The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider

1. The hottest stuff on earth! Physics at the Relativistic Heavy Ion Collider Colloquium Barbara V. Jacak Stony Brook February 18, 2003

2. outline • Why collide heavy ions? • the QCD phase transition • Creating and studying super-dense matter in the laboratory • the Relativistic Heavy Ion Collider • Experiments and observables • What have we learned so far? • A few interesting mysteries…

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

4. QCD Phase Transition • we don’t really understand • how process of quark confinement works • how nature breaks symmetries  massive particles from ~ massless quarks • transition affects evolution of early universe • latent heat & surface tension  • matter inhomogeneity in evolving universe? • equation of state of nuclear matter compression in stellar explosions

5. + +… 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 at short distance: force is weak (probe w/ high Q2, Calculate with perturbation theory) at large distance: force is strong (probe w/ low Q2, calculations must be non-perturbative)

6. Early Universe plasma of free quarks & gluons T (MeV) RHIC 200 SPS AGS baryons <qqq> mesons <q q> Color superconductor? hadrons m (MeV) Baryon density Phase diagram of hadronic matter normal nuclei

7. Phase transition temperature? From lattice QCD Karsch, Laermann, Peikert ‘99 e/T4 T/Tc Tc ~ 170 ± 10 MeV (1012 °K) e ~ 3 GeV/fm3

8. Experimental approach Look at region between the two nuclei to see maximum temperature & density Sort collisions by impact parameter head-on = “central” collisions yield maximum volume

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

10. STAR 4 complementary experiments

11. The challenge • Determine experimentally that conditions are sufficient to search for evidence of new phase of matter • Figure out its properties • Address by correlating different variables • First look at “global” quantities • number, energy flow of produced particles • hadron production and patterns at “end” of collision • Then look at probes of the earliest phase • processes at small distance (high energy) scales • collective behavior • thermal radiation

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

13. How many particles are produced? dNch/dh = 640 Rises somewhat faster than Npart

14. Central Au+Au collisions (~ longitudinal velocity) Density: a first look 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

15. Almond shape overlap region in coordinate space Pressure? 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

16. 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. QM2002 summary slide (Voloshin)

17. Large v2: the matter can be modeled by hydrodynamics v2: Much larger than at CERN or AGS! Hydro. Calculations Huovinen, P. Kolb, U. Heinz STAR PRL 86 (2001) 402 pressure buildup  explosion Happens fast  early equilibration ! first time hydrodynamic behavior seen

18. Thermal Properties measuring the thermal history v2 builds up g, g* e+e-, m+m- p, K, p, n, f, L, D, X, W, d, Real and virtual photons from quark scattering is most sensitive to the early stages. Hadrons reflect thermal properties when inelastic collisions stop (chemical freeze-out).

19. Particle Spectra @ 200 GeV BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% QM2002 summary slide (Ullrich)

20. Spectral shapes <pT> for radially expanding hadron gas with Tth and <b> STAR preliminary F. Wang <pT> in pp with “Tch” = 170 MeV and <b>=0 pp no rescattering, flow or equilibrium

21. _ ¯ early universe 250 RHIC 200  s 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] Can now locate RHIC on phase diagram Antibaryon/baryon Collisions at RHIC approach zero net baryon density Conditions when hadrons freeze out – fit yields vs. mass (grand canonical ensemble) Tch = 175 MeV mB = 51 MeV

22. What does theory say about these data? • To get proper particle yields must tweak models so they no longer agree with pp collisions • Must add some kind of a partonic phase with large scattering cross sections to reproduce v2 • Need QGP-type equation of state to get the v2 (and also radial expansion) correctly • Superposition of pp collisions gives insufficient initial pressure as the “strings” don’t scatter.

23. schematic view of jet production hadrons leading particle q q hadrons leading particle Early density - use a unique probe 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 momentum particles  beam  “jet quenching”

24. nucleons Something new at RHIC? • Compare to baseline: nucleon-nucleon collisions at the • same energy • To zero’th order: Au + Au collisions start as a superposition • of N-N reactions • (modulo effect of • nuclear binding and • collective excitations) • Hard scattering processes scale as the number of N-N binary collisions <Nbinary> • so expect: YieldA-A = YieldN-N. <Nbinary>

25. Preliminary Baseline: p+p collisions Agrees with pQCD predictions (next to leading order)

26. Is Au+Au different? For p0, plot: PHENIX Preliminary Central Peripheral collisions Yes!!

27. So, is there jet quenching? • Suppression observed to 9 GeV/c! (in 3 independent measurements) Theory agrees with data when quark/gluon energy loss is included

28. trigger-jet not much modification (the trigger particles from jets!) Away side: strong jet suppression STAR Look for the jet on the other side! Centrality  • Strong jet suppression  surface emission of jets? • Colored glass back-to-back jets simply not created…

29. p/p at high pT Higher than in p+p collisions or fragmentation of gluon jets in e+e- collisions Vitev & Gyulassy nucl-th/0104066 Can explain by combination of hydro expansion at low pT with jet quenching at high pT

30. Other penetrating probes • Open Charm • J/Y • Dileptons Need (a lot) more statistics in the data But getting a first sniff of physics already

31. J/Y Energy/Momentum Data consistent with: Hadronic comover breakup (Ramona Vogt) w/o QGP Limiting suppression via surface emission (C.Y. Wong) Dissociation + thermal regeneration (R. Rapp)

32. Open charm - Lin about x2 within predicted curves with or w/o energy loss no x4 suppression from peripheral to central, as predicted for dE/dx=-0.5GeV/fm But - Is 40-70% peripheral enough? error bars still big!

33. Some old things and some new things • HBT • High pT baryons • Dijets vs. monojets • Well, there was a prediction but for 10x the pT • Parton saturation

34. HBT – lots of questions Panitkin, Pratt • How to increase R without increasing Rout/Rside? • EOS, initial T and rprofiles (Csőrgó), emissivity? • Why entropy looks low? • Low entropy implies equilibrated QGP ruled out

35. protons p0, h Baryons at high pT Yields scale with Ncoll near pT = 2 – 3 GeV/c Then start to fall Meaning of Ncoll scaling? Accident? Complex hard/soft interplay? Medium modified jet fragmentation function?

36. conclusions • Rapid thermalization & strong pressure gradients • high gluon density, extensive scattering • flow reproduced by hydrodynamics; EOS beyond hadronic • theories requires QGP, “string melting”, large s… • The hot matter is “sticky” – absorbs energy • High pT, high mass data look like pQCD + something • Haveenergy loss and disappearance of back-to-back jets • Colored glass condensate? • Too early to tell, but stuff is dense, hot, and equilibrated • NEED TO KNOW • d+Au “control experiment” results (run underway!) • Tinitial from thermal photon spectrum • Is there deconfinement-driven J/Y suppression?

37. Still flowing at pT = 8 GeV/c? Unlikely!! A puzzle at high pT Nu Xu Adler et al., nucl-ex/0206006

38. Vitev: they can get v2 right • There is a quantitative difference • Calculations/fits with flat • or continuously growing Check against high-pT data (200 AGeV) b=7 fm b~7 fm C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006 Same for 0-50% • The decrease with pT is now • supported by data • For minimum bias this rate is • slightly slower K. Filimonov [STAR Collab.], arXiv: nucl-ex/0210027 See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)

39. v2 of mesons & baryons Au+Au at sNN=200GeV 1) High quality M.B. data!!! 2) Consistent between PHENIX and STAR pT < 2 GeV/c v2(light) > v2(heavy) pT > 2.5 GeV/c v2(light) < v2(heavy) Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001) v2

40. Nbinary ? 2003 ? PHENIX 130 BRAHMS PRL88(02) STAR 130 Npart/2 hch 15% too many particles, baryons over-quenched, but predicted the suppression BUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?

41. kT dependence of R Centrality is in top 30% • Broad <kT> range : 0.2 - 1.2 GeV/c • All R parameters decrease as a function of kT •  consistent with collective expansion picture. • Stronger kT dependent in Rlong have been observed. kT : average momentum of pair

42. Comparison of kaon to pion In the most 30% central

43. Comparison with hydrodynamic model Centrality is in top 30% Recent hydrodynamic calculation by U.Heinz and P. F. Kolb (hep-ph/0204061) Hydro w/o FS • Standard initialization and freeze out which reproduce single particle spectra. Hydro at ecrit • Assuming freeze out directly at the hadronization point. (edec = ecrit) kT dependence of Rlong indicates the early freeze-out?

44. kT dependence of Rout/Rside A. Enikizono QM2002 C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV: @0.25 GeV/c

45. HBT PUZZLE Small Rout implies small Dt P.Kolb Small Rbeam implies small breakup t, ~10 fm/c Large Rside implies large R

46. near-side correlation of charged tracks (STAR) trigger particle pT = 4-6 GeV/c Df distribution for pT > 2 GeV/c signature of jets also seen in g (p0) triggered events (PHENIX) trigger particle pT > 2.5 GeV/c Df distribution for pT = 2-4 GeV/c Jet Evidence in Azimuthal Correlations at RHIC QM2002 summary slide (Peitzmann) M. Chiu, PHENIX Parallel Saturday

47. raw differential yields PHENIX Preliminary 2-4 GeV Identifying Jets - Angular Correlations • Remove soft background • by subtraction of mixed event distribution • Fit remainder: • Jet correlation in f; • shape taken from • PYTHIA • Additional v2 component • to correct flow effects

48. Verify PYTHIA using p+p collisions Df (neutral E>2.5 GeV + 1-2 GeV/c charged partner) Make cuts in  to enhance near or far-side correlations Blue = PYTHIA ||>.35 ||<.35

49. In Au+Au collisions Df (neutral E>2.5 GeV + charged partner) 1-2 GeV partner Correlation after mixed event background subtraction Clear jet signal in Au + Au Different away side effect than in p+p ||<.35 ||>.35 1/Ntrig dN/d 1/Ntrig dN/d

50. jets or flow correlations? fit pythia + 2v2vjcos(2) 1-2 GeV/c partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c 1/Ntrig dN/d Df v2vj Jet strength See non-zero jet strength as partner pT increases!