Physics at RHIC: the Relativistic Heavy Ion Collider

1. Physics at RHIC:the Relativistic Heavy Ion Collider NYS APS Meeting Barbara V. Jacak Stony Brook Oct. 17, 2003

2. outline • Why collide heavy ions? • QCD and the phase transition • the Relativistic Heavy Ion Collider + experiments • What have we learned so far? • Thermalization & pressure build up – early! • (medium-induced) modification of jets • The control experiment: d+Au • Medium effects on fragmentation function? • Conclusions

3. The Physics of RHIC • Create very high temperature and density matter • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • collide heavy ions to achieve maximum volume • Study the hot, dense medium • is thermal equilibrium reached? • transport properties? equation of state? • do the nuclei dissolve into a quark gluon plasma? • Collide Au + Au ions at high energy • s = 200 GeV/nucleon pair, p+p and d+A to compare • Also polarized p+p collisions to study carriers of p’s spin

4. + +… Quantum ChromoDynamics • Strong interaction field theory : colored quarks exchange gluons • Parallels QED but gluons have color charge • unlike E&M where g are uncharged •  they interact among themselves (i.e. theory is non-abelian): curious properties short distance: force is weak (probe w/ high Q2, calculate with perturbation theory) large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative) High temperature: force becomes screened by produced color-charges (gets weak)

5. p-p hep-ex/0304038 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions s = 200 GeV: start with pQCD & pp collisions Works! A handle on initial NN interactions also need: p0

6. EOS Karsch, Laermann, Peikert ‘99 e/T4 Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3 In A+A: QCD in non-perturbative regime Lattice… We look for physics beyond simple superposition of NN: Equilibration Collective effects Energy, color transport in dense medium Deconfinement? T/Tc Physics is soft Lattice QCD says: Create these conditions to look for new physics

7. pT Experimental approach Central region has max temperature & density Head-on = “central” collisions  max volume Thermalization? particle spectra, yields Pressure developed? particle/energy flows Medium properties? effects upon probe particles Deconfinement? c and anti-c remain bound as J/Y?

8. RHIC at Brookhaven National Laboratory RHIC is first dedicated heavy ion collider 10 times the energy previously available!

9. STAR 4 complementary experiments

10. Colliding system expands: Energy  to beam direction per unit velocity || to beam pR2 • e 4.6 GeV/fm3 (130 GeV Au+Au) 5.5 GeV/fm3 (200 GeV Au+Au) 2ct0 well above predicted transition! Is the energy density high enough? PRL87, 052301 (2001)

11. history of heavy ion collisions high e, pressure builds up g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Real and virtual photons from q scattering sensitive to the early stages. Probe also with q and g produced early, & passing through the medium on their way out. Hadrons reflect medium properties when inelastic collisions stop (chemical freeze-out).

12. Central Au+Au collisions (~ longitudinal velocity) Particle production (lots!) sum particles under the curve, find ~ 5000 charged particles in collision final state (6200 in 200 GeV/A central Au+Au) In initial volume ~ Vnucleus Rescattering should be important!

13. Protons show velocity boost  to beam. Expect if pressure build-up due to rescattering Data well fit with: Tfo = 110-120 MeV & <t> = 0.5-0.6 Hadron pT spectra – all 4 experiments! BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% 200 GeV/A Au+Au

14. Evidence for equilibrated final hadronic state • Simple quark counting: K-/K+ = exp(2ms/T)exp(-2mq/T) = exp(2ms/T)(pbar/p)1/3 = (pbar/p)1/3 • local strangeness conservation K-/K+=(pbar/p)a a = 0.24±0.02 BRAHMS a = 0.20±0.01 for SPS • Good agreement with statistical-thermal model of Beccatini et al. (PRC64 2001) w/T=170 MeV From y=0 to 3 At y=0 PRL 90 102301 Mar. 2003

15. More evidence for equilibrated final state Observed hadron ratios in agreement with thermal ratios! T(chemical freeze-out) ~ 175 MeV Strange particles are enhanced vs. p+p collisions by equilibrium

16. early universe 250 RHIC 200 quark-gluon plasma 150 SPS Lattice QCD AGS deconfinement chiral restauration thermal freeze-out 100 SIS hadron gas 50 neutron stars atomic nuclei 0 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV] Locate RHIC on phase diagram fit yields vs. mass (grand canonical ensemble) Tch = 175 MeV mB = 51 MeV These are the conditions when hadrons stop interacting T Observed particles “freeze out” at/near the deconfinement boundary!

17. Almond shape overlap region in coordinate space Early state? a barometer called “elliptic flow” Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy  momentum anisotropy v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

18. Preliminary STAR STAR Preliminary v2 measured by the experiments 200 GeV: 0.2< pt < 2.0 130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.0 4-part cumulants v2=0.05 200 GeV: Preliminary - Consistent results - At 200 GeV better pronounced decrease of v2 for the most peripheral collisions.

19. Hydro can reproduce magnitude of elliptic flow for p, p. BUT must add QGP to hadronic EOS!! Similar conclusion reached by CM Ko, et al., Kapusta, et al., Bleicher, et al., among others… v2 predicted by hydrodynamics Hydro. Calculations Huovinen, P. Kolb, U. Heinz pressure buildup  explosion happens fast  early equilibration ! STAR PRL 86 (2001) 402 Kolb, et al central

20. schematic view of jet production hadrons leading particle q q hadrons leading particle a unique probe for physics of hot medium Probe: Jets from hard scattered quarks Observed via fast leading particles or azimuthal correlations between the leading particles • But, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium • decreases their momentum (fewer high pT particles) • “kills” jet partner on other side • “jet quenching”

21. AA AA If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT AA Nuclear Modification of Hadron Spectra? 1. Compare Au+Au to nucleon-nucleon cross sections 2. Compare Au+Au central/peripheral Nuclear Modification Factor: nucleon-nucleon cross section <Nbinary>/sinelp+p

22. Au-Au s = 200 GeV: high pT suppression! PRL91, 072301(2003) nucl-ex/0304022 Au-Au nucl-ex/0304022

23. near side away side Back-to-back jets are suppressed in central collisions! peripheral central jet correlations: Au+Au vs p+p STAR PRL 90, 082302 (2003) Peripheral Au + Au Central Au + Au

24. Suppression: a final state effect? Hadron gas • Hadronic absorption of fragments: • Gallmeister, et al. PRC67,044905(2003) • Fragments formed inside hadronic medium • Parton recombination (up to moderate pT) • Fries, Muller, Nonaka, Bass nucl-th/0301078 • Lin & Ko, PRL89,202302(2002) • Energy loss of partons in dense matter • Gyulassy, Wang, Vitev, Baier, Wiedemann… Absent in d+Au collisions! d+Au is the “control” experiment

25. probe rest frame r/ ggg Suppression: an initial state effect? • Gluon Saturation • (color glass condensate) Wavefunction of low x gluons overlap; the self-coupling gluons fuse, saturating the density of gluons in the initial state.(gets Nch right!) • Multiple elastic scatterings (Cronin effect) Wang, Kopeliovich, Levai, Accardi • Nuclear shadowing Levin, Ryshkin, Mueller, Qiu, Kharzeev, McLerran, Venugopalan, Balitsky, Kovchegov, Kovner, Iancu … RdAu~ 0.5 D.Kharzeev et al., hep-ph/0210033 Broaden pT :

26. Experiments show NO suppression in d+Au! PHENIX Preliminary p0 STAR Preliminary PHOBOS Preliminary

27. Centrality Dependence Au + Au Experiment d + Au Control PHENIX preliminary • Dramatically different and opposite centrality evolution of AuAu experiment from dAu control. • Jet Suppression is clearly a final state effect.

28. n ZDC p Neutron tagged events enhance peripheral collisions Forward n tagged d+Au <Ncoll> = 5.0 / 3.6 d+Au looks very similar to p+Au

29. Back-to-back jets observed in d+Au STAR • jet pair production • also looks • independent of Ncoll • observe no (big) suppression in back-to back jets! • probably some jet broadening due to initial multiple scattering… PHENIX preliminary

30. R. Fries, et al pQCD spectrum shifted by 2.2 GeV Hydro. expansion at low pT + jet quenching at high pT: Recombination of boosted q’s? Modified fragmentation function INSIDE the medium? Teff = 350 MeV Observe unusual particle mix at RHIC nucl-ex/0305036 (PRL) p/ ~1 at high pT in central collisions Higher than in p+p or jets in e+e- collisions

31. Do the baryons scale with Ncoll? Au+Au Baryon yields not suppresed  Ncoll at pT = 2 – 4 GeV/c nucl-ex/0306007 hard/soft process interplay? Quark recombination? Medium modification of fragmentation function?

32. Other penetrating probes • See that early medium is hot, dense, equilibrated and (probably) induces energy radiation in transiting q,g • What else can we say about it? • J/Y • Test confinement: do bound c + c survive? • or does QGP screening kill them? • Open Charm • Extra heavy quarks from dense gluon gas? • Do the c quarks lose energy like the light quarks? • Dileptons • Tmax from thermal radiation spectrum Need (a lot) more statistics in the data But can take a first look…

33. J/Y suppression was observed at CERN at s=18 GeV/A NA50 collaboration J/Y yield Fewer J/Y in Pb+Pb than expected! Interpret as color screening of c-cbar by the medium Initial state processes affect J/Y too so interpretation is still debated...

34. 0-20% most central Ncoll=779 40-90% most central Ncoll=45 20-40% most central Ncoll=296 R.L. Thews, M. Schroedter, J. Rafelski Phys. Rev. C63 054905 (2001): Plasma coalesence model for T=400MeV and ycharm=1.0,2.0, 3.0 and 4.0. L. Grandchamp, R. RappNucl. Phys. A&09, 415 (2002) and Phys. Lett. B 523, 50 (2001): Nuclear Absorption+ absoption in a high temperature quark gluon plasma Deconfinement at RHIC? Look at J/Y nucl-ex/0305030 A. Andronic et. Al. Nucl-th/0303036 Proton Don’t know yet about deconfinement, but don’t see EXTRA (thermal) J/Y

35. Total charm quark cross section at RHIC Phys.Rev.Lett. 88 (2002) 192303 Cross section fits into expected energy dependence

36. No large suppression as for light quarks! Compare the measurement to (PYTHIA) an event generator tuned for pp collisions… Spectra of electrons from c e + anything

37. conclusions • Rapid equilibration at RHIC! • Strong pressure gradients, hydrodynamics works • Constituent scattering cross section is very large • EOS is not hadronic • The hot matter is “sticky” – it absorbs energy & seems to • transport it efficiently • d+Au data says: final state, not initial state effect • So, the stuff is dense, hot, ~ equilibrated AND NEW! • QGP discovery? • J/Y suppression or not? Next run • Tinitial? direct photon analysis underway • Properties not of perturbative plasma, more like a liquid… • huge interaction cross sections, residual correlations • Mystery: Why so many baryons? In-medium fragmentation?

38. A few mysteries…

39. Hydro describes single + multi-particles But FAILS to reproduce two-particle correlations! • How to increase R without increasing Rout/Rside??? • EOS? • initial T & rprofiles? • emissivity?

40. Elliptic flow of high momentum particles baryons cross mesons (not expected from hydro) v2 pT (GeV/c) Still flowing at pT = 8 GeV/c? Geometry? v2 a bit too big… Mix of flow + jets???

41. Total charm quark cross section at RHIC Cross section fits into expected energy dependence No evidence for strong energy loss of charmed quarks…

42. Why no big energy loss for heavy quarks? no x4 suppression from peripheral to central, as predicted for dE/dx=-0.5GeV/fm! But (we squirm) - Is 40-70% peripheral enough? error bars still big!