeV TLQCD~160-170MeV Small Nch dependence of gs Energy dependence of mB

7. Centrality dependence STAR Preliminary We can describe p-p and Au-Au average ratios. Can we detail the centrality evolution? Look at the particle enhancements. E(i) = YieldAA/Npart Yieldpp /2 Au-Au √sNN = 200 GeV Transition described by E(i) behaviour There is an enhancement E(X) > E(L)

8. Strangeness phase space suppression - gs • Canonical system – p-p • Small system • Lack of phase space available • Strangeness suppressed • Grand Canonical system • – central A-A • Large system • Large phase space available • Strangeness saturated Canonical suppression increases with strangeness decreases with volume

9. Model description of centrality dependence Correlation volume: V= (ANN)·V0 ANN = Npart/2 V0 = 4/3 p·R03 R0 = 1.1 fm proton radius/ strong interactions STAR Preliminary T = 170 MeV T = 165 MeV Au-Au √sNN = 200 GeV Seems that T=170 MeV fits data best – but shape not correct K. Redlich

10. Varying T and R Au-Au √sNN = 200 GeV Calculation for most central Au-Au data Correlation volume: V0  R03 R0 ~ proton radius strong interactions Rapid increase in E(i) as T decreases SPS data indicated R = 1.1 fm K. Redlich

11. Npart dependence Correlation volume: V= (ANN)a·V0 ANN = Npart/2 V0 = 4/3 p·R03 R0 = 1.2 fm proton radius/ strong interactions STAR Preliminary T = 165 MeV a = 1/3 T = 165 MeV a = 1 T = 165 MeV a = 2/3 Au-Au √sNN = 200 GeV Npart is NOT directly correlated to the strangeness volume. K. Redlich

12. More on flavour dependence of E(i) STAR Preliminary PHOBOS: measured E(ch) for 200 and 19.6 GeV Enhancement for all particles? PHOBOS: Phys. Rev. C70, 021902(R) (2004) Au-Au √sNN = 200 GeV Yes – not predicted by model s quark content determines E

13. Moving from the bulk Compare Au+Au with p+p Collisions  RAA A+A yield Nuclear Modification Factor: p+p cross section <Nbinary>/sinelp+p R < 1 at small momenta R = 1 baseline expectation for hard processes R > 1 “Cronin” enhancements (as in pA)R < 1: Suppression

14. Rcp vs RAA √sNN = 200 GeV STAR Preliminary Canonical suppression in p+p √sNN = 200 GeV STAR Preliminary Rcp  RAA Effect increases as strange content of baryon increases.

15. Parton recombination at medium pT • Parton pT distribution is • ~exponential+power-law • 7 GeV particle via : • Fragmentation from high pT • Meson • - 2 quarks at ~3.5 GeV • Baryon • - 3 quarks at ~2.5 GeV Recombination - more baryons than mesons at medium pT

16. RCP -an energy scan √sNN=62 GeV 0-5% 40-60% 0-5% 40-60% √sNN=17.3 GeV NA57, PLB in print, nucl-ex/0507012 √sNN=200 GeV First time differences between L and L B absorption? Baryon meson splitting at all energies

17. The Rcp double ratio NA57: G. Bruno, A. Dainese: nucl-ex/0511020 Baryon/meson splitting at SPS and RHIC is the same 62 GeV Au+Au data also follows the same trend STAR Preliminary Recombination present in all systems?

18. Conclusions • Thermal models give good description of the data as function of energy and centrality. • The enhancement of strangeness as a function of centrality CAN be described– scales with Npart 1/3 NOT Npart • Non-strange particles are enhanced – NOT predicted by phase space models. • The phase space effects of p-p extend into high pT regime. • Baryon/meson splitting energy independent. ReCo at SPS.

19. BACKUP

20. Predictions from statistical model Behavior as expected

21. mT scaling STAR Preliminary p+p 200 GeV No complete mT scaling Au-Au Radial flow prevents scaling at low mT Seems to scale at higher mT p-p Appears to be scaling at low mT Baryon/meson splitting at higher mT – Gluon jets?

22. Gluon vs quark jets in p-p No absolute mT scaling – “data” scaled to match at mT~1 GeV/c Quark jets events display mass splitting Gluon jets events display baryon/meson splitting Way to explore quark vs gluon dominance

23. Recombination and v2 The complicated observed flow pattern in v2(pT) for hadrons is predicted to be simple at the quark level pT → pT /n v2 → v2 / n , n = (2, 3) for (meson, baryon) Works for p, p, K0s, ,  v2s ~ v2u,d ~ 7%

24. ReCo model and Correlations R. Hwa, Z. Tan: nucl-th/0503060 0-10%/40-80% 3 < pTtrigger < 6 The ratio of near side yields in central to peripheral collisions is around 3 at 1 GeV/c and decreases with increasing pTassoc This is in good qualitative agreement with ReCo model predictions though there are some differences to the model (trigger pT, centrality) Long range dη correlations are visible in the STAR data and not taken into account in the plot. This is pT dependent and may reduce any slope.

25. Recent ReCo Model Predictions Premise: The production of Φ and Ω particles is almost exclusively from thermal s quarks even out to 8 GeV/c Observables: 1)The ratio of Ω/Φ yields should rise linearly with pT 2) Any Ω or Φ di-hadron correlations are swamped by the background and not observed Being actively studied, but no results are available as yet

26. Correlations: near side yields STAR Preliminary STAR Preliminary STAR Preliminary No trigger particle dependence in the near side yield/trigger in either d+Au or Au+Au d+Au Au+Au No definite trigger particle dependence vs centrality but meson triggers appear to be systematically below baryon triggers Reason for increase may be due to longe range correlations in η

27. ΔΦcorrelations per trigger particle 3 < pTtrigger < 3.5 GeV/c 1 < pTassoc < 2 GeV/c |η| < 1 Strange Correlations in Au+Au Correlations corrected for TPC acceptance and efficiency of associated particles v2 is then subtracted to give final correlations Near side

28. RAA - A mocked upstring picture does well Are strong color fields the answer? HIJING/BBar + KT ~ 1 GeV Strong Color Field (SCF) qualitatively describes RAA. SCF - long range coherent fields SCF behavior mimicked by doubling the effective string tension Topor Pop et al. hep-ph/0505210 SCF only produced in nucleus-nucleus collisions RAA≠ RCP

29. RAA for central and peripheral data Au-Au √sNN = 200 GeV STAR Preliminary Au-Au √sNN = 200 GeV STAR Preliminary Peripheral and central data both show an enhancement Peripheral data is more enhanced – Cronin effect?

30. Baryons/Mesons nucl-ex/0601042 The Λ/K0S ratio exhibits a peak in the intermediate pT region. The peak highvaries with centrality. At higher pTthe ratios for all centralities converge again. Magnitude and shape of ratio cannot be explained by flow alone.

31. Particle identification a) dE/dx c) Topology  K p d e b) RICH Approx. 10% of a central event

32. Has been studied in e+e- collisions at higher energies Quark jets tend to fragment harder than gluon jets We can study this with identified strange hadrons in p+p collisions in STAR gluon vs quark jets

34. pT reach constrained by p+p data Some hint of splitting in the baryons - RAA ≠ RCP HIJINGBB predicts such a splitting using Strong Colour Fields... See also the Corona effect in EPOS Identified Particle RAA PRC 72: 054901 (TOF)

35. Strange particles at intermediate pT The statistics from Run 4 allow us to go much higher in pT than previously and to study the intermediate pT region in detail Λ K0S

36. Observed suppression of single particle spectra compared to p+p and d+Au Disappearance of back-to-back jets • Baryon/meson puzzle at intermediate pT • Particle production mechanisms • quark vs gluon jets Strange Di-hadron Correlations p, π, Λ, K, Λ Identified Hadrons Coalescence/Recombination or Medium modified jets p, π, Λ, K, Λ Charged Hadrons

37. Multiplicity scaling with log(√s) If I can describe dNch/dh as function of√s dNch/dη- strongly correlated to the entropy of the system! Can we describe other observables in terms of dNch/dη? PHOBOS White Paper: Nucl. Phys. A 757, 28, nucl-ex/0410022

38. HBT and dNch/dh HBT radii ~linear as a function Npart1/3 Even better in (dNch/dh)1/3 power 1/3 gives approx. linear scale Scaling works across a large energy range nucl-ex/0505014 M.Lisa et al.

39. First make a consistency check • Require the models to, in principle, be the same. • Only allow the least common multiple of parameters: T, q, s, s • Use Grand Canonical Ensemble. • Fix weak feed-down estimates to be the same (i.e. at 100% or 0%).

40. The results Au-Au √sNN = 200 GeV after feed-down increase s decrease T 1  error Similar T and s Significantlydifferent errors. Not identical and feed-down really matters

41. Centrality dependence Solid – STAR Au-Au √sNN = 200 GeV Hollow - NA57 Pb-Pb √sNN = 17.3 GeV STAR Preliminary We can describe p-p and Au-Au average ratios. Can we detail the centrality evolution? Look at the particle enhancements. E(i) = YieldAA/Npart Yieldpp /2

42. Tdec = 164 MeV Tdec = 100 MeV Ω- spectra, central Motivation Chemistry Dynamics Summary Ω : central collisions Ideal Hydrodynamics • Data best reproduced with • Tdec ≈ 100 MeV • Same as for π-, K-, p • Agreement holds for entire spectra! • Same results at both energies! P.F. Kolb and U. Heinz, nucl-th/0305084 • Tdec ≈ 164 MeV (decoupling at hadronization): not enough radial flow pT = 2 GeV/c

43. Blast wave fits to data 200 GeV Strong centrality dependence on freeze out parameters for light hadrons Multi-strange hadrons freeze out earlier, with a lower <βT> Indicative of smaller cross-section for interactions of multiply strange hadrons with lighter species. Is this a signature of partonic collectivity?

44. Microscopic picture What interactions can lead to equilibration in < 1 fm/c? Need to be REALLY strong Perturbative calculations of gluon scattering lead to long equilibration times (> 2.6 fm/c) and small v2. R. Baier, A.H. Mueller, D. Schiff, D. Son, Phys. Lett. B539, 46 (2002). MPC 1.6.0, D. Molnar, M. Gyulassy, Nucl. Phys. A 697 (2002). v2 Clearly this is not the weakly coupled perturbative QGP we started looking for. s(trong)QGP 2-2 processes with pQCD s = 3 mb pT (GeV/c)

45. Relativistic Heavy-Ion Collider (RHIC) PHOBOS BRAHMS RHIC PHENIX STAR AGS TANDEMS 1 km v = 0.99995c Au+Au @ sNN=200 GeV

46. Runs so far Run Year Species √s[GeV ] Ldt 01 2000 Au+Au 130 1 b-1 02 2001/2 Au+Au 200 24 b-1 p+p 200 0.15 pb-1 03 2002/3 d+Au 200 2.74 nb-1 p+p 200 0.35 pb-1 04 2003/4 Au+Au 200 241 b-1 Au+Au 62 9 b-1 05 2004/5 Cu+Cu 200 3 nb-1 Cu+Cu 62 0.19 nb-1 Cu+Cu 22.5 2.7 b-1 p+p 200 3.8 pb-1

47. A theoretical view of the collision 4 3 1 2 • Hadronic ratios. • Resonance production. • p spectra. • Partonic collectivity. • High p measurements. Tc – Critical temperature for transition to QGP Tch– Chemical freeze-out (Tch  Tc) : inelastic scattering stops Tfo – Kinetic freeze-out (Tfo  Tch): elastic scattering stops

48. Comparison between p-p and Au-Au Au-Au √sNN = 200 GeV STAR Preliminary p-p √s = 200 GeV STAR Preliminary Canonical ensemble

49. Resonances and survival probability  K* lost  K K*   K* K K K measured • Initial yield established at chemical freeze-out • Decays in fireball mean daughter tracks can rescatter destroying part of signal • Rescattering also causes regeneration which partially compensates • Two effects compete – Dominance depends on decay products and lifetime  lost K* K measured Kinetic freeze-out Chemical freeze-out time Ratio to “stable” particle reveals information on behaviour and timescale between chemical and kinetic freeze-out

50. Chemical to kinetic freeze-out P. Braun-Munzinger et.al.,PLB 518(2001) 41, priv. communication Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) 81. M. Bleicher and Horst Stöcker J. Phys.G30 (2004) 111. Life-time [fm/c] : K(892) ~ 4.0 S(1385) ~ 5.7 L(1520) ~ 13  (1020) ~ 44 Finite time span from Tch to Tfo If only rescattering K(892) most suppressed Need rescattering and regeneration to “fix” the picture.