Hot Matter and Cool Results from RHIC

1. Hot Matter and Cool Resultsfrom RHIC • Every sentence I utter must be understood • not as an affirmation, but as a question. • - Niels Bohr (1885-1962) QCD at the Interface between Particle and Nuclear Physics

2. QCD For Beginners Quarks confined within hadrons via strong force v(r) = a/r + s*r At large r -second term dominates At small r -Coulomb-like part dominates However a function of q( mtm transfer) and a -> 0 faster than q (or 1/r) -> infinity (called asymptotic freedom) This concept of asymptotic freedom among closely packed coloured objects (q and g) has led to one of the most exciting predictions of QCD !! The formation of a new phase of matter where the colour degrees of freedom are liberated. Quarks and gluons are no longer confined within colour singlets. The Quark-Gluon Plasma!

3. Lattice QCD at Finite Temperature q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q q Critical energy density: q q q q q q q q q q q q q q q q q q q q q Recently extended to mB> 0, order still unclear (2nd, crossover ?) Ideal gas (Stefan-Boltzmann limit) F. Karsch, hep-ph/0103314 Tc ~ 150-170 MeV ec ~ 1 GeV/fm3

4. (QCD) Phase Diagram of Nuclear Matter TWO different phase transitions at work! – Particles roam freely over a large volume – Masses change Calculations show that these occur at approximately the same point Two sets of conditions: High Temperature High Baryon Density Deconfinement transition Chiral transition

5. Time Scales of a Relativistic Heavy Ion Collisions soft physics regime e.m. probes (l+l-, g) hard (high-pT) probes Chemical freezeout (Tch  Tc) : inelastic scattering stops Kinetic freeze-out (Tfo Tch): elastic scattering stops

6. 2 concentric rings of 1740 superconducting magnets 3.8 km circumference counter-rotating beams of ions from p to Au RHIC @ Brookhaven National Laboratory Relativistic Heavy Ion Collider h Long Island • 2000 run: • Au+Au @ sNN=130 GeV • 2001 run: • Au+Au @ sNN=200 GeV • polarized p+p @ s=200 GeV (P ~15%)

7. Geometry of Heavy Ion Collisions Preliminary sNN = 200 GeV participants Uncorrected spectators Particle production scales with increasing centrality peripheral (grazing shot) central (head-on) collision Number participants (Npart): number of nucleons in overlap region Number binary collisions (Nbin): number of equivalent inelastic nucleon-nucleon collisions Nbin ≥ Npart

8. Au-Au Central Events at RHIC STAR

9. Charged Particle Multiplicity 200 GeV 19.6 GeV 130 GeV PHOBOS Preliminary dNch/dh Central Peripheral h Central at 130 GeV: 4200 charged particles ! Total multiplicity per participant pair scales with Npart Not just a superposition of pp

10. B/B Ratios RHIC Preliminary Au-Au 130 GeV B - all from pair production B - pair production + transported B/B ratio =1 - Transparent collision B/B ratio ~ 0 - Full stopping, little pair production • All data: • mid-rapidity • ratios from raw yields ~2/3 of proton from pair production First time pair production dominates Still some baryons from beam

11. Do We Reach the Critical Energy Density? Bjorken formula for thermalized energy density: PHENIX EMCAL time to thermalize the system (t0 ~ 1 fm/c) 130 GeV ~6.5 fm pR2 ~30 times normal nuclear density~ 5 times above ecritical from lattice QCD For Central Events: eBjorken~ 4.5 GeV/fm3

12. Is There Collective Motion? Look at “Elliptic” Flow SPS, RHIC AGS Almond shape overlap region in coordinate space Anisotropy in momentum space Interactions v2: 2nd harmonic Fourier coefficient in dN/d with respect to the reaction plane

13. Hydro Calculation of Elliptic Flow V2 Hydrodynamic model SPS AGS PRL 86 (2001) 402 Nch/Nmax • A pressure build up -> Explosion • zero for central events • self quenching • Elliptic flow observable sensitive to early evolution of system • Collective motion + large energy density ->Hydrodynamics • Assumes continuum matter with local equilibrium, “thermalization” Equal Energy Density lines P. Kolb, J. Sollfrank, and U. Heinz Large v2 is an indication of early thermalization Heavy-Ion Collisions create a system which approaches hydrodynamic limit

14. Identified Particle V2 V2 STAR Preliminary Au-Au 200 GeV 1 STAR PRL87 (2001)182301 Hydro-inspired model also predicts mass dependence well

15. Kinetic Freeze-Out and Radial Flow purely thermal source light 1/mT dN/dmT heavy mT explosive source light T,b T 1/mT dN/dmT heavy mT Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion Slope = 1/T Look at pt or mt = (pt2 + m2 )distribution A thermal distribution gives a linear distribution dN/dmt e-(mt/T) If there is radial flow dN/dmt- Shape depends on mass and size of flow Heavier particles show curvature

16. Radial Flow and Hydrodynamical Model PHENIX Preliminary STAR Preliminary Models differ slightly in details but same concept PHENIX: Tfo ~ 104  21 MeV, < t> ~ 0.5  0.1c STAR Tfo ~ 107  8 MeV, < t> ~ 0.55  0.1c

17. Tfo and <br> vs √s • <r > • increases continously • Tfo • saturates around AGS energy • Strong collective radial expansion at RHIC • high pressure • high rescattering rate • Thermalization likely Slightly model dependent here: blastwave model (Kaneta/Xu)

18. Models to Evaluate Tch and B • Statistical Thermal Model • F. Becattini; P. Braun-Munzinger, J. Stachel, D. Magestro • J.Rafelski PLB(1991)333; J.Sollfrank et al. PRC59(1999)1637 • 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

19. Beautiful Agreement Between Model & Data Does the success of the model tell us we are dealing indeed with locally chemically equilibrated systems? This + flow measurements… If you ask me Yes!

20. Phase Diagram from AGS to RHIC early universe 250 RHIC quark-gluon plasma 200 Lattice QCD SPS 150 AGS Chemical Temperature Tch [MeV] deconfinement chiral restauration 100 SIS hadron gas 50 atomic nuclei neutron stars 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Again slight variations in the models QCD on Lattice Tc = 173±8 MeV, Nf=2 Tc = 154±8 MeV, Nf=3 Remember: Measure hadrons not partons so can’t measure T> Tc with this method

21. Particle production is large Total Nch ~ 5000 (Au+Au s = 200 GeV)  ~ 20 in p+p Nch/Nparticipant-pair ~ 4 (central region)  ~2.5 in p+p Vanishing anti-baryon/baryon ratio (0.7-0.8) close to net baryon-free but not quite Energy density is high  4-5 GeV/fm3 (model dependent) lattice phase transition ~1 GeV/fm3, cold matter ~ 0.16 GeV/fm3 System exhibits collective behavior (radial + elliptic flow) strong internal pressure that builds up very early explosive expansion Particles ratios suggest chemical equilibrium Tch170 MeV, mb<50 MeV  near lattice phase boundary Summary on “Soft” (pT < 2 GeV/c) Physics Overall picture: System appears to be in equilibrium but explodes and hadronizes rapidly

22. High-pT Hadrons at RHIC Now even have own pp measurements so detector effects “cancel” All 4 experiments have an impressive array of data out to high pT

23. Why study high pT physics at RHIC ? schematic view of jet production jet production in quark matter hadrons hadrons leading particle leading particle q q q q hadrons leading particle Early production in parton-parton scatterings with large Q2. Direct probes of partonic phases of the reaction • New penetrating probe at RHIC • attenuation or absorption of jets “jet quenching” • suppression of high pT hadrons • modification of angular correlation • changes of particle composition

24. Nuclear Modification Factor N-N cross section <Nbinary>/sinelp+p If no “effects”: R < 1 in regime of soft physics R = 1 at high-pt where hard scattering dominates “Hard” Physics - Scales with Nbin: Number of binary collisions number of equivalent inelastic nucleon-nucleon collisions Nuclear Modification Factor:

25. Hadron Suppression: Au+Au at 200 GeV charged hadrons: p0: PHENIX preliminary Suppression of central yields persists up to pT=10 GeV/c

26. L and p show different behaviour to Ks and p Hadron Suppresion for Identified Particles p Suppression ofLsets in at higher pT p0 Seem to come together at ~6GeV/c - “standard” fragmentation? L K0s Is this a mass effect or a baryon/meson effect ? STAR Prelimimary

27. Azimuthal Anisotropy (v2)of Particle Emission y y x x low pThigh pT Jet Propagation Bulk (Hydrodynamic) Matter Pressure gradient converts position space anisotropy to momentum space anisotropy Energy loss results in anisotropy due to different “length” of matter passed through by parton depending on location of hard scattering

28. Elliptic “Flow” at High-pT Jet propagation through anisotropic matter (non-central collisions) STAR @ 200 GeV • Finite v2: high pT hadron correlated with reaction plane from “soft” part of event (pT<2 GeV/c) • Finite asymmetry at high pT •  Significant in-medium interactions even at 10 GeV/c

29. Jets in Heavy Ion Collisions e+e- q q (OPAL@LEP) pp jet+jet (STAR@RHIC) Au+Au ??? (STAR@RHIC) Jets in Au-Au hopeless Task? No, but a bit tricky…

30. Leading Particle Correlations Dh < 0.5 Dh > 0.5 Leading Particle • Trigger on high pT leading particle • Jet core: Df × Dh ~ 0.5 × 0.5 •  study near-side correlations (Df~0) of high pT hadronpairs • Complication: elliptic flow high pT hadrons correlated with the reaction plane (~v22) • Solution: compare azimuthal correlation functions for • Dh<0.5 (short range)  • particles in jet cone + • background • Dh>0.5 (long range)  • background only incoming partons associated h Near-side correlation shows jet-like signal in central Au+Au

31. Back-to-Back Jets? • away-side (back-to-back) jet can be “anywhere” (Dh~2.5) - can’t use large Dh subtraction “trick“ • Ansatz: correlation function: high pT-triggered Au+Au event = • high pT-triggered p+p event • + • elliptic flow • + • background PHENIX Preliminary pp 2-4 GeV • black = real • green = mixed event • purple = black-green A: from fit to “non-jet” region Df~p/2 v2 from reaction plane analysis

32. Away Side Jets are Suppressed STAR Preliminary near side away side Peripheral Au + Au • Near-side well-described • Away-side suppression in central • collisions STAR Preliminary Central Au + Au Away side jets are suppressed!

33. Charm at RHIC Charm decay is expected to be dominant component of single e- with pT > 1.5 GeV/c: • Large charm production cross section (300-600 mb) which scales roughly with Nbin • Suppression of high pTp’s relative to binary scaling Observe an “excess” in single e-’s over expectation from light meson decays and g conversions  Observation of charm signal at RHIC PHENIX PRL 88 Assuming that all single e- signal is from charm decay and the binary scaling, charm cross section at 130 GeV Data are consistent with s systematics(within large uncertainties)!

34. Summary ? • Soft physics: • System appears to be in equilibrium (hydrodynamic behaviour) • Low baryon density • Explosive expansion, rapid hadronization • Hard physics: • Jet fragmentation observed • Strong suppression of inclusive yields • Azimuthal anisotropy at high pT • Suppression of back-to-back hadron pairs • large parton energy loss and surface emission? • Open charm cross section scales with Nbin • Coming Attractions: • d+Au: disentangle initial state effects in jet production • (shadowing, Cronin enhancement)  resolution of jet quenching picture • J/ and open charm: direct signature of deconfinement? • Polarized protons: DG (gluon contribution to proton spin) • Surprises …

35. Leading Charged Particle Correlations Dh < 0.5 Dh > 0.5 • Jet core: Df × Dh ~ 0.5 × 0.5  study near-side correlations (Df~0) of high pT hadronpairs • Complication: elliptic flow high pT hadrons correlated with the reaction plane (~v22) • Solution: compare azimuthal correlation functions for • Dh<0.5 (short range)  particles in jet cone + background • Dh>0.5 (long range)  background only • Azimuthal correlation function: • Trigger particle pT trig> 4 GeV/c • Associate tracks 2 < pT < pTtrig • Caveat: Away-side jet contribution • subtracted by construction, • needs different method… Near-side correlation shows jet-like signal in central Au+Au

36. Charm and single electron at RHIC Simulation before RHIC PHENIX data (PRL88) • At RHIC, it is expected that charm decay can be the dominant component of single electron in pt > 1.5 GeV/c • Large production cross section of charm ( 300-600 ub) • Production of the high pt pions is strongly suppressed relative to binary scaling • Production of charm quark roughly scale with binary collisions. • PHENIX observed “excess” in single electron yield over expectation from light meson decays and photon conversions  Observation of charm signal at RHIC

37. PHENIX observed excess of single electron yield over the contribution from light meson decays and photon conversoins Spectra of single electron signal is compared with the calculated charm contribution. Charm contribution calculated as EdNe/dp3 = TAAEds/dp3 TAA: nuclear overlap integral Eds/dp3: electron spectrum from charm decay calculated using PYTHIA The agreement is reasonably good. PHENIX single electron data PHENIX PRL88 192303 Assuming that all single electron signal is from charm decay and the binary scaling, charm cross section at 130 GeV is obtained as

38. PHENIX single electron cross section is compared with the ISR data single electron data Charm cross section derived from the electron data is compared with fixed target charm data Single electron cross sections and charm cross sections are compared with Solid curves: PYTHIA Shaded band: NLO QCD Comparison with other experiments Assuming binary scaling, PHENIX data are consistent with s systematics o (within large uncertainties)!

39. Leading Photon Correlations 0 incoming partons associated h trigger  • Select events with a photon of • pt > 2.5 GeV/c. Mostly ’s from decay of a high pt  (leading particle) • Build distributions in delta  -space of the charged hadrons relative to the trigger photons. pp AuAu PHENIX Preliminary 2-4 GeV • black = pair distribution • green = mixed event pair distribution • purple = bkg subtracted distribution In AuAu: add v2 component

41. The “buzz’’ word in the last few months: quark recombination/coallescence Parton recombination and high pT Fries, Mueller, Nonaka,Bass nucl-th/0301087 Hwa & Yang nucl-th/0211010 Greco, Ko, Levai nucl-th/0301093 • Recombination pT(baryons)> pT(mesons)> pT(quarks) (coalescence from thermal quark distribution ...) • Pushes soft physics for baryons out to 4-5 GeV/c • Some exotic explanations (e.g. gluon junctions)

42. PHENIX Axial Field High Resolution & Rates 2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID ~450 Participants • Leptons, Photons, and Hadrons in Selected • Solid Angles • Simultaneous Detection of Various Phase • Transition Phenomena The Two “Large” Detectors at RHIC STAR Solenoidal field Large- Tracking TPC’s, Si-Vertex Tracking RICH, EM Cal, TOF ~420 Participants Silicon Vertex             Tracker Coils Magnet E-M Calorimeter Time Projection           Chamber Time of    Flight Electronics Platforms Forward Time Projection Chamber • Measurements of Hadronic Observables • using a Large Acceptance • Event-by-Event Analyses of Hadrons and • Jets

43. The Two “Small” Experiments at RHIC BRAHMS 2 “Conventional” Spectrometers Magnets, Tracking Chambers, TOF, RICH ~40 Participants PHOBOS “Table-top” 2 Arm Spectrometer Magnet, Si -Strips, Si Multiplicity Rings, TOF ~80 Participants Paddle Trigger Counter TOF Spectrometer Octagon+Vertex Ring Counters • Charged Hadrons in Select Solid Angle • Multiplicity in 4 • Particle Correlations • Inclusive Particle Production Over Large • Rapidity Range

44. Hagedorn (1960’s): Spectrum of excited hadronic states: exponentially increasing level density Heat a hadron gas  excite more massive resonances Hadronic gas has limiting temperature T ~ 170 MeV Phase transition in high (energy-) density matter? But cannot continue to arbitrary energy density: hadrons have finite size  transition to phase of hadronic constituents at T 170 MeV?

45. Exploring the Phases of Nuclear Matter • Can we explore the phase diagram of nuclear matter ? • We think so ! • by colliding nuclei in the lab • by varying the nuclei size (A) and colliding energy (s) • by studying spectra and correlation of the produced particles • Requirements • system must be at equilibrium (for a short time) •  system must be dense and large • Can we find and explore the Quark Gluon Plasma ? • We hope so! • by colliding large nuclei at the highest possible energy

46. Experimental Determination of Geometry Paddles/BBC ZDC ZDC Au Au Paddles/BBC Central Multiplicity Detectors 5% Central Paddle signal (a.u.) STAR

47. RHIC – Runs & Machine Parameters Au+Au integrated luminosity~80 mb-1 2001 2000 Days into RHIC Run Days into RHIC Run PerformanceAu + Aup + p Max snn 200 GeV 500 GeV L [cm-2 s -1 ] 2 x 10261.4 x 1031 Interaction rates 1.4 x 103 s -1 3 x 105 s -1 • 2000 run: • Au+Au @ sNN=130 GeV • 2001 run: • Au+Au @ sNN=200 GeV (80 mb-1) • polarized p+p @ s=200 GeV • (P ~15%, ~1 pb-1)

48. Midrapidity: Centrality Dependence at RHIC hard and soft scaling: _ pp  hard processes are important even for Nch PHOBOS Au+Au |h|<1 200 GeV 130 GeV 19.6 GeV preliminary Kharzeev and Nardi PLB 507, 121 (2001)

49. Nch(sNN) – Universality of Total Multiplicity? Total charged particle multiplicity / participant pair Same for all systems at same s(seff for pp) • pQCD e+e- Calculation (A. Mueller, 1983) Accidental, trivial?

50. pT of Charged Hadrons increase only ~2% STAR preliminary Saturation model: J. Schaffner-Bielich, et al. nucl-th/0108048 D. Kharzeev, et al. hep-ph/0111315 Many models predict similar scaling (incl. hydrodynamic models)